L A N D M A N A G E M E N T H A N D B O O K
36
Silviculture of Temperate and Boreal Broadleaf-conifer Mixtures
1996
Province of British Columbia Ministry of Forests Research Program Silviculture of Temperate and Boreal Broadleaf-conifer Mixtures
P.G. Comeau and K.D. Thomas editors
Province of British Columbia Ministry of Forests Research Program Canadian Cataloguing in Publication Data Main entry under title: Silviculture of temperate and boreal broadleaf-conifer mixtures
(Land management handbook ; 36)
Summary of papers presented at workshop entitled: Silviculture of temperate and boreal broadleaf- conifer mixtures, held Feb. 28 and Mar. 1, 1995 in Richmond, B.C. C.f. Pref. ISBN 0-7726-2806-8
1. Plant competition – British Columbia – Congresses. 2. Conifers – Habitat – British Columbia – Congresses. 3. Forest ecology – British Columbia – Congresses. I. Comeau, P. G., 1954– II. Thomas, K. D. III. British Columbia. Ministry of Forests. Research Branch. IV. Series.
SD397.C7S54 1996 634.9’56 C96-960048-8
Prepared by P.G. Comeau and K.D. Thomas (editors) for B.C. Ministry of ForestsResearch Branch Research Branch Bastion Square Victoria, BC
Published by B.C. Ministry of Forests Forestry Division Services Branch Production Resources Broad Street Victoria, BC
© ␣ Province of British Columbia
Copies of this and other Ministry of Forests titles are available from Crown Publications Inc. Fort Street Victoria, BC LIST OF CONTRIBUTORS
Name Address I.R. Cameron B.C. Ministry of Forests, Research Branch, Columbia Street, Kamloops, BC P.G. Comeau B.C. Ministry of Forests, Research Branch, Bastion Square, Victoria, B..C K. Day University of B.C., Alex Fraser Research Forest, South Seventh Avenue, Williams Lake, BC S.C. DeLong B.C. Ministry of Forests, th Ave., Prince␣ George, BC K. Deschamps Canadian Forest Service - Pacific & Yukon Region, RR #. Site , Comp. - Old Cariboo Hwy, Prince George, BC A. Eastham Canadian Forestry Service, Prince George District Office, RR #, Site , Comp. , Prince George, BC R. Hall Woodlands Superintendent, Canadian Chopsticks Manufacturing Co. Ltd., Mile , Industrial Park; P.O. Box , Fort Nelson, BC G.J. Harper B.C. Ministry of Forests, Research Branch, Victoria, BC D.E. Hibbs Associate Professor, Department of Forest Science, Oregon State University, Corvallis, OR R. Kabzems B.C. Ministry of Forests, Fort St. John, BC M.J. Kelty Department of Forestry and Wildlife Management, University of Massachusetts, Amherst, MA J.P. Kimmins Department of Forest Sciences, University of British Columbia, Vancouver, BC K. Kyllo Kyllo Bros. Guiding and Outfitting, Box , Hudson’s Hope, BC G.B. MacDonald Mixedwood Silviculture Program, Ontario Forest Research Institute, P.O. Box , Sault Ste. Marie, ON D.A. MacIsaac Canadian Forest Service, Northwest Region, Natural Resources Canada, - Street, Edmonton, Alberta, C. Messier Groupe de Recherche en Ecologie Forestière (GREF), Dép. des sciences biologiques, Université du Québec à Montréal, C.P. , Succ. Centre- Ville, Montréal, PQ. K. Mielikäinen Finnish Forest Research Institute, Unioninkatu A, SF- Helsinki, Finland
iii List of Contributors Continued
Name Address S. Navratil Canadian Forest Service, Northwest Region, Natural Resources Canada, - Street, Edmonton, AB C.E. Prescott Department of Forest Sciences, University of British Columbia, Vancouver, BC M. Prévost Ministère des resources naturelles, Direction de la recherche forestière, , rue Einstein, Sainte- Foy, PQ K.J. Puettmann Assistant Professor, Department of Forest Resources, University of Minnesota, St. Paul, MN D. Sachs Forest Science Department, Oregon State University, Corvallis, OR S. Simard Research Silviculturist, Kamloops Forest Region, Columbia Street, Kamloops, BC C. Staley Department of Forest Sciences, University of British Columbia, Vancouver, BC D. Tanner Ministry of Forests, th Avenue, Prince George, BC K.D. Thomas B.C. Ministry of Forests, Research Branch, Victoria, BC R. van den Driessche B.C. Ministry of Forests, Research Station, Glyn Road, Victoria, BC X. Wei Department of Forest Sciences, University of British Columbia, Vancouver, BC L. Zabek Department of Forest Sciences, University of British Columbia, Vancouver, BC
iv Silviculture of Temperate and Boreal Broadleaf-conifer Mixtures
PREFACE
Broadleaved species are a common component agement of mixtures (including appropriate of British Columbia forests. They occur as pure initial densities, espacement, and arrangements stands as well as in mixture with conifer species. as intimate or patchy mixes) and their use. Retaining and managing broadleaves in our for- These proceedings summarize papers pre- ests is desirable for many reasons, including: sented at a workshop entitled: “Silviculture of biodiversity, wildlife habitat, aesthetics, provid- temperate and boreal broadleaf-conifer mix- ing nurse crops for conifers, improving nutrient tures” which was held in Richmond B.C. Febru- supplies, ameliorating or reducing risk of forest ary and March , . The purpose of this health problems, diversifying forest end prod- workshop was to review current knowledge of ucts, and potentially increasing yield. However, the silviculture of temperate and boreal problems can be encountered when the broadleaf-conifer mixtures and the conse- broadleaf species has a more rapid initial quences of growing mixed stands. Nineteen pa- growth rate than the conifers. Critical questions pers were presented during the workshop and exist about the feasability of growing species twelve posters displayed. Total attendance was mixtures and their consequences, and the man- approximately .
vii ACKNOWLEDGEMENTS
The success of this workshop was due to the Boateng, Alan Vyse, Pasi Puttonen, Chris contributions made by numerous individuals. Thompson, and Dave Coates. Our appreciation Dwight Yochim and Shirley Sato of the Univer- is also extended to the following for assistance sity of British Columbia Forestry Continuing with the review of manuscripts: Catherine Studies program were instrumental in arranging Bealle-Statland, Lorne Bedford, Jacob Boateng, the facilities, advertising, and administering of Dave Coates, Dave Coopersmith, Paul Courtin, this workshop. Speakers willingly agreed to George Harper, Richard Kabzems, Christian make presentations and to provide manuscripts Messier, Cindy Prescott, Pasi Puttonen, Paul for publication in this proceedings. Their co- Sanborn, Mel Scott, and Alan Vyse. We are operation facilitated the organization of the grateful to Susan Bannerman for assistance in workshop and the preparation of these proceed- the editing of this proceedings and to Paul ings. Several individuals helped to prepare for Nystedt and David Izard for their assistance the event or assisted during the workshop. I with this publication. would like to acknowledge: George Harper, Financial support for this workshop and for Balvinder Biring, Marilyn Blache, and Cheryl publication of these proceedings has been pro- Pehl, and the session moderators: Jacob vided by Forest Renewal BC.
viii LIST OF PARTICIPANTS
Name Address Graham L. Allison Pine Grass Silviculture Ltd., PO Box , Mile Ranch, BC Jim Annunziello B.C. Ministry of Forests, Kootenay Lake Forest District, RR #, Site , Comp. , Highway A, Nelson, BC Bev Atkins Forestry Assistant, Alex Fraser Research Forest - U.B.C., South th Avenue, Williams Lake, BC Rob N. Baron Ainsworth Lumber Co. Ltd., Box , Savona, BC Catherine Bealle Statland B.C. Ministry of Forests, Research Branch, Forest Productivity & Decision Support, - Douglas Street, Victoria, BC Andy Becker Rustad Bros. & Co. Ltd., Pacific Street, Prince George, BC Lorne Bedford B.C. Ministry of Forests, Silviculture Practices Branch, Bastion Square, Victoria, BC Daniel T. Biggs Renewable Tenures Forester, B.C. Ministry of Forests, Mackenzie Forest District, Bag , Mackenzie, BC Norm Bilodeau B.C. Ministry of Forests, Kispiox District, Box , Hazelton, BC Dr. Jacob Boateng B.C. Ministry of Forests, Silviculture Practices Branch, Bastion Square, Victoria, BC Keith R. Boyes B.C. Ministry of Forests, Vernon Forest District, th Avenue, Vernon, BC Kevin R. Brown B.C. Ministry of Forests, Research Station, Glyn Road, Victoria, BC Robert (Robin) Brown B.C. Ministry of Forests, Silviculture Branch, Bastion Square, Victoria, BC Phil Burton U.B.C. Faculty of Forestry, - Main Mall, Vancouver, BC Ian R. Cameron B.C. Ministry of Forests, Kamloops Forest Region, Columbia Street, Kamloops, BC Dan Carter Integrated Silviculture Services, PO Box ( Foley Cres), Prince George, BC A. Paul Chalifour B.C. Ministry of Forests, Duncan Forest District, Hawkes Place, Duncan, BC Dave Cheyne Canadian Forest Service - TDU, nd Street, Edmonton, AB
ix List of Participants Continued
Name Address Dave Coates B.C. Ministry of Forests, Prince Rupert Forest Region, Bag , Alfred Avenue, Smithers, BC Merle D. Collinge B.C. Ministry of Forests, Kamloops Forest District, Dalhousie Drive, Kamloops, BC Timothy S. Conlin B.C. Ministry of Forests, Research Branch, Glyn Road, Research Station, Victoria, BC Dave Coopersmith B.C. Ministry of Forests, Prince George Forest Region, - th Ave., Prince George, BC Paul J. Courtin B.C. Ministry of Forests, Vancouver Forest Region, Labieux Road, Nanaimo, BC John Steven DeGraag Inland Timber Management Ltd., - nd Avenue North, Williams Lake, BC Brian A. DeGusseme Greater Vancouver Regional District, Kingsway, Burnaby, BC Kerry C. Deschamps Canadian Forest Service, Prince George District Office, RR#, Site , Comp. , Prince George, BC Hongyun Dong B.C. Ministry of Forests, Fort Nelson Forest District, RR#, Mile , Alaska Hwy., Fort Nelson, BC Ronald D Donnelly B.C. Ministry of Forests, RR #, Mile , Alaska Hwy., Fort Nelson, BC Brenda Dyck Evans Forest Products Ltd., General Delivery, Malakwa, BC VE J H. Arnold Erickson B.C. Ministry of Forests, Arrow Forest District, Columbia Avenue, Nelson, BC Leonard L. Feldes B.C. Ministry of Forests, Chilliwack Forest District, PO Box , Rosedale, BC Robert L. Furness B.C. Ministry of Forests, Duncan Forest District, Duncan Street, Duncan, BC Terry Gordon Sylvan Forestry Limited, Pirates Lane, Nanaimo, BC Malcolm Graham Craigievar Forest Services, West th Avenue, Vancouver, BC Ronald R. Gray B.C. Ministry of Forests, Kamloops Forest Region, Columbia Street, Kamloops, BC Marvin A. Grismer Contractor, RR #, Box , Site , Comox, BC
x List of Participants Continued
Name Address Gordon Grunerud B.C. Ministry of Forests, Kootenay Lake Forest District, RR #, S-, C-, Nelson, BC Robert S.D. Hall Canadian Chopstick Manufacturing Co., PO Box , Fort Nelson, BC Ian Hamilton Ainsworth Lumber Co. Ltd., PO Box , Mile House, BC Paul Hanna B.C. Ministry of Forests, Kispiox Forest District, PO Box , Hazelton, BC Norman A. Holt Cenwest Ent. () Ltd., RR #, Site , Comp. , Vanderhoof, BC Jim F. Horkoff Manager, Strathinnes Forestry Consultants, PO Box , Nelson, BC John Howell Greater Vancouver Water District, Kingsway, Burnaby, BC Dr. Hamish Kimmins U.B.C. Faculty of Forestry, Department of Forest Sciences, - Main Mall, Vancouver, BC Charles B. Klasen B.C. Ministry of Forests, Vancouver Forest Region, Labieux Road, Nanaimo, BC Marek Krasowski B.C. Ministry of Forests, Red Rock Research Station, RR #, RMD , Prince George, BC Robert H. Kyle Kyle Forest Management Ltd., #, Auckland Street, New Westminster, BC Ken Kyllo Box , Hudson’s Hope, BC Phil Lepage B.C. Ministry of Forests, Bag , Smithers, BC Tony Letchford B.C. Ministry of Forests, Red Rock Research, RR #, RMD , Prince George, BC Steve Lindsey B.C. Ministry of Forests, Fort Nelson Forest District, RR #, Mile , Alaska Hwy., Fort Nelson, BC Debbie Long Evans Forest Products Ltd., General Delivery, Malakwa, BC A. Kelly Low Weyerhaeuser Canada Ltd., PO Box , Kamloops, BC Tim McCarthy Stewart & Ewing Associates Ltd., (Seafor), - Simpson Road, Richmond, BC Leanne Marcy McKinnon U.B.C. Faculty of Forestry, Dept. of Forest Sciences, - Main Mall, Vancouver, BC Daniel Mailly U.B.C. Faculty of Forestry, - Main Mall, Vancouver, BC
xi List of Participants Continued
Name Address Barbara E. Marshall B.C. Ministry of Forests, Box , McBride, BC Robert J. Martin B.C. Ministry of Forests, Clearwater District, Box , R.R. #, Clearwater, BC Simon Martin Strathinnes Forestry Consultants, PO Box , Nelson, BC Janet Mitchell FERIC, East Mall, Vancouver, BC Kenneth J. Mitchell B.C. Ministry of Forests, Research Branch, - Douglas Street, Victoria, BC Helmuth Mueller SloCanada Forest Products Ltd., Tackama Division, RR #, Fort Nelson, BC Reinhard Muller B.C. Ministry of Forests, Duncan Forest District, Duncan Street, Duncan, BC Ross Munsey International Ecosynectics Corp., Martin Street, Penticton, BC Shawn O’Brien Evans Forest Products Ltd., General Delivery, Malakwa, BC Phil James O’Connor Coast Mountain Hardwoods Inc., Hopcott Road, Delta, BC F. Mark Palmer B.C. Ministry of Forests, Port Alberni Forest District, th Avenue, Port Alberni, BC Michael C. Pedersen B.C. Ministry of Forests, Silviculture Practices Branch, Bastion Square, Victoria, BC Cheryl A. Power Silviculture Forester, Malcolm Knapp Research Forest - U.B.C., RR #, Maple Ridge, BC Cindy Prescott U.B.C. Faculty of Forestry, Department of Forest Sciences, Rm. , Main Mall, Vancouver, BC Don Purdy B.C. Ministry of Forests, Vernon Forest District, -th Avenue, Vernon, BC Ray Raatz B.C. Ministry of Forests, Mile House Forest District, PO Box , S. Cariboo Hwy, Mile House, BC Gerald Reichenback B.C. Ministry of Forests, Nelson Forest Region, Lake Street, Nelson, BC John S. Robertson Proprietor, Aspen Silviculture Ltd., Box , Invermere, BC Paul T. Sanborn B.C. Ministry of Forests, Prince George Forest Region, Fourth Avenue, Prince George, BC Mel Scott B.C. Ministry of Forests, Vancouver Forest Region, Labieux Road, Nanaimo, BC
xii List of Participants Concluded.
Name Address Paul Sears Rustad Bros. & Co. Ltd., Pacific Street, Prince George, BC Mark Seilis B.C. Ministry of Forests, Mile House Forest District, PO Box , S. Cariboo Hwy, Mile House, BC Derek Sidders Canadian Forest Service - TDU, nd Street, Edmonton, AB, Elaine H. Smith B.C. Ministry of Forests, Chilliwack District, PO Box , Rosedale, BC Ken Smith B.C. Ministry of Forests, Kispiox Forest District, Box , Hazelton, BC Craig C. Stemmler Strathinnes Forestry Consultants, PO Box , Nelson, BC Christopher Thompson B.C. Ministry of Forests, Nelson Forest Region, Lake Street, Nelson, BC Allan Todd Integrated Silviculture Services, PO Box ( Foley Cresc.), Prince George, BC Lou Tromp B.C. Ministry of Forests, Prince Rupert Forest Region, Bag , Alfred Ave., Smithers, BC William L. Voelker Oregon Department of Forestry, State Street, Salem, OR Alan Vyse B.C. Ministry of Forests, Kamloops Forest Region, Columbia Street, Kamloops, BC Jian R. Wang U.B.C. Faculty of Forestry, - Main Mall, Vancouver, BC Adrian Weber U.B.C. Faculty of Forestry, NRC Conservation Program, West th Avenue, Vancouver, BC Xiaohua Wei U.B.C. Faculty of Forestry, Forest Sciences Department, - Main Mall, Vancouver, BC Lisa M. Zabek Graduate Student, U.B.C. Faculty of Forestry, Forest Sciences Department, Rm. - Main Mall, Vancouver, BC Barb J. Zimonick B.C. Ministry of Forests, RR , Site , Comp. , Kamloops, BC Robert van den Driessche B.C. Ministry of Forests, Research Branch, Glyn Road, Victoria, BC Bodo von Schilling von Schilling Forest Management, Box , Merville, BC
xiii
CONTENTS
List of Contributors ...... ii
Preface ...... vii
Acknowledgements ...... viii
List of Participants ...... ix
Why Mixedwoods Phil Comeau ...... 1
Approaches to Managing Birch-dominated Mixed Stands in Finland Kari Mielikäinen ...... 8
Mixtures of Paper Birch and Conifers: An Ecological Balancing Act Suzanne Simard ...... 15
Competition Dynamics in Juvenile Boreal Hardwood-Conifer Mixtures D.A. MacIsaac and S. Navratil ...... 23
Silvicultural Systems for Managing Deciduous and Mixedwood Stands with White Spruce Understorey S. Navratil ...... 35
Stand Dynamics and Silviculture of Mixed Conifer-hardwood Stands in Southern New England Matthew J. Kelty ...... 47
Managing Light and Understorey Vegetation in Boreal and Temperate Broadleaf-conifer Forests Christian Messier ...... 59
Ecology and Dynamics of Mixed Red Alder–conifer Stands Klaus J. Puettmann and David E. Hibbs ...... 82
Mixedwood Management Research and Practice in Quebec Marcel Prévost ...... 97
Mixedwood Management Research and Practice in Ontario G. Blake MacDonald ...... 102
Investigations of Planting White Spruce Under a Trembling Aspen Canopy David Tanner, S. Craig DeLong, and Andrea Eastham ...... 114
Vertical Stratification in Some 50-Year-Old Mixed-Species Stands in the Interior Cedar-Hemlock Zone Ian R. Cameron ...... 122
xv Habitat Management for Game and Parasite Control by Prescribed Burning Ken Kyllo ...... 126
Simulation of the Long-term Impacts of Alder–Douglas-fir Mixtures on the Sustainability of Site Productivity Using the Ecosystem Management Model FORECAST Xiaohua Wei and J. P. (Hamish) Kimmins ...... 129
Summaries of Posters ...... 139
Control of Spruce Weevil Damage by Introduction of Alternative Commercial Species (FRDA II Project 93-03) Ken Day ...... 140
Boreal Mixedwood Understory Protection Demonstration Trial in Fort Nelson, B.C. Kerry C. Deschamps and Robert Hall ...... 142
Effects of Red Alder on Stand Dynamics and Nitrogen Availability (EP-1121) P.G. Comeau, G. Harper, B. Biring, and K.D. Thomas ...... 144
Response of Hybrid Poplar to Phosphorus Fertilization and Water Supply R. van den Driessche ...... 149
Simulation of the Growth of Mixed Stands of Douglas-fir and Paper Birch Using the FORECAST Model D. Sachs ...... 152
Mixedwood Forests: Implications for Litter Decomposition C. Prescott, R. Kabzems, S. Simard, C. Staley, L. Zabek, and P.G. Comeau ...... 159
xvi Why Mixedwoods?
PHIL COMEAU
Abstract Introduction
Mixedwood forests are found throughout much of A mixedwood stand includes a mixture of different British Columbia. Six major broadleaf species species. Broadleaf-conifer mixtures generally (trembling aspen, paper birch, balsam poplar, black consist of combinations of deciduous cottonwood, red alder, and bigleaf maple) are (“hardwood”) and coniferous species. Examples found in mixture with conifer species. Available include mixtures of aspen (Populus tremuloides data suggest that mixedwood forests represent Michx.) with spruce (Picea glauca [Moench] Voss), approximately % of the area of productive forests and red alder (Alnus rubra Bong.) with Douglas-fir in British Columbia. (Pseudotsuga menziesii [Mirb.] Franco). Reasons to manage for mixedwoods include: A stand is generally identified as mixedwood if it () mixedwoods occur naturally in British␣ Columbia; has recognizable component of a second species. () they provide a valuable visual resource; For example, a Douglas-fir stand should be () mixedwood forests are more diverse and considered mixedwood if it has red alder per support a greater diversity of other organisms; hectare; a stand of aspen should be considered () mixedwood stands may suffer reduced impacts mixedwood if it has understorey spruce from insect and disease problems and reduced risk per␣ hectare. of loss; () broadleaves can serve as a valuable Mixedwood stands may be multistoreyed, at nurse crop for conifers; () broadleaves can least in early stand development stages, or may improve nutrient availability in mixed stands; have only a single canopy layer. The structure of () mixed stands may provide greater wood yield mixedwood forests found in British Columbia than pure stands; () growing mixedwood stands tends to change over time. Broadleaves native to may be more readily sustainable than growing British Columbia are early seral species that have single-species stands; () growing mixtures may rapid rates of initial height growth and reach give better economic returns than pure stands; and maturity at young ages. Consequently, during the () regulations may require management early stages of stand development most stands have of␣ mixtures. a dominant broadleaf overstorey with a coniferous A knowledge of the ecology and silvics of the understorey. As stands age, conifers develop species in the mixture is fundamental to successful underneath the broadleaf canopy and in gaps in mixedwood management. We need to embrace the canopy. As mixedwood stands mature, conifers more complex models of stand development than may gradually grow into the main canopy and are used in the management of coniferous stands. eventually overtop the broadleaves. Better information on the dynamics of mixedwood As well as occurring as mixed stands, stands, including information on growth patterns mixedwood forests that are mosaics of stands as well as on key processes, is required to support dominated by broadleaves, conifers, or mixtures of management decisions. the two are common in British Columbia.
1 Broadleaved and mixedwood forests are found briefly discusses several reasons why mixedwood predominantly in four biogeoclimatic zones: management should be considered on appropriate Coastal Western Hemlock ₍CWH₎, Interior Cedar– sites in British Columbia Hemlock ₍ICH₎, Sub-Boreal Spruce ₍SBS₎, and Boreal Mixedwood Forests Are a Natural Feature of the White and Black Spruce ₍BWBS₎. Landscape British Columbia has approximately Six major broadleaved species are native to ␣ million ha of productive forest land (Still British Columbia. Red alder and bigleaf maple et␣ al.␣ ). Hardwoods are the lead species on % (Acer macrophyllum Pursh) are largely restricted to of this land ( million ha) with aspen forests southwestern British Columbia. Paper birch (Betula covering over million ha (Figure ). Most of these papyrifera Marsh.) is found throughout the interior forests have a coniferous component either in the and northeastern portions of the province. understorey, in secondary canopies, or in the Trembling aspen is most abundant in the central main␣ canopy. and northern interior and in the northeast. Black cottonwood (Populus balsamifera ssp. trichocarpa Mixedwood (estimate) [T. & G.] Brayshaw) occurs throughout southern (11 million ha) and central British Columbia, while balsam poplar (Populus balsamifera ssp. balsamifera L.) is found in the province’s northeast. 23.9% These broadleaved species occur in mixed stands with many of the major conifers found in Broadleaved British Columbia. In the CWH zone, red alder, 10.9% (5 million ha) bigleaf maple, cottonwood, and paper birch may be found in mixture with Douglas-fir, western 65.2% hemlock (Tsuga heterophylla [Raf.] Sarg.), western Conifer redcedar (Thuja plicata Donn ex D. Don in (30 million ha) Lamb.), Sitka spruce (Picea sitchensis [Bong.] Carr.), and grand fir (Abies grandis [Dougl. ex D. Forested area (Crown land) = 46 million ha Don in Lamb.] Lindl.). In the ICH and SBS zones, paper birch, black cottonwood, and trembling Composition of British Columbia’s productive aspen may be found in mixture with Douglas-fir, forest land base (after Still et␣ al. 1995). western hemlock, western redcedar, western larch (Larix occidentalis Nutt.), lodgepole pine (Pinus contorta var. latifolia Engelm.), Engelmann spruce Detailed statistics are not available describing (Picea engelmannii Parry ex. Engelm. and hybrids the extent of mixedwood forests where hardwoods with P. glauca [Moench] Voss), subalpine fir (Abies are a secondary component. Personal observations lasiocarpa [Hook.] Nutt.), and western white pine suggest that approximately % of conifer- (Pinus monticola Dougl. ex D. Don in Lamb.). dominated stands in the CWH zone and % of White spruce (Picea glauca [Moench] Voss), black conifer-dominated stands in the ICH, SBS, BWBS, and spruce (Picea mariana [P. Mill.] B.S.P.), subalpine the Spruce-Willow-Birch ₍SWB₎ zones are actually fir, lodgepole pine, and tamarrack (Larix laricina mixedwood. Based on these observations, I [DuRoi] K. Koch) may be found in mixture with estimate that mixedwood stands cover another trembling aspen, paper birch, and balsam poplar in ␣ million ha (Figure ). These estimates suggest the BWBS zone. that mixedwood and broadleaved forests make up about % of the province’s productive forests. Why Mixedwoods? Visual Resource Mixedwood stands are an Historically, mixedwood forests have developed important visual resource. The presence of a simply by virtue of natural succession. Virtually no broadleaf component helps to create variety during intentional management of mixedwood stands has spring, summer, and winter, as well as providing occurred in the province. The following section autumn colour.
2 Biodiversity The presence of a species mix gives Nutritional Role Red alder can contribute to site greater species and structural diversity. Mixed nitrogen capital through symbiotic nitrogen stands and forests provide important habitat for fixation. In pure stands of red alder, nitrogen birds and other wildlife (Enns et␣ al. ; Lance fixation rates are usually between and and Phinney ). Mixedwood stands generally ␣kg·ha- ·yr- (Binkley et␣ al. ). In mixed support much greater numbers and greater stands, fixation rates of – kg · ha- ·yr- have diversity of wildlife species than do coniferous been reported (Miller and Murray ; Binkley stands (McComb ). Mixed stands also support et␣ al. ). Preliminary evidence suggests that a greater diversity of understorey species than do rates of nitrogen fixation by free-living soil bacteria pure stands. Understorey diversity can create better are higher under paper birch and mixed stands habitat for many wildlife species. Since broadleaves than under coniferous stands (Simard ). grow and reach maturity faster than conifers they In some instances the presence of deciduous can be important to many species, including litter can accelerate decomposition and turnover of cavity-nesting birds. conifer litter. Fyles and Fyles ₍₎ reported that a small component of red alder mixed with Douglas- Forest Health By growing a mixture of species it is fir litter can significantly increase rates of litter possible to reduce the risk of stand loss due to decomposition. insects, disease, or other factors. Sitka spruce growing under or near red alder Yield A recent analysis (Massie et␣ al. ) generally have substantially reduced incidence of indicates that we are currently harvesting leader weevil damage (McLean , ). Side approximately . million m of aspen and shade and overtopping broadleaves also reduce .␣ million m of red alder in British Columbia. weevil damage to white spruce in British Columbia While some aspen is used for chopsticks and other (Taylor et␣ al. ) and white pine (Pinus strobus) products, most is chipped and used for pulp or in central and northeastern North America (Stiell oriented strandboard. Red alder is used primarily and Berry ; Katovich and Morse ). for saw lumber, while some red alder chips are Red alder is resistant to laminated root rot exported for pulping. Massie et␣ al. () suggested (Phellinus weirii) (Harrington et␣ al. ). This that more than million m of hardwood from fungus can live in stumps of harvested conifers and stands identified as predominantly deciduous could can infect conifers planted on the site. The be used each year. If all species are used efficiently presence of red alder appears to reduce the spread during commercial thinning or multiple harvesting of Phellinus and also appears to accelerate stump entries, additional volume might be realized from decay, potentially leading to “sanitation” of the site. mixedwood stands. Paper birch is immune to Phellinus weirii and is Species mixtures have the potential to provide resistant to Armillaria root rot (Morrison et␣ al. greater total yields and diversity of end products. ). The presence of birch in mixture with Simulations using the FORCYTE- ecosystem model conifers may reduce the impact of Armillaria suggest that maximum yield of Douglas-fir and red (H.␣ Merler, B.C. Ministry of Forests, pers. comm., alder stands would occur when the species are ), as well reducing economic risk. mixed in a proportion of % ( trees per hectare) Douglas-fir and % ( trees per Nurse Crop The presence of some deciduous cover hectare) red alder (Figure ). This set of can reduce the vigour of other vegetation. Lieffers simulations suggests that such a mixture would and Stadt ₍₎ provided an excellent illustration yield % more wood than pure stands of either of the beneficial effects of aspen cover in reducing species. Other simulation studies have suggested competition from bluejoint reedgrass. Similarly, that between and red alder per hectare McLennan and Klinka ₍₎ reported that black would contribute to productivity without cottonwood can serve as a nurse crop for western negatively affecting yield of Douglas-fir (Miller and redcedar by reducing the vigour of salmonberry Murray ; Comeau and Sachs ) (Figure ). and other shrubs. When densities or proportions become too high, Aspen (Stathers ) and birch (Perala and broadleaved species may substantially reduce light Alm␣ ) canopies can protect conifer seedlings (Figure ) and growth of understorey conifers. from␣ frost.
3 300 1.0
Birch 250 log(t)= -0.0927*ba 0.8 n= 30 , R2= 0.77
200 Red alder log(t)= -0.1058*ba 0.6 n= 12 , R2= 0.58 150 Dr biomass Fd biomass 0.4 100 Total biomass
50 Fractional transmittance 0.2 Harvested biomass (Mg/ha)
0 0 20 40 60 80 100 0 0 10203040 Douglas-fir (%) Basal area (m2/ha) Simulated effects of mixing red alder (Dr) and Douglas-fir (Fd) in differing proportions. The fraction of diffuse sunlight penetrating paper Simulation runs were done using FORCYTE-11 birch and red alder canopies decreases (Comeau and Sachs 1992); runs completed for a exponentially as tree basal area increases. medium site; Douglas-fir harvested at 80 years; Graphs are based on field measurements red alder harvested at 40 and 80 years. collected at several locations in British Columbia using LICOR LAI-2000 plant canopy analyzers and measurement of broadleaves in small plots 300 around each light measurement point. Red alder Douglas-fir 250 Long-term Sustainability Hardwoods are important in forest succession, commonly 200 occurring as early seral species after disturbance. Contributions of hardwoods to nutrient supplies 150 and their effects on nutrient cycling will likely help to ensure that forest productivity does not decline 100 over time. Comeau and Sachs () found that simulated yields declined with successive rotations 50 of pure Douglas-fir on a medium site. However,
Harvested stemwood biomass (Mg/ha) 0 the simulations suggest that managing for mixed 0 50 100 200 300 400 600 1000 2000 stands of Douglas-fir and red alder with red Initial red alder density (stems per hectare) alder per hectare will result in stable and sustainable yields over successive rotations. Predicted effects of initial red alder density on cumulative stemwood biomass yield over 80 Economics It is important to balance establish- years of Douglas-fir and red alder in mixed ment and stand-tending costs with financial stands. Results are based on simulations using returns at harvest. Preliminary analyses (Pearce FORCYTE-11 for a site with Douglas-fir SI =30m at ; Vyse ) suggest that better financial 50 years and red alder SI = 17 m at 50 years; returns may be possible in some circumstances red alder harvested at 40 and 80 years; (for␣ aspen and birch) by managing for mixedwoods Douglas-fir were harvested at 80 years (from rather than investing large amounts of money in Comeau and Sachs 1992). converting broadleaved or mixedwood stands to␣ conifers.
4 Regulations The Forest Practices Code of British information about trembling aspen, paper birch, Columbia requires that a minimum number of black cottonwood, balsam poplar, red alder, and broadleaves be retained where these occur maple mixtures. naturally. Section ₍₎ of the regulations (B.C. Managing mixedwood stands requires careful Ministry of Forests ) prescibes that a thought. Mixtures must be managed on the basis mixedwood stand should be established where one of the species present, site conditions, and forest existed before harvesting or where a mix of species management objectives. A good knowledge of is ecologically suited to the area. ecology and silvics is fundamental to success. As Smith ₍₎ suggests, “What we need to do is to fit Where Should We Grow Mixedwoods silviculture to the stand development process in British Columbia? rather than always trying to get the stand to fit the simple model.” Areas that currently support mixedwood stands Mixedwood stands are dynamic ecosystems will be the focus of future mixedwood involving many linked processes. Spatial variability management activities. These areas include exists within stands, as does temporal variation in substantial portions of the CWH, ICH, SBS, and BWBS stand characteristics both through the year and zones. Specific sites within these zones must be from year to year. In the face of such complexity, selected on the basis of their suitability for each we can learn much from observation and from species as well as for mixedwood management. The small-scale trials. We need to do research to fill biogeoclimatic ecosystem classification system important knowledge gaps and we need to docu- provides us with an excellent framework for species ment and share our observations and experiences. selection and mixedwood management decisions. However, information on the growth of broadleaf References species and broadleaf-conifer mixtures on each major site classification unit is required as a basis Armson, K.A. . The boreal mixedwood for- for decision making. ests of Ontario: past, present and future. In Management and utilization of northern How Do We Manage and Grow mixedwoods. J.K. Samoil (editor). North. Mixedwood Stands? For. Cent. Can. For. Serv. Edmonton, Alta. pp. –. Broadleaved species are generally shade-intolerant Binkley, D., K. Cromack, Jr., and D.D. Baker. early seral species that tend to outgrow associated . Nitrogen fixation by red alder: biology, conifers. When the dominant canopy becomes too rates, and controls. In The biology and man- vigorous or casts too much shade, growth and agement of red alder. D.E. Hibbs, D.S. survival of understorey conifers will decline. DeBell, and R.F. Tarrant (editors). Oreg. Similarly, the growth of broadleaves such as red State Univ. Press, Corvallis, Oreg. pp. –. alder, aspen, or paper birch will decline when they British Columbia Ministry of Forests and Minis- are overtopped by conifers. The challenge is to try of Environment. . Forest Practices recognize when one particular component will Code of British Columbia. Regulations. become dominant and to take appropriate steps. April␣ . Armson ₍₎ pointed out that “well tried Comeau, P.G. and D. Sachs. . Simulation of silvicultural practices for the treatment of the consequences of red alder management mixedwood stands are not available yet.” While we on the growth of Douglas-fir using FORCYTE- have made progress during the intervening 7 years, . B.C. Min. For. and For. Can. Victoria, notably in the management of boreal aspen B.C. FRDA Rep. No. . p. mixedwood forests, a substantial need exists for
5 Enns, K.A., E.B. Peterson, and D. McLennan. McLennan, D.S. and K. Klinka. . Black . Impacts of hardwood management on cottonwood: a nurse species for regenerating British Columbia wildlife: problem analysis. western redcedar on brushy sites. B.C. Min. B.C. Min. For. and For. Can. Victoria, B.C. For. and For. Can. Victoria, B.C. FRDA Rep. FRDA Rep. No. . p. No. . p. Fyles, J.W. and I.H. Fyles. . Interaction of Massie, M.R.C., E.B. Peterson, N.M. Peterson, Douglas-fir with red alder and salal foliage and K.A. Enns. . An assessment of the litter during decomposition. Can. J. For. Res. strategic importance of the hardwood re- :–. source in British Columbia. B.C. Min. For. Harrington, C.E., J.C. Zasada, and E.A. Allen. and For. Can. Victoria, B.C. FRDA Rep. No. . Biology of red alder. In The biology . p. and management of red alder. D.E. Hibbs, Miller, R.E. and M.D. Murray. . The effects D.S. DeBell, and R.F. Tarrant (editors). Oreg. of red alder on growth of Douglas-fir. In State Univ. Press, Corvallis, Oreg. pp. –. Utilization and management of alder. D.G. Katovich, S.A. and F.S. Morse. . White pine Briggs, D.S. DeBell, and W.A. Atkinson (edi- weevil response to oak girdling: results from tors). U.S. Dep. Agric. For. Serv. Gen. Tech. a -year-old study. N. J. Appl. For. :–. Rep. PNW-. Lance, A., R. Pojar, and M. Phinney. ₍₎. Bird Miller, R.E., D.L. Reukema, and T.A. Max. . diversity and abundance in aspen forests in Size of Douglas-fir trees in relation to northern British Columbia. In Ecology and distance from a mixed red alder–Douglas-fir management of British Columbia hardwoods. stand. Can. J. For. Res. : -. Dec. –, , Richmond, B.C. P.G. Comeau, Morrison, D., H. Merler, and D. Norris. . G.J. Harper, M. Blache, J.O. Boateng, and Detection, recognition and management of K.D. Thomas (editors). B.C. Min. For. and Armillaria and Phellinus root disease in the For. Can. Victoria, B.C. FRDA Rep. In press. southern interior of British Columbia. B.C. Lieffers, V.J. and K.J. Stadt. . Growth of Min. For. and For. Can. Victoria, B.C. FRDA understory Picea glauca, Calamagrostis Rep. No. . p. canadensis, and Epilobium angustifolium in Pearce, C.V. . Some thoughts on the econom- relation to overstorey light transmission. ics of mixedwood management. In Northern Can. J. For. Res. :–. Mixedwood ’. Proc. symp. Fort St. John, McComb, W.C. . Red alder: interactions with B.C. Sept. –, . A. Shortreid (editor). wildlife. In The biology and management of For. Can. Victoria, B.C. FRDA Rep. No. . red alder. D.E. Hibbs, D.S. DeBell, and R.F. pp. –. Tarrant (editors). Oreg. State Univ. Press, Perala, D.A. and A.A. Alm. . Regeneration Corvallis, Oreg. pp. –. silviculture of birch: a review. For. Ecol. McLean, J.A. . Effect of red alder overstorey Manage. :–. on the occurrence of Pissodes strobi (Peck) Simard, S. . Mixtures of paper birch and during establishment of a Sitka spruce plot. conifers: an ecological balancing act. These In Insects affecting reforestation: biology and proceedings. damage. R.I. Alfaro and S.G. Glover (editors). Smith, D.M. . Ideas about mixed stands. In For. Can. Victoria, B.C. pp. –. The ecology and silviculture of mixed-species .␣ . Silvicultural control of the white forests. M.J. Kelty, B.C. Larson, and C.D. pine weevil at the UBC Malcolm Knapp Oliver (editors). Kluwer Academic Publishers. Research Forest. In The white pine weevil: Dordrecht, The Netherlands. For. Sci. :– biology, damage and management. Proc. . symp. Jan. –, , Richmond, B.C. R.I. Stathers, R.J. . Summer frost in young forest Alfaro, G. Kiss, and R.G. Fraser (editors). plantations. B.C. Min. For. and For. Can. Can. For. Serv. and B.C. Min. For. Victoria, Victoria, B.C. FRDA Rep. No. . B.C. FRDA Rep. No. . pp. –. Stiell, W.M. and A.B. Berry. . Limiting white pine weevil attacks by side shade. For. Chron. :–.
6 Still, G., A. MacKinnon, and R. Planden. . Vyse, A. ₍₎. Should we grow mixtures of Forest, range and recreation resource analy- paper birch and Douglas-fir in the Interior sis, . B.C. Min. For. Victoria, B.C. Cedar Hemlock Zone? In Ecology and man- Taylor, S.P., R. Alfaro, C. Delong, and L. Rankin. agement of British Columbia hardwoods. . The effects of overstorey shading on Dec. –, , Richmond, B.C. P.G. Comeau, white pine weevil damage to interior white G.J. Harper, M. Blache, J.O. Boateng, and spruce. In The white pine weevil: biology, K.D. Thomas (editors). B.C. Min. For. and damage and management. Proc. symp. Jan. For. Can. Victoria, B.C. FRDA Rep. In press. –, , Richmond, B.C. R.I. Alfaro, G. Kiss, and R.G. Fraser (editors). Can. For. Serv. and B.C. Min. For. Victoria, B.C. FRDA Rep. No. . pp. –.
7 Approaches to Managing Birch-dominated Mixed Stands in Finland
KARI MIELIKÄINEN
Abstract Finland is located between –° N. Wood production, and indeed life in general this far Scots pine (Pinus sylvestris), Norway spruce (Picea north is made possible by the warmth brought by abies), and birch (Betula pendula and B. pubescens) the Gulf Stream. Its influence is transmitted to form both single- and multistoreyed mixed stands Finland from the Norwegian coast. The mean in Finland. Pine and birch are pioneer species that temperature during the growing season varies regenerate only on sites cleared by fire, storm, or between and °C and annual precipitation man. Spruce is a shade-tolerant species that averages – mm. Due to modest evaporation, regenerates either at the same time as other species about a one-third of the forest area (i.e., over or later in the understorey. Spruce is often planted million ha) consists of mires. Roughly half of this under a birch nurse crop to avoid frost damage. peatland area has been drained for forestry. According to studies conducted in Fennoscandia, Finland falls within the boreal coniferous zone mixtures of birch and conifers are both ecologically where fire has always played a decisive role in and economically competitive with pure conifer forest stand succession. Research has revealed that stands. The state of health, wood production, and the country’s natural forests have been burnt over monetary yield of a mixed stand can clearly exceed at average intervals of – years. The areas that of a pure stand. This is the case especially burnt have varied between a few hectares and when growing two-storeyed stands of birch and thousands of hectares, and this has ensured the spruce. Birch can produce a remarkable amount of presence of pioneer tree species. valuable timber for the plywood industry without Management practices in Finnish forests aim at negatively affecting the spruce understorey. The mimicking the natural dynamics of northern abundant regeneration of birch on open lands after boreal forests. This is done by thinning of dying final felling is valuable in reducing conifer planting trees and regenerating forests using clearcuts of – costs. The long-term effects of broadleaved species ␣ ha. Without open land, the climax stage in forest on soil, biodiversity, and landscape can be expected stand succession in Finland is almost without to gain increasing prominence in future forest exception a low-productive, spruce-dominated management. forest stand on acid soil (Parviainen a).
History of Natural Forest in Finland Human Impact on Forests
Following the retreat of glaciation or a natural Apart from hunting, which is a far older forest use, disturbance, light-demanding pioneer tree species the human impact on forests began to be felt as are the first to colonize the land. In Europe, the recently as years ago. Slash-and-burn birches of the genus Betula are an example of such cultivation, which was practised for centuries, pioneers. This genus was the first to establish itself emulated the large cycle of forest stand succession in present-day Finland following the end of the (Figure ). This land use ensured the continued most recent Ice Age, some years ago. The next dominance of Scots pine and birches in the forests. (now commercially valuable) species to follow More to the north, forests were a source of the tar birch was Scots pine (Pinus sylvestris). The last to used to impregnate wooden ships. In the s, arrive, some years ago, was Norway spruce Finland was Europe’s leading tar producer. (Picea abies).
8 LARGE CYCLE
Area free of forest as a consequence of Large-scale destruction catastrophes Immigration of of forest mostly caused by fire pioneer tree species
SMALL CYCLE Phase of decay
Regeneration Pioneer forest stage Aging phase Climax forest phase (on small areas)
Optimum phase
Immigration of climax tree species Elimination of the pioneer species
Transition phase
Development cycles in boreal forests (after Parviainen 1994b).
Slash-and-burn cultivation and tar distillation Commercially Used Tree Species in Finland came to a sudden end in the s. Ships began to be made of iron, and wood was increasingly used Scots pine has traditionally been regarded as the in the emerging wood-based industries. most valuable tree species in Finland. This is Initially, industrial demand for wood focused on explained by the excellent sawlog quality of pine sawlogs. Apart from firewood and the use of spruce when growing on barren soils (sandy soils and the as raw material for groundwood pulp in the poorest peatland sites). In recent years pine quality manufacturing of paper, there was no demand for has declined because pine plantations were small-diameter trees. Consequently, forest established on soils too rich for the species and management consisted of the selection felling of were too widely spaced. the biggest trees. The young age of the forests Scots pine is a light-demanding pioneer species. established on slash-and-burn sites and the poor On the most appropriate sites, its yield over the regeneration of stands treated with selection felling rotation averages – m per hectare and year. The gave cause for concern over the sufficiency of wood rotation for pine in southern Finland varies between during the first few decades since the Declaration and years by which time individual trees of Independence in . reach a dbh of about cm and a height of ␣m.
9 Norway spruce is in its element on the more Development of Finland’s Forest Resources fertile sites. There its mean increment may rise to m per hectare per year over the rotation. The Finnish Forest Research Institute ₍METLA₎ was The␣ corresponding current annual increment can founded in . The first task for the newly exceed m per hectare. Norway spruce is also founded institute’s head of forest mensuration, a␣ sawtimber species, but the product (called Prof. Yrjö Ilvessalo, was to plan and implement the “whitewood”) is not as profitable as that world’s first national forest inventory based on obtained␣ from pine (called “redwood”). statistical and objective sampling. Since the first In Finland, large amounts of spruce wood are inventory, completed in the early s, another used to produce mechanical pulp (groundwood) seven have been carried out in Finland (Tomppo for making paper. The relatively high price of long- and Siitonen ). fibred fine paper means that the paper industry is The survey methods have changed over the capable of paying about the same price for large- years. Survey lines crossing the country from side sized spruce as the sawmilling industry. Therefore, to side have been replaced by a combination of an increasing proportion of sawlog-sized spruce satellite imagery, map and field data, and predictive ends up in pulp and paper mills. models. Despite developments in the methods Alongside aspen (Populus tremula), the used, the inventory data thus far collected form an biologically most aggressive tree species to be unbroken series covering years and reflect the found in Finnish forests are silver birch (Betula development of the country’s forest resources. pendula) and downy birch (B. pubescens). Birch When combined with comprehensive felling regenerates with ease on open land in places where statistics, these data provide us with opportunities the mineral soil is exposed. The advantage of this to examine the effect of forest use on the growing aggressiveness is that birch provides a ready stock resource. supplement in young coniferous stands in cases Apart from demonstrating the significance of where natural calamities have created gaps. the territories lost to the former Soviet Union In its juvenile years, birch is clearly faster during World War II, the survey series show the growing than spruce or pine. A plantation of silver rapid increase in the volume and increment of the birch may attain a height of m in years, a growing stock during the past few decades. The mean dbh of over cm, and a total yield of more primary contributing factor to this increase is the than m per hectare. Downy birch favours bogs intensification of silvicultural activities since the and it has a yield that is about % lower than that s. A minor contributing factor may be the of silver birch. deposition of airborne nitrogen. For Europe as a During the past few decades, birch has become whole, the same two phenomena are estimated to more valuable than pine. The reason for this is that have promoted forest growth by about % since birch of good quality is in great demand by the (Kauppi et al. ). plywood industry. Light-coloured birchwood is During the past decade, the timber harvests popular among interior decorators and furniture from Finland’s forests have been clearly below their makers, and the consumers are prepared to pay for growth. Unless the consumption of wood can be it. This in turn has increased the popularity of significantly increased, Finland’s forest resources breeding and planting birch, especially among can be expected to almost double during the next small-scale woodlot owners. The problem with years (Tomppo and Siitonen ). birch is that of various natural damages caused by The development observed in Finland’s forests moose, hares, and voles. indicate that the proportions of the various tree During the past three decades, birch has also species have remained much the same regardless gained the respect of the pulp industry because of of␣ the changes in silviculture. Both pine and its pulping properties. The demand for birch spruce␣ currently account for about % of the pulpwood is such that – million m have been growing stock, while broadleaves, led by birch, imported annually from neighbouring countries, account for␣ the remaining %. As less than % mainly Russia and the Baltic countries. of␣ the broadleaves occur in pure stands, a considerable proportion of birch is found in conifer-dominated stands.
10 Attitudes Toward Mixed Stands Previous Nordic Studies on Mixed Stands
In Finland, Scots pine, Norway spruce, and birch When compared to the occurrence and significance (silver birch and downy birch) form both single- of mixed stands, few yield studies of their growth and multistoreyed mixed stands. An even-aged have been undertaken. Apart from the difficulty of stand generally results when an open tract of land the task itself, this has been due to the prolonged is colonized by pine and birch. A two-storeyed trust of forestry practitioners in the economic mixed stand is formed when spruce establishes superiority of pure conifer stands. underneath an overstorey of these pioneer In the first Nordic growth study focusing on a species—either naturally or when planted by man. mixed stand, Lappi-Seppälä ₍₎ found that pine The prevailing attitude toward mixed stands in and birch, when unmanaged and grown together Finland has always been cautiously positive. There produced more wood than pure stands of are several reasons for this, the foremost being that corresponding areal size. Lappi-Seppälä also the formation of mixed stands is next to impossible observed that a mixed stand grew no better than to prevent. Allowing birch to fill gaps in stands of the more productive of the two species. The short- conifers reduces costs. Moreover, replanting coming of Lappi-Seppälä’s study is that he did attempts frequently fail. not include managed commercial stands in his work. Mixed stands are generally understood to be Jonsson ₍₎ studied competition between tree healthier than pure stands. Individual trees of the species on temporary sample plots in Sweden. His same species have less root and crown contact in results show that a mixture of species improved mixed stands, reducing the possibility of disease both height and diameter growth, except in cases transmission (Mielikäinen ). Mixed stands are where the trees were in contact with their also more likely to survive should a species-specific neighbours. Fries ₍₎ calculated the mutual pest appear. Indeed, some tree species in a mixed profitability of a stand of spruce and a mixed stand stand may benefit from the damage to others. of spruce and birch. He observed that a mixed In␣ this way, they make up for part of the loss stand of spruce and birch can provide the same in␣ productivity. economic return as a stand of spruce if the spruce The litter of broadleaved tree species is less acidic establishes itself naturally underneath the birch. than that of conifers. This is significant where acid However, to avoid “whipping” damage, the birch rain speeds up the acidification process in the soil should be – years older than the spruce. underlying a coniferous forest. Mixed stands are believed to grow better than pure stands because Recent Nordic Mixed Stand Studies the tree roots of different species take in nutrients from different soil depths (Laitakari ). Single-storeyed Stands Together with the public’s Competition at the canopy level can also restrict growing environmental awareness, a marked tree growth. If the tree species that make up a increase in the demand for veneer logs and birch mixed stand differ in their light requirements, a pulpwood (i.e., birch shortage) has promoted the two-storeyed stand may make better use of the cause of growing mixed stands throughout available light than a stand composed of one Fennoscandia. The numerous studies of the s tree␣ species. and s are proof of this trend. Yet another reason behind the popularity of The mixture effect on stand growth has been growing mixed stands in Finland is that the most interpreted as follows: a positive mixture effect numerous tree species (pine, spruce, and birch) occurs only when the mixed stand yields more enjoy industrial demand. As well, veneer logs of wood than a pure stand of any one of the species the best quality tend to be found in densely grown making up the mixed stand. In reality, the effect is mixed stands composed of spruce and birch. always positive when two species grown on a Birch has always been a symbol of the Finnish certain area of land yield more wood as a mixed landscape. This being so, it is only natural that stand than they do as pure stands (Figure ). birch is also favoured because of its landscape value. The reasons why mixed stand yield is not
11 110 The optimal proportion of birch is usually between 100 and %. Such proportions of birch do not 90 constrain conifer growth and, because of the 80 intraspecies competition, birch grows better in mixed stands than in pure stands. 70 Mixture effect 60 50 40 30 7
Relative net present value (%) 20 30 yr 10 6 150 m3/ha 0 0 20 40 60 80 100 Birch percentage of MAI (%) 5 40 yr 150 m3/ha
The mixture effect (after Valsta 1993). 4 50 yr 200 m3/ha generally the average of the yields of pure stands 3 are the changing competition between tree species and the influence that broadleaved species have on 70 yr 2 250 m3/ha the soil. The studies referred to in the following are
based on temporary sample plots. Consequently, Annual increase in stand volume (%) 1 Norway spruce – Silver birch the results obtained do not include the positive Scots pine – Silver birch long-term effects of the broadleaved component in Birch percent 0 the growing stock. 0 50 100 Contrary to previous knowledge on the subject, the results of mixed-stand studies conducted in the Spruce stand Silver birch stand s in Finland show that the canopy-level Pine stand competition by birch had a negative influence on single-tree growth in mixed stands (Mielikäinen Annual volume growth in mixed stands of silver , ). Because it grows more rapidly, silver birch and conifers (after Mielikäinen 1980, 1985). birch reduced the growth of both pine and spruce when compared to the growth achieved in pure The total yield over the rotation was simulated conifer stands of equal density. Birch itself also for a single-storeyed stand, starting with thinning showed signs of suffering from competition as its carried out at age – years and ending with proportion in the stand increased. Downy birch, final felling at age . The alternatives assessed on the other hand, is not an aggressive competitor were to retain the mixed-stand structure or to and was not observed to reduce the growth remove the birch in conjunction with thinning. of␣ conifers. Results suggest that the addition of birch Stand growth models and models for the growth increases the total yield of both pine and spruce of individual trees were constructed to facilitate stands by –%. For stands of pine, thinnings stand-level growth predictions. Wide variability in should focus more on removing birch as stand age species composition enabled the use of these increases. For spruce plantations, the ideal mixture models to predict growth in pure and mixed has a % component of birch. stands. Figure shows growth estimated for pure In mixed stands of spruce and birch, the yield of and mixed stands using these models. The results sawtimber was almost % higher than in pure indicate that mixed stands (excluding old, pine- stands of spruce. The high prices paid for birch dominated stands) grow better than pure stands. veneer logs and the rapid diameter increase of
12 birch provided monetary yields that were % Finland to grow technically high-quality birch over higher for mixed stands than those for stands of top of spruce until the birch reaches veneer log spruce. Having an admixture of downy birch did dimensions. not promote stand growth when compared to that of pure conifer stands. Apart from this, Mixed Stands and Practical Forestry downy birch is also a valuable species for supple- menting otherwise understocked stands of conifers From Selective Felling to Intensified Forestry At and for its ameliorating influence on the soil. the end of World War II, Finland’s forests were in a Two-storeyed Stands The nurturing of spruce poor state because the practice of selective felling underneath birch has been a common silvicultural followed by poor regeneration had produced practice for decades and a subject of studies in the understocked stands. In , selective felling was Nordic countries (see Andersson ). Andersson banned and thinnings began to favour the best observed that naturally established thickets of trees in the stand (thinning from below). As there birch␣ can be profitably thinned and grown to were no thinning models available at the time, it commercial dimensions (at least to pulpwood size). was possible for thinnings to result in stands that When the stump-sprouting of birch is kept under were too sparse. control, spruce continues to benefit from shelter About years ago, national forest inventory against frost, and a considerable volume of data revealed that the rate of timber harvesting merchantable wood is produced. posed a threat to sustainable wood production. Using the permanent experiments established by This signalled the beginning of a period of Andersson as a data source, Tham ₍₎ simulated intensified forestry that lasted about years. development of a two-storeyed mixed stand to the Underproductive stands were regenerated, peatland age of years. Tham assumed that birch would be sites were drained for forestry, forests were removed at age years, and thereafter the stand fertilized, and tree breeding programs were started. would carry on as a pure stand of spruce. Tham’s These activities encompassed millions of hectares simulations showed that – birch per hectare and resulted in a great increase in the country’s had no influence on the growth of spruce, and the forest resources and a % rise in the forest growth birch produced nearly m of wood per hectare rate. Currently, the total harvest from Finland’s within years (Figure ). forests amounts to no more than –% of the Growing two-storeyed mixed stands of birch and annual forest growth of about million m. spruce in Sweden has produced results similar to During these years, pure conifer stands were those in Finland. However in Sweden, birch is not believed to be economically superior to mixed used by the plywood industry. It is profitable in stands. However, mixed stands have been preserved because % of Finland’s forests are in private, non-industrial ownership. This owner group has 400 always practised forestry on a small scale and has )
3 favoured mixed stands. 300 Environmental Awareness in Forestry In the mid- 200 s, the environmental movement began to recognize the importance of biodiversity in forest 100 ecosystems. Disputes over the protection of old- Total volume (m Total growth forests and of threatened species are well 0 0000 0600 0800 1000 1200 known from those years. Clear felling and planting Number of birch in the shelter have also become less popular. Some years later Finland, like so many other Norway spruce Silver birch countries, found itself in a deep recession. Economic recession, an abundance of wood, and Total yield of Norway spruce and silver birch in the increasing environmental awareness of years with five combinations of managed consumers have helped to make every forestry birch shelters (after Tham 1988). organization favour the growing of mixed stands.
13 The birch that establishes itself on regeneration Jonsson, N. . Om barrblandskogens sites following site preparation has been received volymproduktion. Summary: Yield of mixed with gratitude. The current practice is to thin coniferous forests. Medd. Stat. Skogsforskn. young stands when they reach the height of – m Inst. Band :–. and to retain a species mixture when doing so. Kauppi, P., K. Mielikäinen, and K. Kuusela. . Aerial application of herbicides is not allowed in Biomass and carbon budget of European the Nordic countries. forests, to . Science :–. The first commercial thinning of birch is done Laitakari, E. . Koivun juuristo. Summary: when the stand is – m high. The stem number The root system of birch. Acta For. Fenn. is then reduced to per hectare. The thinned :–. material is harvested either by applying the cut-to- Lappi-Seppälä, M. . Untersuchungen ueber length system or the whole-tree system; in the die Entwicklung gleichaltriger Mischbestände latter case, the tops of trees can be burnt to aus Kiefer und Birke. Commun. Inst. For. generate energy in heating plants or pulp mills. Fenn. :–. About % of Finland’s total energy requirement is Mielikäinen, K. . Mänty-koivusekametsi- satisfied from bioenergy sources—this is a world köiden rakenne ja kehitys. Summary: record for an industrialized country. Structure and development of mixed pine Following the first thinning, stands are thinned and birch stands. Commun. Inst. For. Fenn. once or twice more before final felling. Final felling ₍₎:–. takes place when the mean stand diameter is – .␣ . Koivusekoituksen vaikutus kuusikon cm. Large-diameter pine goes to sawmills, birch is rakenteeseen ja kehitykseen. Summary: Effect sought out by the plywood industry, part of the of an admixture of birch on the structure spruce harvest is sawn, while another part goes and development of Norway spruce stands. into papermaking. Commun. Inst. For. Fenn. :–. Parviainen, J. a. Finnish silviculture: manag- Conclusions ing for timber production and conservation. J. For. ₍₎:–. Birch is now Finland’s most valuable commercial .␣ [b]. Silviculture, management prac- tree species and is likely to be in the future. High- tices, techniques and sustainability. Nordic quality birch can have a commercial value double Workshop on Sustainable Forest Manage- that of medium-quality pine. ment, Rolighed, Finland, – November Pure or mixed birch will be the main species in . In press. the afforestation of abandoned farm lands because Tham, Å. . Yield prediction after heavy thin- of its short rotation time and aesthetic value. ning of birch in mixed stands of Norway Natural regeneration of birch will increase. This spruce (Picea abies (L.) Karst) and birch is necessary to reduce the regeneration costs of (Betula pendula Roth and Betula pubescens conifer plantations and to establish healthy Ehrh.). Swedish Univ. Agric. Sci. Dep. For. mixtures having high value for biodiversity. Yield Res. Rep. No. (Three publications separately paginated). References Tomppo, E. and M. Siitonen. . The National Forest Inventory of Finland. Paperi ja Puu - Andersson, S-O. . Treatment of young mixed Paper and Timber :‒. stands with birch and conifers. In Valsta, L. . Stand management optimization Broadleaves in boreal silviculture: an obstacle based on growth simulators. Finn. For. Res. or an asset. Swedish Univ. Agric. Sci. Dep. Inst. Res. Pap. (Academic dissertation). Silv. Rep No. :–. Fries, J. . Björk och gran. Framtidsskogen - Skogsproduktionens mål och medel. Rapp. Uppsats. Instn. Skogsprod. Skogshögsk, Finland. :–.
14 Mixtures of Paper Birch and Conifers: An Ecological Balancing Act
SUZANNE SIMARD
Abstract B.C. Ministry of Forests’ free-growing stocking standards discriminate against mixed species Paper birch–conifer mixtures are normal stands. The requirement that conifers be free of components of Interior Cedar–Hemlock ₍ICH₎ overtopping vegetation within a m radius has biogeoclimatic zone ecosystems. However, often resulted in attempted removal of all traditional timber-oriented silviculture regulations, broadleaves, rather than simply those that threaten such as the British Columbia Ministry of Forests survival of neighbouring conifers. This practice, in free-growing standards, have motivated removal of combination with planting of root disease paper birch from young conifer plantations. The susceptible conifers, in some cases has reduced tree potential ecological risk, high management costs, species diversity and increased disease-related and low social acceptability of attempts to eradicate mortality in ICH conifer plantations. birch are providing incentives for management of Relative to single species stands, there is paper birch–conifer mixtures. Mixture manage- increasing evidence that mixed paper birch–conifer ment represents an ecological balancing act, where stands are associated with lower root disease the negative effects of resource competition are incidence (Morrison et␣ al. ), lower insect pest weighed against positive attributes such as outbreaks (Watt ), greater and more rapid resistance to root disease, maintenance of nutrient cycling (Wang et␣ al. a; Prescott et al., biodiversity, and enhanced nutrient cycling. The in progress), greater sustainable biomass pro- transition from pure conifer to mixedwood duction and long-term productivity (Sachs ), management represents a positive step toward and greater biodiversity (Jones and Durall, in “ecosystem management,” and requires progress). The complexity of tree species mixtures modification of the current free-growing standards. likely confers stability on ICH ecosystems, which is Some recommendations are provided for a crucial in this era of unprecedented anthropo- “mixture-friendly” standard. morphic stress (Perry et␣ al. ). Complexity in species, structure, and function may indeed be Introduction fundamental to sustainability in the ICH zone. The complexity of mixed-species forests Mixed species stands are characteristic of the precludes successful application of simple Interior Cedar–Hemlock ₍ICH₎ zone. Paper birch regulations such as the current free-growing (Betula papyrifera) and early seral conifers, such as standards. A new approach, which embraces rather Douglas-fir (Pseudotsuga menziesii), western larch than rejects complexity, is needed to ensure the (Larix occidentalis), and western white pine (Pinus future stability of British Columbia’s forests. monticola), grow in mixtures on sites disturbed by This␣ approach should allow development of wildfire and logging. Paper birch also grows in root situation-specific prescriptions based on an disease and windthrow pockets in climax stands understanding of␣ stand development patterns dominated by western redcedar (Thuja plicata) and and␣ ecosystem processes. western hemlock (Tsuga heterophylla). The current
15 Patterns in Paper Birch–Conifer Mixtures disperse seed long distances, and survive fire due to its low foliar flammability. Succession Complex vegetation patterns The complex pattern of mixed ICH forests is characteristic of the ICH zone can partly be traced being simplified by traditional forest management to patterns of disturbance and ecological practices such as fire suppression, clearcutting, and succession. Communities in the ICH typically weeding. The free-growing standard is a reflection include species from several seral stages. Seeds or of a singular conifer management focus, which has seedlings of all tree species are usually present soon contributed to pattern simplification in new forests after disturbance (within years), and species by favouring preferred conifer species. Pattern dominance in a particular sere reflects variation in simplification could have profound implications seed abundance, germination, and growth rates. for ecosystem processes, such as movement of Following a stand-destroying wildfire, paper birch wildlife, spread of disease and fire, and migration commonly dominates sites in various combinations of species. with Douglas-fir, western white pine, lodgepole Interspecific Interactions in Paper Birch–Conifer pine (Pinus contorta), trembling aspen (Populus Mixtures The type and intensity of interspecific tremuloides), and, in the southern portions of the interactions in paper birch–conifer mixtures are zone, western larch and ponderosa pine (Pinus highly dependent on the spatial scale of ponderosa). Abundant seed, tolerance of extreme observation. For example, paper birch and conifers microclimatic conditions, and rapid growth of may be fierce competitors for light on an early seral species, particularly paper birch, individual basis, and concurrently benefit from effectively prevent germination or suppress growth each other’s nutrient cycling abilities on a stand of later successional species, such as western basis. Most research has focused on individuals or redcedar and western hemlock. As early seral communities (< – ha plots) over short time species die and the canopy opens, however, western periods (< – years) because of limitations of redcedar and western hemlock slowly assume measurement and direct applicability to traditional dominant canopy positions. Paper birch and some forest management. In addition, the focus has been early seral conifers persist in climax stands where on pairs of species, specifically paper birch and a root disease, windthrow, and selective cutting commercially valuable conifer species. The focal create suitable conditions for their survival. conifer species in the ICH zone has been Douglas- Spatial Pattern Paper birch contributes to fir, simply because it dominates planting programs. complexity of vegetation patterns in the ICH zone Ecological attributes of paper birch make it both across a wide range of hierarchically and a competitive and beneficial species in ICH temporally organized scales, from communities communities. Table summarizes selected charac- (<␣ –s ha; – years) to landscapes (– teristics that should be considered when developing ha, –s years). Within communities, paper birch–conifer mixture prescriptions paper birch and conifers often occur in a dispersed (Simard␣ ). spatial pattern, where individuals of different Paper birch disperses abundant seed, germinates species are intimately mixed. This pattern can at temperatures near freezing, sprouts prolifically occur following wildfire or logging where surviving from cut stumps, and grows quickly. Compared to paper birch and conifer species successfully neighbouring Douglas-fir, paper birch also disperse to disturbed seedbeds. Across landscapes, maintains high net photosynthesis rates, high water paper birch and conifers appear to occur in a use efficiency and high nitrogen use efficiency, clumped spatial pattern, where wildfire, root even under moderate drought conditions (Wang disease, and succession interact to create a dynamic et␣ al. ). These characteristics allow paper birch mosaic of relatively pure paper birch and conifer to colonize disturbed sites and quickly overtop patches. Relatively pure stands develop where neighbouring conifers. predominantly paper birch or conifers survived in Paper birch foliage is also rich in macronutrients large patches. Paper birch is particularly successful (Wang et␣ al. ) and decomposes rapidly at forming pure fire-seral stands because of its (Prescott et al., in progress). Nitrogen-fixing ability to sprout from buried adventitious buds, bacteria associated with paper birch rhizospheres
16 Selected ecological attributes of paper birch
Attribute Source
Reproduction Frequently produces abundant seed Fowells 1965 Sprouts from adventitious buds Klinka and Scagel 1984 Germinants tolerate low temperatures Zasada et␣ al. 1983 Germinants require high soil moisture Marquis 1966 Germination best on mixed mineral/organic soil Marquis 1966 Sprouts stimulated by high temperatures Johannson 1985 Resource use Shade intolerant Krajina et␣ al. 1982 Rapid photosynthesis rate Wang et␣ al. 1995 Moderately drought tolerant Ranney et␣ al. 1991 High water use efficiency Wang et␣ al. 1995 High nitrogen use efficiency Wang et␣ al. 1995 Best performance on sites rich in N, Ca, Mg Post et␣ al. 1969 Growth Rapid juvenile height growth Simard and Vyse 1992 (seedlings 40 cm · yr-1, sprouts 70–200 cm · yr-1) Height growth mostly complete by 50–60 years Simard and Vyse 1992 Above-ground biomass 7 t · ha-1 ·yr-1 Wang et␣ al. 1996 Nutrient cycling Leaf litter rich in N, P, K, Mg, Ca Prescott et al. (in progress) Rapid leaf litter decomposition rate Prescott et al. (in progress) Associative nitrogen fixation Simard et␣ al. (in progress) Resistance to root disease Resistant to Armillaria ostoyae Morrison et␣ al. 1991 Immune to Phellinus weirii Morrison et␣ al. 1991 Rhizosphere micro-organisms Diversity of ectomycorrhizal fungi Jones et al. (in progress) Associative nitrogen-fixing bacteria Simard et␣ al. (in progress) Iron-chelating Fluorescent pseudomonads Simard and Li (in progress)
also contribute to site nitrogen capital (Simard or as an alternate rotation in temporal mixtures et␣ al., in progress). These characteristics are with susceptible conifers. In addition, paper birch important to nutrient cycling and long-term contributes to floral, faunal, fungal, and microbial productivity in species mixtures. Using the diversity (e.g., Jones ), which may be FORECAST model, Sachs ₍₎ postulated that paper important to stability of ICH ecosystems (Perry birch–conifer mixtures could sustain higher total et␣ al. ). yield than pure conifer stands. Equally important to the development of Paper birch is resistant to infection by Armillaria mixture prescriptions is the autecology of ostoyae and immune to Phellinus weirii (Morrison associated conifer species. Information on the et␣ al. ). Consequently, paper birch is attractive ecological characteristics of ICH conifer species is either as a neighbouring species in spatial mixtures presented in Krajina et␣ al. ().
17 Paper Birch–Conifer Mixtures: An Ecological Where Armillaria ostoyae is present, disease Balancing Act spread may be discouraged by creating intimate mixtures (e.g., alternating individuals or rows) Mixtures of paper birch and conifers represent an of paper birch and resistant (e.g., western larch), ecological balancing act for forest ecosystem moderately susceptible (e.g., lodgepole pine, managers. The competitive effects of paper birch western redcedar, western hemlock), or must be weighed against positive attributes such as susceptible (e.g., Douglas-fir, spruce) conifers resistance to root disease, preservation of (Morrison et␣ al. ). biodiversity, and maintenance of long-term Where Phellinus weirii is present, consider productivity. The natural complexity of species alternating rotations of pure paper birch or mixtures may preclude effective application of intimate mixtures of paper birch and resistant simple silvicultural prescriptions and regulations (western redcedar) or tolerant (e.g., lodgepole across a broad range of circumstances. Rather, pine) conifers with rotations of susceptible (e.g., species composition, density, proportion, spatial Douglas-fir) or moderately susceptible (e.g., pattern, and rotation length must be prescribed on western larch, spruce, western hemlock) conifers a situation-specific basis. Each situation must be (Morrison et␣ al. ). evaluated on the basis of, among other factors: site Patches of pure paper birch should be included series, history of disturbance, incidence of root in a landscape plan. Pure birch stands, for disease, end product, and position in landscape. example, provide habitat for many bird, The following are some guidelines for the ungulate, and small mammal species (Haeussler development of paper birch–conifer mixture et␣ al. ). Pure stands may also serve as prescriptions in the ICH zone. They are based on vegetation breaks to large-scale disturbances our understanding of tree species ecology, stand such as wildfire. development patterns, and ecosystem processes, Changing our narrow focus from one that which were acquired through retrospective studies, manages complexity at only one spatial- surveys, the literature, personal observations, and temporal scale (mid-seral community or stand) some empirical growth data. The rationale to one that addresses complexity across a wide underlying many of these guidelines is more fully range of hierarchically organized scales (microbial developed by Simard ₍₎. to landscape) will facilitate ecosystem management. Species Composition Species Density and Proportion Multiple species mixtures should be encouraged. Paper birch–conifer mixtures can be maintained Mixtures should include both early and late at a higher total density than pure stands due to seral species. Western redcedar and western each species’ ability to occupy separate root and hemlock, for example, may be encouraged under crown growing spaces (Mielikäinen ). a pure canopy of paper birch or a mixed canopy Approximately stems per hectare of paper of paper birch and early seral conifers. Note that birch can be mixed within operational planting mixtures of paper birch with shade-tolerant densities (– stems per hectare) of conifers may require fewer cleanings or Douglas-fir without significant growth losses to thinnings than mixtures with shade-intolerant Douglas-fir (i.e., approximately –% paper conifers. birch) (Simard ). This recommendation Site characteristics should be considered when may be generalized to conifers of similar shade choosing a mixture of conifer and broadleaved tolerance, such as lodgepole pine. Appropriate tree species. For example, spruce should be paper birch proportions may be lower in mixed with paper birch on subhygric or cold mixtures with more shade-intolerant conifers sites in all subzones in the ICH where spruce and such as western larch or western white pine birch occur together. Lodgepole pine may be the (e.g., –%), and higher in mixtures with most successful conifer in mixture on dry sites. more shade-tolerant conifers such as western Douglas-fir, western larch, lodgepole pine, redcedar or western hemlock (e.g., –%). western redcedar, and western hemlock are Where Armillaria ostoyae is present, the suitable for circum-mesic sites. proportion of paper birch in intimate mixture
18 with susceptible conifers may be increased to coincide with that of the disease pocket. % (e.g., stems per hectare paper birch . Where pure paper birch patches are established and stems per hectare Douglas-fir). The for wildlife habitat, the patch size, shape, proportion of paper birch should be lower proximity to neighbouring patches, and position (e.g.,␣ –%), however, in mixture with in the landscape should be based on wildlife western␣ larch. species requirements, existing land cover, and Where Phellinus weirii is present, pure alternate existing and proposed land use patterns. rotations of paper birch can be established Rotation Length either by encouraging natural regeneration or In intimate mixtures, paper birch can be managed by␣ planting. Where paper birch regenerates on a shorter rotation (e.g., – years) than naturally, stands should be thinned to neighbouring conifers (e.g., years or more). approximately – stems per hectare Commercial thinning of paper birch can help between – years of age (Safford ). offset associated management costs. Seedlings should be favoured over sprouts, since . Where pure paper birch patches are thinned, sprouts frequently have heartrot or are rotation length for sawlogs may be – years mechanically weak at the stump-sprout on good sites. Otherwise, paper birch reaches its interface. Where paper birch is planted, regional pathological rotation at – years. stocking standards for shade-intolerant conifers Occasionally an individual may survive for more should be adopted. Those standards can be than years. adapted specifically for paper birch as performance of operational and research paper Free-growing Standard birch plantations is evaluated. Where pure paper birch patches are planned, Regulations under the British Columbia Forest Act variable density stands can be readily be encouraged intensive conifer plantation manage- established using appropriate site preparation ment and discouraged broadleaf-conifer mixture techniques. Mechanical site preparation, management. In particular, the free-growing depending on the tool and intensity, can result standard required that every hectare harvested be in densities ranging from to stems reforested with a minimum number of healthy, per hectare, and broadcast burning can result in well-spaced crop trees, and that those trees be free densities averaging stems per hectare of overtopping non-crop vegetation within a (Simard and Vyse ). specified number of years following harvest. The Spatial Pattern standards required that a conifer crop tree be taller Intimate mixtures of paper birch and conifers by % than all neighbouring non-crop vegetation can be established by alternating planted within m radius of its stem. individuals or rows, or by encouraging natural Arguably, the free-growing standard could be regeneration of paper birch through seedbed applied to maintain a small component of disturbance. Naturally regenerated paper birch broadleaves in conifer plantations. In most may require thinning or point cleaning to situations, however, this has not occurred because ensure survival of neighbouring conifers. Birch of cost savings realized by complete versus selective can be thinned to a uniform overstorey density broadleaf removal. Complete removal has also been (e.g., stems per hectare) or point cleaned supported by short-term research, which in many with successively larger free-growing radii as the cases demonstrated optimum seedling performance stand ages. The size and shape of the free- in the absence of competing vegetation (e.g., growing area should be based on individual tree Simard ; Comeau et␣ al. ). Hence, a performance based on measures such as height, common strategy considered successful for crown radius, and stem diameter. Sprouts will Douglas-fir sawlog production in the southern eventually acquiesce as the overstorey crown interior ICH often includes: () clearcut; () closes. broadcast burn within years of harvest; () weed Where Phellinus weirii is present, the size and to reduce competition, preferably before paper shape of the pure paper birch patch should birch overtops Douglas-fir; and () space conifers
19 to favour Douglas-fir. This high-input strategy has with a minimum light availability requirement resulted in relatively pure Douglas-fir plantations (P.G. Comeau, B.C. Ministry of Forests, pers. of low species, structural, and spatial diversity comm., ). Photosynthetically active (Simard and Vyse ). Because Douglas-fir is radiation ₍PAR₎ reaching each target conifer can susceptible to infection by Armillaria and Phellinus easily be measured using a Sunfleck Ceptometer root disease fungi, disease incidence in these (Decagon, Inc.). Where light cannot be plantations is often greater than % (Morrison measured, an empirically related characteristic, et␣ al. ). Similar trends occur in plantations of such as percent cover of overtopping vegetation other susceptible conifer species, including or birch basal area, could be used instead to lodgepole pine and Engelmann spruce. estimate light. Standards for minimum The traditional intensive conifer management acceptable light and overtopping cover must be strategy in the ICH should be replaced with a established on a species-specific basis and can be strategy that emphasizes mixtures of paper birch derived from field research and the literature. and conifers. This transition is necessary if the B.C. • Where point cleaning is required to meet Forest Service is indeed committed to ecosystem minimum PAR, the size of the free-growing management rather than timber management. A radius should be flexible to account for revised mixedwood strategy would recognize the differences in individual conifer performance. high investment costs, high biological risk, This is particularly important on disease- uncertainty over usable wood production, and affected sites, where overlapping birch lower non-timber values associated with intensive rhizospheres may benefit susceptible conifers. conifer plantation management, as well as intense • Certain sites should be excluded from the free- public opposition to the use of herbicides. growing requirement to meet health or Broadleaved species are now starting to be landscape ecology objectives. Pure paper birch recognized as acceptable crop trees. Under the new stands may be suitable, for example, in root Forest Practices Code only tree species that are disease pockets, for wildlife habitat, or as ecologically adapted to the site and that can meet natural disturbance breaks. management objectives are used in regeneration or retained as crop trees. This can be interpreted as Summary either conifer or broadleaf species. In addition, mixed-species stands must be established on sites Paper birch–conifer mixtures make ecological, where they were previously present. With the new economic, and social sense in the ICH zone. There forest practices code, stocking guidelines and new is increasing evidence that mixtures are less prone free-to-grow standards are available for various to disease and insect outbreaks, more biologically commercially valued broadleaved species. and structurally diverse, and more productive in the long term. Biological diversity may confer Recommendations for a “Mixture-Friendly” stability on ICH ecosystems, which could be crucial Standard to sustainability in this period of unprecedented anthropomorphic disturbance. The current free-growing standard is at odds with Transition from primarily conifer to mixture paper birch–conifer mixture management. Several management requires modification of the free- positive steps can be taken to remedy this situation: growing standard. The new standard should include • Paper birch should be allowed as an “acceptable” paper birch as an acceptable or, in some cases, pre- or, where root disease is present, “preferred” ferred crop species. The free-growing radius could crop species in ICH ecosystems. A few forest be replaced with a minimum light requirement. In districts already favour a low density of paper addition, some sites should be exempted from the birch during weeding or spacing operations on standard to allow establishment of pure paper diseased sites. birch stands in critical wildlife corridors, root • The m free-growing radius could be replaced disease pockets, or as natural disturbance breaks.
20 References Mielikäinen, K. . The structure and development of pine and spruce stands with Comeau, P.G., T.F. Braumandl, and C.Y. Xie. birch mixture. In Broadleaves in boreal silvi- . Effects of overtopping vegetation on culture: an obstacle or an asset? B. Hagglund light availability and growth of Engelmann and G. Peterson (editors). Swedish Univ. spruce (Picea engelmannii) seedlings. Can. J. Agric. Sci., Dep. Silv. Umea, Sweden. For. Res. :-. pp. -. Fowells, H.A. (compiler) . Silvics of forest Morrison, D., H. Merler, and D. Norris. . trees of the United States. U.S. Dep. Agric. Detection, recognition and management of For. Serv. Handb. No. . Armillaria and Phellinus root diseases in the Haeussler, S., D. Coates, and J. Mather. . southern interior of British Columbia. B.C. Autecology of common plants in British Min. For. and For. Can. Victoria, B.C. FRDA Columbia: a literature review. For. Can. and Rep. No. . p. B.C. Min. For. Victoria, B.C. FRDA Rep. Morrison, D., G.W. Wallis, and L.C. Weir. . No.␣ . Control of Armillaria and Phellinus root Johansson, T. . Treatment of broadleaved diseases: -year results from Skimikin stump trees in stands. In Broadleaves in boreal removal experiment. Can. For. Serv. Pac. For. silviculture: an obstacle or an asset? B. Cent. Victoria, B.C. Inf. Rep. BC-X-. Hagglund and G. Peterson (editors). Swedish Perry, D.A., T. Bell, and M.P. Amaranthus. . Univ. Agric. Sci., Dep. Silv. Umea, Sweden. Mycorrhizal fungi in mixed-species forests pp. -. and other tales of positive feedback, Jones, M.D. . Belowground interactions redundancy and stability. In The ecology of between Betula papyrifera and Pseudotsuga mixed-species stands of trees. M.G.R. menziesii in the ICH zone of the southern Cannell, D.C. Malcom, and P.A. Robertson interior of British Columbia: Prog. Rep. (editors). Br. Ecol. Soc. University Press, B.C. Min. For. Kamloops, B.C. Cambridge, U.K. Spec. Publ. No. . Jones, M.D., D.M. Durall, M. Harniman, D. Post, B.W., W.H. Carmean, and R.O. Curtis. Classen, and S.W. Simard. [n.d.] The . Birch soil-site requirements. Birch ectomycorrhizal communities of Betula Symp. Proc., U.S. Dep. Agric. For. Serv. NE papyrifera and Pseudotsuga menziesii For. Exp. Sta. Upper Darby, Pa. pp. –. seedlings planted in single-species and mixed Prescott, C.E., R. Kabzems, S. Simard, C. Staley, plots in the southern interior of British L. Zabek, and P.G. Comeau. . Mixed- Columbia. In prep. wood forests: implications for litter decom- Klinka, K. and A. Scagel. . Selected ecological position. These proceedings. characteristics and the role in natural and Ranney, T.G., R.E. Bir, and W.A. Skroch. . managed forest ecosystems of some tree and Comparative drought resistance among six shrub species in British Columbia: genera species of birch (Betula): influence of mild Juniperus, Taxus, Populus, Alnus, Betula, water stress on water relations and leaf gas Quercus, Prunus, Rhamnus, Acer, Cornus, and exchange. Tree Physiol. :–. Arbutus. Canadian Cartographics Ltd., Sachs, D.L. . Simulation of the growth of Coquitlam, B.C. mixed stands of Douglas-fir and paper birch Krajina, V.L., K. Klinka, and J. Worrall. . using the FORECAST model. These proceedings. Distribution and ecological characteristics of Safford, L.O. . Silvicultural guide for paper trees and shrubs of British Columbia. Fac. birch in the northeast. U.S. Dep. Agric. For. For. Univ. B.C., Vancouver, B.C. Serv. NE For. Exp. Sta. Upper Darby, Pa. Res. Marquis, D.A. . Germination and growth of Paper NE-. Revised. paper birch and yellow birch. U.S. Dep. Simard, S. . A retrospective study of compe- Agric. For. Serv. NE For. Exp. Sta. Upper tition between paper birch and planted Darby, Pa. Res. Pap. NE- Douglas-fir. B.C. Min. For. and For. Can. Victoria, B.C. FRDA Rep. No. .
21 .␣ []. Some ecological and silvicultural Simard, S., C.Y. Li, and D.A. Perry. [n.d.] Asso- characteristics of paper birch in the southern ciative nitrogen fixation by paper birch and interior of British Columbia. In Ecology and Douglas-fir seedlings grown in mixture in management of British Columbia hardwoods. soils collected from the southern interior of Dec. –, , Richmond, B.C. P.G. Comeau, British Columbia. In preparation. G.J. Harper, M. Blache, J.O. Boateng, and Wang, J.R., S.W. Simard, and J.P. Kimmins. K.D. Thomas (editors). B.C. Min. For. and []. Above-ground biomass production For. Can. Victoria, B.C. FRDA Rep. In press. and nutrient accumulation in an age sequence Simard, S. and A. Vyse. . Ecology and of paper birch in the Interior Cedar–Hemlock management of paper birch and black zone in British Columbia. For. Ecol. Manage. cottonwood in southern British Columbia. In press. B.C. Min. For. Victoria, B.C. Land Manage. .␣ . Physiological responses of paper Rep. No. . birch to thinning in the ICHmw subzone of .␣ . Paper birch: weed or crop tree in British Columbia. For. Ecol. and Manage. the interior cedar hemlock forests of south :–. British Columbia. In Interior cedar-hemlock, Watt, A.D. . Insect pest population dynamics: white pine forests: ecology and management. effects of tree species diversity. In The ecol- Symp. proc. D.M. Baumgartner, J.E. Lotan, ogy of mixed-species stands of trees. M.G.R. and J.R. Tonn (editors). Dep. Nat. Resour. Cannell, D.C. Malcom, and P.A. Robertson Sci. Wash. State Univ. Pullman, Wash. (editors). Br. Ecol. Soc. University Press, pp. –. Cambridge, U.K. Spec. Publ. No. . Simard, S. and C.Y. Li. [n.d.] Fluorescent Zasada, J.C., R.A. Norum, R.M. van Veldhuizen, pseudomonads associated with roots of paper and C.E. Teutsch. . Artificial regeneration birch and Douglas-fir grown in Interior B.C. of trees and tall shrubs in experimentally soils, and their role in suppression of burned upland black spruce/feather moss Armillaria ostoyae and Armillaria sinipina. stands in Alaska. Can. J. For. Res. :–. In␣ preparation.
22 Competition Dynamics in Juvenile Boreal Hardwood-conifer Mixtures
D.A. MACISAAC AND S. NAVRATIL
Abstract sites, maintaining a sufficient conifer component is a difficult problem in the Prairie provinces. The There are concerns about the shift from conifer to trembling aspen and balsam poplar competition is mixedwood regeneration in young stands in the seen as one of the major factors limiting coniferous Prairie provinces. Aspen and balsam poplar rapidly growth in these regenerating stands. outgrow spruce and pine and can seriously affect Recent free-to-grow standards developed in the growth and survival of associated conifers in Alberta (Alberta Forest Service ) and other these regenerating stands. The Canadian Forest jurisdictions reflect this concern by incorporating Service has conducted three studies in Alberta and acceptable conifer growth standards, such as a Manitoba to quantify the relationship between required minimum height for –-year-old conifer growth and broadleaf competition levels to seedlings, in their regulations. Conifer release assist in stand-tending decision making. These programs are being implemented to bring studies are of Alberta lodgepole pine–aspen, regenerated stands to the targeted standards. Alberta white spruce–broadleaf, and Manitoba However, selecting stands that are most economical conifer-broadleaf mixtures. Results from the three to tend and will have the best potential for good studies are presented. The basal diameter ratio conifer post-treatment response is difficult because competition index is shown to be most appropriate of limited information on biological efficacy. for the lodgepole pine–aspen in Alberta. Height Treatment decisions have been (in some cases) and distance thresholds were developed for the arbitrary and quantitative tools are required by Manitoba conifer–broadleaf sites. For the Alberta foresters to assist in these decisions. white spruce-broadleaf study, the effects of the Research scientists with the Canadian Forest spatial distribution of competitors is described. Service ₍CFS₎ have conducted a series of studies that The effects of neighbouring competition on conifer aim to quantify the effects of broadleaf tree growth are discussed along with the competition on growth of major commercial development and potential applications of conifer species in Alberta and Manitoba, competition indices in mixedwood forests. specifically lodgepole pine (Pinus contorta Loudon), jack pine (P. banksiana Lamb.), white Introduction spruce, and black spruce (P. mariana [Mill.] BSP). Throughout the western boreal mixedwood forests, This paper presents some results about the effects trembling aspen (Populus tremuloides Michx.) and of neighbouring competition on conifer tree balsam poplar (P. balsamifera L.) often dominate growth and the effects of site conditions on these regenerating stands. On sites planted to spruce and interactions. It also gives a brief discussion of the pine, these fast-growing broadleaf species often development and potential applications of negatively affect both the growth rate and survival competition indices in mixedwood forests. of the associated conifers. For example, historically Concepts of Conifer-broadleaf Competition up to two-thirds of white spruce (Picea glauca [Moench] Voss) plantations in the region have In boreal aspen mixedwood forests in the Prairie reverted to mixedwood or broadleaf status (Brace region, there appears to be a critical period in and Bella ). While it is desirable to manage for stand development between and years after conifer-broadleaf associations on these mixedwood harvest (the actual age depending on the site),
23 during which the conifer component often Competition Indices Several methods have been declines. This decline in conifer growth may be proposed for describing interspecific competition associated with the crown closure of the broad- (between plants of different species) and for leaved competitors. Several studies suggest that estimating the magnitude of its effects. The conifer response to increasing broadleaf compe- objective of our studies was to develop or select tition is species specific and differs for survival, competition indices which best quantified the height growth, and diameter growth (e.g.,␣ Carter relationship between aspen or broadleaf and Klinka ; Klinka et␣ al. ). For shade- competition and conifer growth response. These intolerant species such as pine, under increasing indices had to be applicable to release decisions competition (shade) levels, radial growth and readily applied in an operational setting. declines before height growth. This is manifested Interspecific competition indices are based on in the elevated height:diameter ratios of spindly either stand characteristics, or individual stemmed, etiolated lodgepole pine growing under competitor and target tree measurements, or a these conditions (Navratil and MacIsaac ). For combination of both. Stand measures can include moderately shade-tolerant conifers such as white density, percent cover, amount of overtopping by spruce, height growth is suppressed while the surrounding vegetation, and light interference growth of laterals is somewhat maintained. This (Mugasha ; MacDonald et␣ al. ). produces a short seedling with a relatively large Neighbourhood measurements may include: size crown surface. While the spruce may survive under ratios of competitor to target tree, competitor size, very low light levels (Sims et␣ al. ), their␣ low and distance and dispersion around the target tree. rate of radial increment makes them susceptible to Competition is usually measured within a specific stem clipping by herbivores or to damage from distance from the target tree, using either fixed or vegetation press or falling debris. variable radius plots. Most competition indices developed for mixed- Retrospective Neighbourhood Approach wood regeneration are individual-tree centred, and describe “neighbourhood competition” adjacent to The new free-to-grow standards are based on the the target (crop) tree. They may be distance depen concept of a “competition neighbourhood” around dent (spatial) or distance independent (non-spatial), a target conifer tree. At this scale, individual tree- extensive (one measure, many plots) or intensive based models of competition and growth can be (many measurements, fewer plots), and based on developed. In many cases, microsite effects may be plotless or fixed/variable radius plots. Competition more important than the observed above-ground indices incorporate a wide variety of information competition effects and can be incorporated into on the attributes of neighbouring vegetation. these neighbourhood models. Examples include: available growing space around Neighbourhood competition analysis has been the target tree, crown, or adjusted crown overlap; used to estimate the effects of neighbouring competitor tree density or basal area; diameters or vegetation on tree growth and survival (e.g., distances of neighbouring trees; and measured or Simard ; Wagner and Radosevich a, b; inferred shade by competitors (e.g., light trans- Comeau et␣ al. ). This approach is useful in mittance or height ratios). Comprehensive reviews situations where species “A” negatively affects the of competition indices, as well as discussions on growth of species “B,” but species “B” has a lesser their utility can be found in Alemdag (), or no detrimental effect on the growth of species Mugasha (), and Burton (). “A” (described as “asymmetric competition” by Pacala and Weiner [ ]). Because of the rapid Competition Studies growth of young broadleaved trees compared to conifers on boreal mixedwood sites, the above- Three competition studies are presented here. The ground competition is asymmetric in these Alberta lodgepole pine–aspen competition study situations. Neighbourhood analysis is an efficient comprised , m (. m radius) plots, approach, as it concentrates the study on the measured in blocks – years after harvest species of interest. within the Upper and Lower Foothills natural
24 regions (Anon. ). The Alberta white spruce– broadleaf competition study was based on , ★ .␣m ( m radius) plots in blocks, – years ★ after␣ harvest, within the Lower Foothills and Central Mixedwood natural regions. The Manitoba The Pas conifer (white spruce, black spruce and jack pine)- broadleaf competition study was based on , . m ( m radius) plots, measured in blocks – years after harvest within the Boreal Mixedwood, Manitoba Lowland and Lower English ▲ River forest regions (Rowe ). Study locations Swan for the Alberta and Manitoba studies are shown in River Figures and , respectively. In all cases, the ■ selected blocks were in sites where the dominant ■ ■ competition was broadleaved trees. ■ ■ ■ Winnipeg ■ Black spruce ★ Jack pine ▲ White spruce
Location of sites for the conifer-broadleaf study in Manitoba.
Field Methods The field methods and measure- ments were similar for all three studies. They were ▲ all based on target tree-centred circular plots, . or m in radius. These plots were located along a ▲★ ▲ regular grid pattern, with spacing ranging from × m to × m, with at least plots in ★ each block. In each plot, a “retrospective survey” ■ (Simard ) was completed. Target tree growth ★ ★ measurements were taken, along with detailed Edmonton measurements of broadleaf tree and shrubby ★ competitor stem density, crown cover, proximity, size, and the amount of overtopping of the target ▲★ tree by surrounding vegetation. Height increments were measured for the target tree and closest and tallest broadleaf and conifer, and then the target conifer and competitors were harvested for radial Calgary increment and age measurements. For the
■ Validation of basal diameter Manitoba conifer and Alberta white spruce studies, ratio competition index for additional information was collected. This included lodgepole pine-aspen cover and height information on all trees and other ★ Lodgepole pine –␣ aspen vegetation in the plot, and an evaluation of micro- site conditions, using both subjective classification ▲ White spruce – broadleaf variables (e.g., moisture class) and continuous measurement variables (e.g., litter depth). Location of conifer-broadleaf studies in Alberta.
25 Selection of Competition and Growth Variables A determination (R), lowest relative mean square limited set of variables was tested in the Alberta residual, and most homogeneous variance. Catego- lodgepole pine–aspen study. These included six rical evaluation was also conducted using t-tests published competition indices, four single and multiple means tests. Most of the analysis was competition variables (based on density and conducted using SAS statistical software (SAS cover), and four growth response variables. In the Institute ). Manitoba conifer-broadleaf study, a much wider range of competition indices and variables were Competition Index for Lodgepole Pine–Aspen evaluated. They included published and in West-central Alberta unpublished intensive and extensive competition variables (using ratios, sums, and multivariable For lodgepole pine–aspen data in Alberta, the basal indices) and microsite variables. The competition diameter ratio (BDR) competition index had the variables were based on stem density, cover, height, strongest Spearman’s rank-order correlation values distance, shading, and distribution of competitors of the tested competition indices and variables around the target tree. The growth response with lodgepole pine growth. The correlations were variables included simple variables and ratios, strongly negative (P < .), with absolute corre- both␣ size dependent and size independent. For the lation values as high as . (Navratil and MacIsaac Alberta white spruce–broadleaf study, a smaller set ). This index, which is a simplification of of more recently developed competition indices Lorimer’s () index, is: was tested, along with single competition variables BDR basal diameter of tallest aspen within based on cover and density. competition = 1.8 m of the target tree The competition indices tested in these studies index basal diameter of target pine included those by Daniels (), Lorimer (), Martin and Ek (), Brand (), Wagner and The BDR index and basal area increment Radosevich (, a, b), Braathe (), relationship approximated a negative hyperbolic MacDonald et␣ al. (), Delong (), Towill and curve (Figure ). The steep slope between BDR . Archibald (), Comeau et␣ al. (), MacDonald and . indicated high sensitivity of pine radial and Weetman (), and Navratil and MacIsaac growth to these competition levels. The regression (). Specific equations for these indices as used model accounted for and % of the variation in in this research are described in Navratil and basal area increment, for age groups – and – MacIsaac (), MacIsaac (a), and Navratil years, respectively. A concern raised by this analysis and MacIsaac (). was that the high correlations and coefficient of determination (R) values in the regression models Analysis Methods may occur because pine size was included in both the competition index and growth response. To In general the analytical approach has followed the address this concern, the regression analysis was same pathway for each study, with refinements as rerun with the pine diameters stratified into -mm indicated by the specific requirements and study size classes. This caused the variations in BDR objectives. Initial exploratory data analysis was index to be almost exclusively due to changes in conducted using Spearman’s rank-order correlation aspen size. Overall, some R values were reduced, (rs) analysis (Zar ) and multiple linear stepwise but some individual size classes had R values as regression (Neter et␣ al. ). A ranking method high as ., which was greater than for the other modified from Mugasha () was used to competition indices. determine which growth and competition variables Pine height increment was not affected at the were best suited for more detailed analysis. Model- levels of competition up to BDR values of . for building with subsets of these variables was – year and – year pine age groups (Figure ). completed with multiple linear, curvilinear, and Competition levels at greater BDR values resulted non-linear regression. As well, the parameter in decreased height growth by about % with a estimates and final form of the selected models unit increase in BDR. While this may be partly due were derived on the basis of highest coefficient of to a shorter height:diameter ratio of seedlings, it
26 may also be because low amounts of aspen cover 60 could be beneficial to pine seedlings (e.g., protec- Age class 55 5–10 years tion from radiant frost). 11–16 years Additional regression analyses were conducted 50 to determine the effects of pine and aspen density, and the distance and quadrant location of the 45 tallest aspen within the plot on this competitive relationship. In general, the relationship between 40 BDR index and pine growth was consistent for the range of stand characteristics encountered in the 35 study (Navratil and MacIsaac ). This research indicated that stand-tending treatments should be 30 aimed at reducing competition before BDR index values of . are reached. Height increment in current year (cm) 25
20 10 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Basal diameter ratio values in previous year )
2 9 Age class 8 Relationship of lodgepole pine height increment 5–10 years to basal diameter ratio competition index, from 7 11–16 years the Alberta lodgepole pine–aspen study. Each point represents 19 observations. Vertical lines 6 indicate standard error for height increments in 5 basal diameter ratio competition index values (from Navratil and MacIsaac [1993]). 4
3 Validation of the Basal Diameter Ratio Competition Index for Lodgepole Pine-Aspen 2 in␣ Alberta
Basal area increment in current year (cm 1 A -year study was initiated in in the Foothills 0 Model Forest to test the usefulness of the BDR 0 1 2 3 4 5 Basal diameter ratio values in previous year competition index in making stand-tending decisions for regenerating lodgepole pine–aspen blocks in west-central Alberta. Three –-year-old Relationship of lodgepole pine radial increment blocks were selected northwest of Edson, Alberta, to basal diameter ratio competition index, from in the Lower Foothills natural region (see Figure ). the Alberta lodgepole pine–aspen study. For age class 5–10 years the model fit to these data is A total of ( . m radius) plots in three blocks BAINC = exp(1.19 – 0.74ln(BDRATIO)– ( each) were sampled. A mixed-nested experi- 0.09BDRATIO); n = 185, R2 = 0.56, F = 115.2. ment with three blocks and four levels of aspen For age class 11–16 years the model fit to these removal (treatments) was designed. The four data is BAINC = exp(1.71 – 0.44ln(BDRATIO)– treatments (aspen removal within . m of pine) 0.37BDRATIO); n = 175, R2 = 0.52, F = 99.2. were defined as follows: Vertical lines show mean and 95% standard no aspen removed error for basal area increment with basal aspen with diameter greater than pine diameter diameter ratio competition index values (BDR > .) removed between␣ 0.26 and 0.75 (from Navratil and aspen with diameter greater than three-quarters MacIsaac [1993]). of the pine diameter (BDR > .) removed all aspen removed
To determine the predictive ability of the basal diameter ratio, analysis was based on the relationship between BDR index values calculated for the previous year versus the conifer growth response in the current year. In an operational setting, both BDR and growth would be based on non-destructive, concurrent measurements.
27 In , initial vegetation competition and growth has accelerated for trees under full release, conifer measurements were made and then the especially compared to the control (no aspen aspen were removed. These were followed by removal). While these trends were encouraging, growth response measurements in . The initial they were not statistically significant year after vegetation conditions and first-year growth treatment using Ryan’s multiple range test (SAS response have been analyzed and are reported in Institute ). MacIsaac (b). Table illustrates the levels of aspen competition 31 before and after each treatment in (the post- ●●All removed ■■ treatment results are shown only for the two 30 BDR>0.75 removed ■ a ▲ ▲ intermediate treatment levels). The remaining BDR>1.0 removed ● a ▼ ▼ ▲ aspen competition was similar in terms of density, 29 No aspen removed a height, and cover for the two intermediate treatments (removal with BDR > . and 28 BDR > . ). One year after treatment, there were 27 ▼ a significant differences (P < .) in both pine radial increment and pine root collar diameter 26 between no removal and full removal of aspen within . m of the conifer (MacIsaac b). 25 ■ Root collar diameter (mm) a However, no significant differences occurred in 24 a root collar diameter growth response and basal ▼▲ a 23 diameter between the two intermediate treatments. a ● This may be because the level of post-treatment aspen competition was similar for both 22 intermediate treatments (Table ). 1993 1994 Table presents an analysis of covariance (used Year to control for initial pre-treatment pine size) for first-year radial increment response. The most First-year radial growth response for lodgepole significant effects in the model were treatment pine after aspen removal. Standard error shown (lower interval only). Similar letters for the same (P = .) and initial pine size (P = .). year indicate means are not significantly Even after only year, the pine were responding different at P = 0.05 using Ryan et␣ al.’s multiple to␣ release with increased radial growth. Figure range test (SAS Institute 1990). indicates how the rate of root collar diameter
Aspen competition levels before and after aspen removal in 1993 for the basal diameter ratio competition index validation study, Foothills Model Forest. Data are for the three blocks combined (n = 320). Values shown are the mean and the standard error of the mean. Number in brackets is % change from pre-treatment levels.
Aspen before removal Aspen after removal
BDR > 0.75 removed BDR > 1.0 removed
Aspen Density (1000s) 25.60 ± 0.71 8.90 ± 0.93 (-65%) 10.50 ± 0.85 (-59%) Height (m) 2.43 ± 2.56 0.87 ± 3.56 (-64%) 1.01 ± 4.45 (-58%) Cover (%) 45.80 ± 0.97 4.20 ± 0.48 (-91%) 5.20 ± 0.51 (89%) Pine Height (m) 1.01 ± 2.25
28 Analysis of covariance on first-year post-treatment radial increment response for lodgepole pine in the basal diameter ratio competition index validation study, Foothills Model Forest
Source df Mean square F value P > F
1993 root collar diameter 1 16.47 6.614 0.0155 Block 2 15.16 3.5579 0.0904 Sub-block (Block) 6 4.52 2.5120 0.0216 Removal 3 14.74 8.1922 0.0001 Block*Removal 6 2.17 1.2059 0.3026 Error 343 1.80 – –
There were no significant statistical differences variable in the analysis (Table ). Distance to the in first-year pine height growth response between closest broadleaf competitor stem was more highly treatments; however, some graphical trends were correlated with white spruce and jack pine growth noted by MacIsaac (b). The response of the than were other distance variables. Of the growth height increment may be less pronounced than for response variables tested, radial increment had the radial growth because of the general trend for strongest correlation with competition variables. In released trees to respond with accelerated radial the regression analysis, R values were not as high growth before height growth. Trends in height for growth variables based on total target tree size growth will be tracked closely in subsequent years. (height, root collar diameter, etc.) as for radial increment and height increment variables. Results of the Manitoba Conifer-broadleaf Analysis had shown that competitor height Competition Study relative to target tree height was one of the best competition variables in “predicting” conifer The Manitoba study permitted analysis over a growth, and that stem-to-stem distance of the wider range of competition and growth variables closest broadleaf was also important. These two than the Alberta lodgepole pine–aspen study. variables were used to test for competitor distance Table␣ provides a summary of the growth and and height thresholds. In determining the distance competition variables and indices that revealed the thresholds the following question was addressed: highest correlation or explained the most amount At what distance from the conifer do broadleaved of variation in conifer tree growth, as shown in the trees begin to significantly affect conifer growth? analysis by MacIsaac (a). Published (Figure ). Because the size and location of all the competition indices that include target tree size, broadleaved competitors in each plot had been such as those by Martin and Ek ₍₎, Comeau recorded, this analysis was possible. Trees were et␣ al. ₍₎, and Navratil and MacIsaac ₍₎, assessed as “open-grown” (relatively free from perform better than those that do not. This finding broadleaf competition) and “ not open-grown” was consistent even when stratified analysis was (relatively not free from broadleaf competition) performed to control for the effect of target tree using different criteria for broadleaf competitor size in the indices. Very simple variables such as distance. The average growth difference of the broadleaf density and cover provided greater target trees in these classes was compared using a correlation with growth response than a variety of series of t-tests and the difference plotted against more sophisticated competition indices. In general, the tested distances (Figure ). In each test, a relative height of the average competitor versus the specific distance criterion was used to group the target tree was the most consistent competition target trees from each plot into “open-grown” and
The term “open-grown” in this analysis labelled trees with greater and lesser competition into two groups based on competitor distance. It did not refer to the actual tree morphology.
29 Significant growth and competition variablesa for the Manitoba conifer-broadleaf competition study
Species/region Growth variable Competition variables and competition indices
Black spruce/ radial increment • relative target tree:broadleaf height Pine Falls height increment • broadleaf cover • Navratil and MacIsaac (1993) basal diameter ratio competition index Black spruce/ radial increment • relative target tree:broadleaf height Duck Mountain height increment • broadleaf density • broadleaf cover • Comeau et␣ al. (1993) competition indexb White spruce/ radial increment • relative target tree:broadleaf height Porcupine Hills • stem-to-stem distance between closest broadleaf and target conifer • broadleaf density Jack pine/The Pas radial increment • relative target tree:broadleaf height total diameter • stem-to-stem distance between closest broadleaf and target conifer • Martin and Ek (1984) competition indexc a These were variables that consistently had the highest R2 in the regression analysis. b Comeau’s index is the sum of the average cover × average height of the competing species, divided by the target tree height. c Martin and Ek’s competition index is a distance-weighted, root collar diameter ratio of the target tree and broadleaved competitors.
At what disance from the conifer do broadleaved trees begin to significantly affect conifer growth?
5 m
decreasing conifer-broadleaf stem-stem distance
10 m
Concept of the effect of changes in conifer-broadleaf competitor inter-tree distance on conifer growth. In a young regenerating stand, the conifer seedling is potentially affected by the presence of nearby broadleaf competitors. In upland mixedwood stands on mesic sites, the effects would be mostly due to shading. At a certain distance, the broadleaf tree has little or no effect on the conifer (distance relationships may vary for different conifer-broadleaf orientations).
30 open-grown In the Pine Falls area of Manitoba, the analysis showed that broadleaved trees within cm of the difference target black spruce seedling had the largest in growth negative impact on conifer growth; in plots where broadleaved trees were at a stem-to-stem distance greater than cm, the impact on conifer growth was less (Figure ). Seventy-five centimetres is not open-grown considered to be the critical distance threshold. Similar analysis was done to determine relative conifer:broadleaf height thresholds and is described in MacIsaac (a). Table summarizes the over- 25 50 75 100 125 150 175 all results of height and distance threshold analyses, Broadleaf-conifer inter-tree distance (cm) for each species in each region, tested with six growth response variables. Additional information on this analysis is found in MacIsaac (a).
Results of the Alberta White Spruce–Broadleaf Competition Study
The results of the Alberta white spruce–broadleaf study show some similarities to the Manitoba work. The complete results are shown in Navratil and MacIsaac ₍₎. Of the growth response
grown conifer trees Conifer tree size variables tested, radial increment consistently had the highest coefficient of determination in the open-grown and not open- % growth difference between regression analysis. Published competition indices
25 50 75 100 125 150 175 Broadleaf-conifer inter-tree distance (cm) 40
Graphical representation of methodology used to 30 determine broadleaf-conifer inter-tree distance thresholds. The top diagram illustrates the 20 average tree sizes for “open-grown” and “not open-grown” trees for each distance threshold based on individual t-test results. The circles in 10 % growth difference the diagrams represent the critical distance thresholds. The trees in the lower diagram (open-grown vs not open-grown) 0 25 050 075 100 125 150 175 represent the stem distances associated with the Broadleaf-conifer stem distance threshold (cm) x-axis values and a constant competitor-target tree ratio of 0.66 (from MacIsaac [1995a]). Effect of broadleaf-conifer stem distance on growth response between “open-grown” and “not open-grown” categories. Each test was run “not open-grown” black spruce from Pine Falls, independently of the others. In other words, the Manitoba. Based on overall average of separate grouping of target trees into “open-grown” and t-tests performed for each age class (5–9, 10, 11–15 years) and growth variable (radial “not open-grown” sets was based on the unique increment, root collar diameter, height, and competition distance threshold; a tree could be height increment). The critical distance threshold defined as “open-grown” in one test and “not is circled on the line. Tests based on broadleaf- open-grown” in the next. conifer height ratio of 0.66 (from MacIsaac [1995a]).
31 Distance and height thresholds for the Manitoba trees was for seedlings with no competitors to the conifer-broadleaf competition study (see east and southeast, and was significant for root MacIsaac [1995a] for details) collar diameter (Figure ). This growth difference between shaded and unshaded trees was main- Species/ Threshold Threshold tained, even with competitors in the west and region broadleaf:conifer distance (cm)b southwest. Solar radiation received in the morning height ratioa appears to be more critical than that received in the afternoon, perhaps because of early soil warm- Black spruce/ ing in the former case. This may have implications Pine Falls 0.66 75 for mixedwood management. Stand tending could Black spruce/ remove broadleaved competitors to the east, while Duck Mountain 0.66 75–100 retaining those to the west; this would maintain a White spruce/ mixture of broadleaves and conifers in the stand Porcupine Hills 0.66 100 and young conifer growth. Conifer growth could Jack Pine/ even be enhanced because of the reduced mid- The Pas 0.66 125 afternoon sunlight, which can lead to desiccation on some sites. a The height thresholds refer to the ratio of broadleaf height to target conifer height. b Distance thresholds are based on stem-to-stem distance Conclusions on the Use of Competition Indices between broadleaved trees and target conifer. in Conifer-broadleaf Competition
Some researchers believe that competition indices that explained the highest amount of variation in have serious limitations, and their use may be regression analysis included: overrated (e.g., Caza and Kimmins ; Burton ). On the spectrum from detailed process • ratio of volume of target tree to sum of models to mensurational empirically based broadleaf competitor volume (MacDonald , mathematical models, many competition indices cited by MacDonald and Weetman ); are very simplistic. They are empirically based and • distance-weighted root collar diameter ratio of often developed with only minimal knowledge of the target tree and broadleaf competitor (Martin underlying causal processes. As such, they must be and Ek ); and used with caution. When testing for appropriate • distance-weighted basal area of the broadleaf indices using regression analysis, the average R competitor (MacDonald et␣ al. ). must be examined rather than the highest R . For Aspen and broadleaf density were often shown as example, the relationship between a specific important competition variables, as was broadleaf competition index and radial increment is tested and total vegetation cover. for a particular species in various young conifer A number of microsite variables were measured age classes and sites. It is also very important to for Alberta including moisture class, drainage class, recognize when high R results are partly due to aspect, slope, micro-topography class, slash size dependence. Notwithstanding the above abundance, and litter depth. Multivariate analysis concerns, competition indices are efficient to use. indicated that three groups of variables added If an operationally useful and simple competition significantly to the competition models for some index has a high correlation with a more physio- locations. These were: () combined slope and logically based measurement, then using the index aspect; () moisture and drainage; and () slash, may be more valid. This was Delong’s approach micro-topography, and litter depth. These groups in␣ the development of his light interception of variables are important in determining microsite index␣ (). moisture and nutrient status. In British Columbia, competition indices have The effect of the location of broadleaved been applied successfully to mixedwoods in very competitors around the target trees was tested young stands (e.g., DeLong ; Comeau et␣ al. (MacIsaac and Navratil ). The greatest ), where the competitors are mostly herbaceous difference in growth of shaded versus unshaded with high leaf areas. However, use of competition
32 35 important, competition indices are only one approach to quantifying the relationship between 30 conifer growth and broadleaves in regenerating boreal conifer-broadleaved mixed stands in the Prairie Provinces. 25 Acknowledgements
20 Funding for this research was provided by the Canada-Manitoba Partnership Agreement in 15 Forestry (Project ), the Canada-Alberta Partnership Agreement in Forestry (Project ), Root collar diameter (mm) and the Foothills Model Forest–Green Plan 10 Unshaded Canada. The review comments provided by Shaded Dr.␣ Phil Comeau and two anonymous reviewers are␣ appreciated. 5 10 11 12 13 14 15 Age References Alberta Forest Service. . Free-to-grow stand- Difference in root collar diameter growth ards. Reforestation Branch. Edmonton, Alta. between shaded and unshaded white spruce Alberta Ministry of Environmental Protection. seedlings, Calling Lake, Alberta. Values shown . Natural regions and subregions of are mean ± standard error of the mean. Alberta. Land Inf. Serv. Div., Alta. Environ. Unshaded trees are those with greater than 40% Protection. Edmonton, Alta. full sunlight to the south and southeast of the Alemdag, I. . Evaluation of some competition tree (measured with a sunfleck ceptometer in late summer, 1.5 m from the seedlings at mid- indices for the prediction of diameter incre- crown height). Based on 22 unshaded and 99 ment in planted white spruce. Can. Dep. shaded trees, respectively. Environ. Can. For. Serv., For. Manage. Inst. Ottawa, Ont. Inf. Rep. FMR-X-. p. indices has not been as successful with aspen Braathe, P. . Development of regeneration (H.␣ Cullen, B.C. Ministry of Forests, pers. comm., with different mixtures of conifers and broad- ). This is partly because of the diffuse, open- leaves. II. Proc. IUFRO Conf. on Treatment of grown, asymmetric crown form of aspen. Young Forest Stands. June - , , Dres- Competition indices and variables based on aspen den, GDR. IUFRO Working Party S␣ . - . crown characteristics have been less accurate than Brace, L. and I. Bella. . Understanding the those derived from less-ambiguous parameters understory: dilemma and opportunity. In such as density and the BDR competition index. Management and utilization of northern In spite of the drawbacks mentioned by Burton mixedwoods. Proc. symp. April – , , ₍₎, researchers and those involved in vegetation Edmonton, Alta. J.K. Samoil (editor). Can management continue to search for the appropriate For. Serv. North. For. Cent., Edmonton, Alta. indices to use in stand evaluation and tending Inf. Rep. NOR-X- . pp. – decisions. One cannot expect to develop a univer- Brand, D. . A competition index for predict- sally applicable index. Indices must be developed ing the vigour of planted Douglas-fir in for each combination of species in each ecoregion. southwestern British Columbia. Can. J. For. When used for assessment, a particular Res. : – . competition index may only be appropriate in Burton, P.J. . Some limitations inherent to portions of the block because of variations in static indices of plant competition. Can. J. slope, aspect, and competition situations. Although For. Res. : – .
33 Carter, R.E. and K. Klinka. . Variation in Mugasha, A. . Evaluation of simple competi- shade tolerance of Douglas fir, western hem- tion indices for the prediction of volume in- lock, and western redcedar in coastal British crement of young jack pine and trembling Columbia. For. Ecol. Manage. :–. aspen trees. For. Ecol. Manage. :–. Caza, C.L. and J.P. Kimmins. . Problems with Navratil, S. and D.A. MacIsaac. . Competi- the development and application of competi- tion index for juvenile mixed stands of tion indices in complex, multispecies commu- lodgepole pine and aspen in west-central nities. In Vegetation management: an Alberta. For. Can. North. For. Cent. integrated approach. Proc. th Annual Vegeta- Edmonton, Alta. For. Manage. Note . tion Management Workshop, Nov. –, , .␣ []. Juvenile growth of white spruce Vancouver, B.C. E. Hamilton (compiler). B.C. and deciduous on mixedwood sites in Alberta. Min. For. and For. Can. Victoria, B.C. FRDA Can. For. Serv. North. For. Cent. Edmonton, Rep. No. , pp. – Alta. Canada-Alberta Partnership Agreement Comeau, P.G., T.F. Braumandl, and C.Y. Xie. in For. Rep. In press. . Effects of overtopping vegetation on Neter, J., W. Wasserman, and M.H. Kutner. . light availability and growth of Engelmann Applied linear regression models. nd ed. spruce (Picea engelmannii) seedlings. Can. J. Irwin. Boston, Mass. For. Res. :–. Pacala S.W. and J. Weiner. . Effects of competitive Daniels, R. . Simple competition indices and asymmetry on a local density model of plant their correlation with annual loblolly pine interference. J. Theor. Biol. :–. tree growth. For. Sci. :–. Rowe, J.S. . Forest regions of Canada. Can. For. Delong, S.C. . The light interception index: a Serv. Dep. Environ. Ottawa, Ont. Publ. No. . potential tool for assisting in vegetation man- SAS Institute Inc. . SAS/STAT user’s guide. Version agement decisions. Can. J. For. Res. :–. . th ed. SAS Institute Inc., Cary, N.C. vols. Klinka, K., Q. Wang, G.J. Kayahara, R.E. Carter, Simard, S. . A retrospective study of compe- and B.A. Blackwell. . Light-growth re- tition between paper birch and planted Doug- sponse relationships in Pacific silver fir (Abies las-fir. For. Can. and B.C. Min. For. Victoria, amabilis) and subalpine fir (Abies lasiocarpa). BC. FRDA Rep. No. . Can. J. Bot. :–. Sims, R.A., H.M. Kershaw, and G.M. Wickware. Lorimer, C. . Tests of age-independent compe- . The autecology of major tree species in tition indices for individual trees in natural the north-central region of Ontario. Ont. hardwood stands. For. Ecol. Manage. :–. Min. Nat. Resour. Thunder Bay, Ont. Publ. MacDonald, B., D.M. Morris, and P.L. Marshall. No. , COFRDA Rep. No. . NWOFTDU Tech. . Assessing components of competition Rep. No. . indices for young boreal plantations. Can. J. Towill, W.D. and D.A. Archibald. . A compe- For. Res. :–. tition index methodology for Northwestern MacDonald, G.B. and G.F. Weetman. . Func- Ontario. Ont. Min. Nat. Res., NW Ont. For. tional growth analysis of conifer seedling re- Tech. Devel. Unit, Rep. No. TN-. sponses to competing vegetation. For. Chron. Wagner, R. and S. Radosevich. . Interspecific :–. competition indices for vegetation management MacIsaac, D.A. a. Competition and juvenile decisions in young Douglas-fir stands on the growth in mixed regeneration in Manitoba. Siuslaw National Forest. Dep. For. Sci. Oreg. Can. For. Serv. North. For. Cent. Edmonton, State Univ. Corvallis, Oreg. Rep. No. . p. Alta. Canada-Manitoba Partnership Agree- .␣ a. Interspecific competition and ment in For. Rep. other factors influencing the performance of .␣ b. Validation of basal diameter ratio Douglas-fir saplings in the Oregon Coast competition index for lodgepole pine–aspen. Range. Can. J. For. Res. :–. Establishment and progress report. Foothills .␣ b. Neighbourhood predictors of Model Forest. Hinton, Alta. interspecific competition in young Douglas-fir Martin, G.L. and A.R. Ek. . A comparison of plantations. Can. J. For. Res. :–. competition measures and growth models for Zar, J.H. . Biostatistical analysis. Prentice- predicting plantation red pine diameter and Hall. Englewood Cliffs, N.J. height growth. For. Sci. ():–.
34 Silvicultural Systems for Managing Deciduous and Mixedwood Stands with White Spruce Understorey
S. NAVRATIL
Abstract overstorey on release of the white spruce understorey (Frohning ). Results of these trials The white spruce understorey in deciduous and demonstrated promising growth response of spruce deciduous-conifer stands is a valuable conifer in single-tree release treatments (Yang ), and resource in the boreal forest. Numerous options are led to the development of a two-stage harvesting available for managing these stands and protecting and stand-tending model (Brace and Bella ) the white spruce understorey. Selecting the most (Figure ). Applying this model along with the appropriate silvicultural and harvesting system proper silvicultural prescriptions for natural requires clear definition of the management regeneration of white spruce could ensure the objectives and evaluation of the stand suitability, continuity of both white spruce and aspen on the site conditions, and risk of wind damage. Expected same land base and avoid the risks and costs gains in aspen regeneration, white spruce natural incurred by regenerating pure spruce stands on regeneration, growth response of released spruce, mixedwood sites (Navratil et␣ al. ). and aspen and spruce yield at the second harvest Harvesting techniques that protect the white are discussed in this paper. spruce understorey, while harvesting the aspen overstorey, were evaluated in a set of the trials Background established in west-central Alberta in – (Brace ; Brace Forest Services ; Sauder ). On sites containing boreal mixedwoods that are difficult and expensive to reforest by conventional methods, it is economically and ecologically Hardwood overstorey responsible to regenerate forests by extensive Softwood overstorey management of species adapted to specific sites Harvest 1 (Benson ; Lieffers and Beck ). An essential requirement of extensive management is the adaptation of silvicultural systems to encourage Harvest 2 and take advantage of natural regeneration. Approximately % of fire-origin, deciduous
and deciduous-coniferous stands in the Prairie Yield provinces have a significant white spruce understorey resulting from natural regeneration. In␣ these stands, white spruce naturally regenerates shortly after fire or gradually at later stages of stand development. The predominant deciduous species in the overstorey is aspen, with variable amounts of balsam poplar. The forest industry, provincial managers, and 000 020 040 060 080 100 120 140 research community have been challenged to Age (years) develop the most appropriate methods for managing and utilizing these stands. Early research Generalized two-stage harvesting and tending trials examined the effects of harvesting aspen model (from Brace and Bella 1988).
35 In the same trials other forest management objec- greater variability in understorey heights and ages tives, such as the growth potential of released white results from white spruce establishment after local spruce understorey, wind damage to released spruce, disturbances such as endemic windthrows or aspen and quality of aspen and balsam poplar regen- die-back at advanced age. Examples of the different eration, have also been monitored and evaluated patterns of spruce establishment are evident in the (Brace Forest Services ; Navratil et␣ al. ). age class distribution of the white spruce The results confirmed the feasibility and advan- understorey from deciduous and mixedwood tages of saving the white spruce understorey. stands in the Whitecourt area of Alberta (Figure ). However, windthrow damage to the released under- Stands with an aspen overstorey and white storey spruce is a major concern, and operating spruce understorey can also be artificially strategies to minimize this damage are needed to produced. Underplanting aspen stands with white manage this␣ resource. spruce can be successful and provides a viable Consequently, methods for using conventional alternative for enhancing the white spruce feller-buncher/grapple skidder technology were component on sites where seed sources are devised to minimize windthrow losses. A field trial inadequate or have been removed. that involved an array of silvicultural and harvesting In northeastern British Columbia, DeLong systems and represented a range of harvesting () recommended underplanting of –-year- difficulty was established in northern Alberta to old aspen stands. Waldron ₍₎ documented test incremental wind protection levels (Navratil excellent results with underplanting and seeding of et␣ al. ). scalped strips prepared in –-year-old aspen This paper addresses the steps required to design stands. The yield potential of underplanted white and select an appropriate silvicultural system spruce was estimated to be m/ha at years. specific to site and stand conditions, and to Underplanting white spruce takes advantage of mixedwood and deciduous stands with white the natural dynamics of mixedwood ecosystems. spruce understorey. The management objective Since the spatial distribution of planted spruce can involves protecting the white spruce understorey be controlled, the white spruce understorey is and managing the stand for white spruce and protected when the aspen overstorey is harvested in hardwood production using a two-stage – years after underplanting (DeLong ). silvicultural and harvesting model. The potential lack of natural regeneration due to insufficient seed or poor seedbed conditions is also Stand Structure and Stand Development of addressed by underplanting. Aspen Stands with White Spruce Understorey Selecting a Silvicultural System By definition a stand with an aspen-dominated overstorey and lower stratum or strata of white Selecting the most appropriate silvicultural system spruce are two- or multistoreyed stands. The white to manage a deciduous or mixedwood stand with spruce stratum may be in the regeneration stratum white spruce understorey must be done on a site- (consisting of seedlings and saplings of variable and stand-specific basis. From a silvicultural heights) or in the understorey stratum, having perspective, stands with white spruce provide some trees forming intermediate or codominant numerous stand management options. These crown classes. options may vary from “doing nothing” to complex White spruce in these stands originates during shelterwood systems (Table ). By definition, the the stand or understorey initiation phase that “do nothing” alternative falls into the category of follows a large-scale disturbance (Oliver and natural shelterwood. Alternatives B, C, and D Larson ). Spruce regeneration can occur involve clearcutting systems or clearcutting with immediately after fire if seed is available (Zasada retained seed trees. Plantation technology ), or may not occur until or more years (alternative B) has been attempted many times in after the disturbance (Youngblood ), with the the past, prevailing during the –s in the gradual ingress of white spruce occurring during Prairie Region. It involves clearcutting, site the understorey initiation phase. The latter creates preparation, planting spruce, and variable levels of an understorey with varying heights and ages. Even stand tending and competition control. Many cases
36 20 20 24 Stand U6 Aspen age Stand U6 42 yrs (41–48) 15 15
10 10
5 5
0 0
20 20 Stand U7 Aspen age Stand U7 45 yrs (37–50) 15 15
10 10
5 5 Number of trees Number of trees
0 0
20 20 Stand U11 Aspen age Stand U11 64 yrs (61–68) 15 15
10 10
5 5
0 0 >25 <1.3 6–10 2.6–5 11–15 16–20 21–25 26–30 31–35 36–40 41–45 46–50 51–55 56–60 61–65 66–70 71–75 7.6–10 1.3–2.5 5.1–7.5 Age classes 12.6–15 17.6–20 22.6–25 10.1–12.5 15.1–17.5 20.1–22.5 Height classes
Age class and height class distribution of white spruce understorey in three mixedwood stands, Whitecourt, Alberta (V. Lieffers and S. Navratil, unpublished data).
Management and silviculture options for deciduous and mixedwood stands with white spruce understorey
Management option Silvicultural system
A. “Do nothing” • extended rotation or use of other silvicultural systems at later stages of stand development • natural shelterwood systems B. Plantation technology • clearcutting, site preparation, planting, and tending C. Deciduous production • clearcutting and no treatments D. Deciduous production with • clearcutting with retained white spruce seed trees natural regeneration of white spruce E. White spruce understorey protection, • two-stage harvesting model conifer and deciduous production • array of systems with the incremental levels of harvesting • protection of understorey, wind protection of released spruce, • harvesting difficulty, and conifer yield
37 exist where these methods have resulted in poor Successful application of the two-stage harvesting coniferous stocking or high-cost plantation system requires decision-making steps that involve establishment (Samoil ; Shortreid ). the evaluation of stand suitability, site factors, and The last alternative (E) of white spruce wind risk. understorey protection while harvesting aspen uses the two-stage harvesting model. This model can Stand Suitability involve an array of silvicultural systems (see Density and Spatial Distribution of White Spruce Navratil et al. [] and Navratil [] for Understorey Results of pre-harvest and post- details), ranging from clearcutting to several harvest assessments of white spruce densities in the variants of shelterwood systems: Alberta harvesting trials show that – % of white spruce understorey is destroyed or damaged • Clearcutting with or without seed trees during harvesting of the aspen overstorey when • Clearcutting with or without windbreaks intermediate and high levels of protection are • Alternate strip clearcutting employed. The amount of immature spruce • Progressive strip clearcutting protected is influenced by its pre-harvest density, • Patch clearcutting the harvesting equipment used, the operating • Uniform shelterwood techniques, and the levels of planning and • Strip shelterwood supervision (Brace Forest Services ; Sauder • Combined strip shelterwood ). Conventional roadside harvesting equipment • Irregular shelterwood. protected more understorey than cut-to-length All of these systems can provide incremental Scandinavian equipment. The differences were levels of the understorey protection, wind directly related to felling and skidding methods. protection of released spruce, harvesting difficulty, Roadside harvesting equipment left well-defined yield and value potential, biodiversity, and skid trails with islands of relatively undamaged aesthetics. The overriding management objective understorey between the trails. Cut-to-length applied here, and which we will also use in our equipment left skid trails that were less visible, but discussion, is to enhance softwood production more of the understorey between the trails was while sustaining mixedwoods on the site. damaged (Brace Forest Services ; Sauder ). The density of white spruce clumps may also Two-stage Silvicultural and Harvesting Model influence the degree of understorey protection. High protection levels were observed in stands In this model (Figure ), the first harvest takes with dense clumps that restricted equipment entry place when aspen is – years old and for aspen removal. The operator may also leave understorey spruce is about – years old. All marginally merchantable spruce standing if these aspen forming the overstorey and all spruce over a are surrounded by dense immature spruce dbh utilization limit is harvested, leaving a released (Navratil et␣ al. ). spruce understorey. Following harvest, aspen Based on these estimates, the pre-harvest resuckers in the available spaces, which results in a understorey density should be about double the stand compromised of species clumps as well as targeted post-harvest densities. Targeted post- broadleaf-conifer mixtures. Broadleaf regeneration harvest densities will greatly depend on may also contain suckers of balsam poplar and management objectives and wind-risk level. seedlings of both aspen and balsam poplar. If the Management objectives could specify the highest objective of softwood production is to be level of spruce protection during harvest to achieve maximized, conifers could be planted in areas the maximum spruce yield or thermal cover for inadequately stocked by the spruce understorey. wildlife. Maximum spruce yield (expressed as total The second harvest is taken approximately volume at the second harvest) may be achieved ␣ years later, when both aspen and spruce are when post-harvest densities exceed spruce trees harvested. During the time between the first per hectare. If the management objective is to harvest at age and the second harvest at age , sustain production of both hardwoods and spruce natural regeneration of spruce could occur on the same site and to harvest aspen and spruce at (Navratil et␣ al. ).
38 140 the second harvest, lower spruce densities may be Y = 0.0189X + 98.75 desired to enhance vigorous aspen regeneration. R2 = 0.73 130 Tree Morphology of Understorey Spruce Resistance of a tree to windthrow results from a combination 120 of several tree characteristics. Height is important
because wind speed, and therefore windload on a Height per dbh 110 tree, increases exponentially with distance from the ground. In the pooled data from the Alberta 100 harvesting trials, cumulative windthrow damage 0300 0500 0700 0900 1100 1300 1500 White spruce trees per hectare (>1.3 m in height) ␣ years after release for spruce trees with heights up to m was less than %. However, trees taller than Linear regression of slenderness coefficient of m had the most windthrow damage during the white spruce understorey trees and number of first years after release. The lack of damage in white spruce per hectare (from Navratil et al the – years after release indicates an improve- 1994). ment in tree stability resulting from crown, stem, and root system growth after release, which is a function of increased light and wind stimulus (Navratil et␣ al. ). effectiveness of root anchorage, which in turn is Crown morphology, shape, and size affects the mainly governed by the depth and size of struc- centre of gravity and windload on a tree. Crown tural roots. Depth of roots (shallowness of root morphology is influenced by the intensity and system) and biological depth of soil is often used to quality of light in the understorey. Shade-tolerant rate wind risk (Stathers et␣ al. ). spruce may be more affected by side shade than by Windthrow risk is expected to be higher on high shade cast by the upper canopy. We found moist and wet sites. Wet soils in general have lower that higher spruce density and volume in the stands shear strength and lower cohesion. Clay is very with understorey was associated with higher cohesive when dry, but its cohesion becomes slenderness coefficient of understorey spruce, increasingly weaker as moisture content increases. presumably because of greater side shade cast by White spruce is a shallow-rooted species and neighbouring spruce (Navratil et␣ al. .) forms flat root plates on moist soils. On sites with (Figure␣ ). a high water table, the root systems often have a The slenderness coefficient, expressed as a flat table-like appearance on the bottom. When the height per dbh ratio, is correlated with the crown water table fluctuates, spruce roots may be killed size and particularly crown length, which often during temporary wet periods. The presence of serves as an indication of wind damage resistance. endemic windthrows with flat root systems and The slenderness coefficient has been intensively signs of gleying in upper soil horizons may help to studied in Europe where the importance of diagnose these sensitive sites (Navratil ). maintaining well-tapered trees for protection On sites with wet soils, the extent of uprooting against wind and snow damage is emphasized. The is more affected by the duration of wind storms desirable height per dbh ratios vary with species than on drier sites. When a tree sways, movement and site. In central Europe, a ratio of to is transferred to the root plate, which rises and (or less) is acceptable for Norway spruce sinks. In the process, water is mixed with soil (Navratil␣ ). particles and washes soil particles from and below At present, in the absence of more specific local the root plate. As a result, the roots are pressed data for white spruce, we consider white spruce deeper into the soil, swaying is greater, and finally understorey trees with the values equal or greater the tree is uprooted. than and taller than m as a high-risk category. Understorey protection trials in Alberta were established mainly on mesic sites, where wind Site Evaluation Site, primarily soil characteristics, damage to residual spruce may be a less critical directly affects windthrow hazard of released issue because of deeper spruce rooting than on spruce. Susceptibility to windthrow is related to the
39 moist to wet sites. On sites with higher soil AnnualAnnual extremesextremes moisture, root anchoring strength is reduced. Root expansion and growth may be inhibited by anae- robic soils (Navratil et al. ; Urban et␣ al. ).
Wind Risk Assessment Wind gusts produce most of windthrow. Expected return periods for maximum gusts (the average length of time between gusts of a given wind speed) can be calculated from long-term meteorological records 79% and are useful for identifying the areas that will 80% 91% require special attention in wind protection planning (Flesch and Wilson ). In complex Length of bars terrain and topography, particularly in mountain represents regions that modify the wind direction and speed, frequency of occurrence in each the occurrence of high-speed winds is less direction. predictable and probability calculations may have less applicability and reliability. Directional analysis of maximum gusts (Figure␣ ) is also essential in planning for sheltering effects and cutblock layouts. The sheltering effect of stands that are located upwind from the stands Each concentric ring 94% represents a 10% increment requiring protection have long been recognized. in frequency (i.e., 45% However, little quantitative information is available occurrence of westerly on the speed change of winds leaving forest stands maximum gust). and entering open cutblocks (McNaughton ). Figure provides a simplified illustration of Directional frequencies for the annual extreme wind wind behaviour where a sheltering stand is on the gusts in Alberta (from Flesch and Wilson 1993). windward side of the open area. The open area may represent a cutblock with released white aspen overstorey in aspen-dominated stands where spruce understorey. Wind speed changes depicted a good supply of viable root suckers exists. It is in the figure show that when a wind leaves the estimated that approximately – aspen trees per stand it accelerates to about %; at m into the hectare with uniform distribution are needed to clearing it reaches %; and at about m it fulfil aspen regeneration targets (Doucet ; reaches % of the original speed. Navratil ). Different wind speeds and the roughness of the Deterrents to adequate aspen regeneration could forest stand can affect the extent of turbulence come from two sources: a lack of soil warming and␣ the width of the protection zone. Therefore, because of shading by retained spruce, and soil the above values cannot and should not be disturbance on skid trails and landings. Soil broadly applied. warming is essential for root suckering, particularly on mixedwood sites with thick insulating forest Targeted and Expected Gains floor layers. For stands of retained spruce with gaps of irregular size and shape, the effect of Aspen Regeneration The density, stocking, and shading on soil temperatures, as well as on the growth of aspen and balsam poplar established light levels available for vigorous growth of after the first harvest is of importance in sustaining emergent suckers has not yet been determined. a mixedwood stand and ensuring expected Observations of the effects of harvesting on hardwood yield at the second harvest. aspen regeneration in hardwood cutblocks The prerequisites for successful aspen (Kabzems ; Navratil ; Shepperd ) can regeneration are largely met by harvesting the likely be extended to aspen harvesting with
40 Wind direction 98% 91% 60% 55% 45% 22% 7% 2% 30% 80%
100
80
60
40
20 Relative windspeed (%)
0
40 20 0 30 50 90 140 210 250 300 50 100 Horizontal distance from stand edge (m)
Relative wind speed in a forest stand and adjacent open areas (after Vicena et al. 1979, from Navratil 1995). understorey protection. On the harvested blocks aspen and balsam poplar at the first harvest. with protected white spruce understorey in the trials in west-central Alberta, overall aspen Natural Regeneration of Spruce after the First stocking and densities had attained acceptable Harvest When shelterwood systems are applied in levels – years after harvesting. The overall aspen aspen-dominated mixedwoods, –% stocking stocking was greater than %, and aspen densities levels of white spruce regeneration are commonly varied from to trees per hectare produced by the partial removal of the overstorey (Navratil et␣ al. ). Shading by retained spruce and site preparation. Adequate surface soil and its effect on soil temperature (if any) did not exposure directly affects the establishment of white significantly affect the thresholds vital for suckering. spruce seedlings (Waldron ; Wurtz and Zasada Height growth of aspen regeneration, however, was ; Zasada ). less than the expected growth rate for the same In white spruce understorey stands after aspen general area (Navratil et␣ al. ). This could be overstorey has been harvested, stand and forest attributed to the reduced light caused by spruce floor conditions are variable and favour white shading. Whether or not the observed initial slow spruce natural regeneration. The removal of aspen growth will be expressed in long-term stand always generates some degree of surface development and yield is not clear. Balsam poplar disturbance and mineral soil exposure. In Alberta’s regeneration showed a consistent increase in harvesting trials with understorey protection, stocking during the first – years after harvesting, ground disturbances with mineral soil exposure particularly on exposed mineral soil. This confirms averaged about –% (Navratil et␣ al. ). the affinity of this species to colonize areas with Spatial distribution of retained spruce on harvested heavy ground disturbance. The same was not blocks is highly irregular, varying from very open observed for aspen. Balsam poplar is more versatile to very dense conditions. The resulting variability than aspen in both vegetative and seed-origin in soil moisture and light is also high. A seed reproduction (Zasada and Phips ). source is available from cone-producing released Aspen and balsam poplar regeneration will likely trees or adjacent stands and could be further fill the openings. Aspen and balsam poplar growth enhanced by intentionally retaining white spruce and yield at the second harvest should be affected seed trees in the first harvest. Preliminary obser- more by the density, stand development patterns, vations show that in the existing understorey and spatial distribution of retained spruce under- protection trials white spruce regeneration is storey than by the initial regeneration potential of common on disturbed soil surfaces such as skid
41 trails and landings. volume gains as well as for improvement of tree Spruce regeneration established between the first stability. The slenderness coefficient for the and second harvest has substantial benefits for the released spruce was consistently reduced. This ecosystem, but primarily for sustaining conifer reduction indicated gradual improvement in tree production in the second cycle, which is after the stability and coincided with the observed lack of second harvest of the two-stage harvesting model. significant windthrow in or more years after After the second harvest, the existing advanced release (Navratil et␣ al. ). Decreasing wind spruce regeneration will develop into the next damage with time since release is a similar trend to stand concurrently with newly regenerated aspen those reported for other conifers and thinning and form a merchantable spruce component in the treatments (Navratil ). For example, first harvest of the second cycle. Lohmander and Helles ₍₎ found a rapid decline in windthrow probability of Norway spruce in Growth Response of Spruce after Release The years to , a slight decline between years and , two-stage harvesting model is based on the and a very slight decline or no change between unobstructed growth of retained understorey years and after thinning. spruce after the first harvest. Improvement of tree stability does not come The tree’s condition at the time of release from the changes in taper (slenderness coefficient) governs how rapidly and how great the growth alone but is probably related to strengthening and response will be after release. The crown size and expansion of the root system after wind exposure. condition are the most critical factors in this An increase in the growth of structural roots is process (Oliver and Larson ). White spruce has believed to counterbalance sway and prevent a remarkable ability to respond to release at all blowdown (Coutts ). ages. For white spruce saplings, there is no evidence of reduced photosynthetic capacity following the Yield of Aspen and Spruce At First Harvest and removal of overtopping hardwoods (Lieffers et␣ al. Second Harvest The volume removed in the first ). The potential for white spruce growth harvest depends on the volume of overstorey response is greatest in the range of – years aspen, volume of spruce in the canopy, volume of (Jarvis et␣ al. ; Johnson ). Yang ₍₎ merchantable understorey spruce, and the type of found␣ the best growth response in the age range of silvicultural system prescribed. – years for released individual understorey After the removal of aspen in the first harvest, trees in aspen stands. Substantial increases in the released understorey spruce is expected to diameter growth also occurred in -year-old develop along with aspen regenerating from the spruce after the seed-cut of a shelterwood system root systems of harvested aspen. The second (Youngblood ). harvest is expected years later. Since aspen can Remeasurement of the spruce understorey only develop into the spaces unoccupied by spruce, ␣ years after the removal of aspen canopy in west- aspen yield would be lower in proportion to the central Alberta’s trials showed positive and uniform spruce density increase. diameter growth response across a range of Brace Forest Services ₍₎ estimated the densities. A –% increase in diameter occurred proportion of aspen yield years after the – years after release (Navratil et␣ al. ). understorey release compared to the white spruce Growth response, and particularly the response post-harvest densities (assuming trees are in diameter growth, are equally important for uniformly distributed across the site), as follows:
42 White spruce post-harvest Percentage of unoccupied canopy space at 100 density (trees per hectare) years in simulationsa of a white spruce stand with different densities (trees per hectare) and spatial – – – – distribution at age 40 % aspen yield of total yield Density Clumpiness Random Low High According to these estimates, aspen yields in stands with post-harvest white spruce densities 250 26 51 64 greater than trees per hectare would be 600 13 21 44 insignificant because the site would be fully occupied by spruce years after first harvest. 900 7 12 31 Crown closure at the rotation age of years predicted from simulations using the Tree and a TASS simulation by Forest Productivity Section, Research Branch, B.C. Ministry of Forests. Stand Simulator ₍TASS₎ model compares well to the above estimates. In the TASS simulations, the canopy Therefore, insignificant aspen volume will be closure (or, in reverse, unoccupied canopy space) produced at the second harvest. varied with the number of spruce trees and also Stands with spruce densities lower than trees with the spatial distribution of trees expressed in per hectare or stands with a high clumpiness of three levels of clumpiness: random, low, and high retained spruce will have a higher yield of aspen (Table ). Stands that started with initial white though, at present, we lack the tools to forecast spruce densities of trees per hectare had (white these proportions. Higher yields of aspen can also spruce age = years) an unoccupied canopy be expected in harvested stands with larger open space of , , and % for random distribution, areas such as landings, if soil disturbance in these low clumpiness, and high clumpiness, respectively, areas can be minimized. This, for example, may be at second harvest. done by winter harvesting. Since aspen is expected to regenerate after the Examples of the expected white spruce yield at first harvest into gaps in retained understorey the second harvest compared to understorey spruce, it is unlikely that all available space would density (in stands harvested by conventional be filled fully with aspen. In addition, Johnson clearcutting with white spruce protection) are ₍₎ calculated that if spruce is taller than . m presented in Table . The simulations also show at the time of release, newly regenerated aspen will that spruce yields are significantly influenced by not grow fast enough to overgrow the spruce. For the level of clumpiness of white spruce. Higher the levels of unoccupied canopy space in the range levels of understorey clumpiness produce lower of –%, very few aspen will form dominant or spruce yield at the second harvest. codominant crown classes of the canopy.
43 White spruce yield (in m3/ha) at second harvest in relation to understorey density
White spruce per hectare after first harvest 250 580 850
TASS simulationsa 230 369 452 Spruce age = 100 years Trials in Drayton Valleyb 162 346 468 Whitecourt 142 310 428 Hinton 132 294 411 estimated spruce age = 110 years a TASS simulations by Forest Productivity Section, Research Branch, B.C. Ministry of Forests. TASS simulations based on random distribution of white spruce and densities at age . b Projections based on calculations of -year PAI (the first years after release) and compounded mortality (% per year) as reported in Navratil et␣ al. ₍₎. Densities of , , and trees per hectare were used, which are identical to those used in the TASS simulations.
Conclusions Section, Research Branch, British Columbia Ministry of Forests, for the TASS simulations of My introductory remarks placed the silviculture of understorey development. We appreciate the stands with understoreys within the context of technical and editorial reviews by our peers. extensive and ecosystem management. A well- designed and properly implemented silvicultural References system that protects spruce understorey does provide an extensive management option that is Benson, C.A. . A need for extensive forest affordable, ecologically sound, and can provide management. For. Chron. :–. high yields. Brace Forest Services. . Protecting white Managing stands with understorey is more spruce understories when harvesting aspen. complex than the clearcutting approach practised For. Can. and For. Lands Wildl., Alta. For. in the past. Successful management of these stands Serv. Edmonton, Alta. Canada-Alberta Part- requires new knowledge and a change in attitudes. nership Agreement in For. Rep. No. . However, the rewards and long-term gains, in Prog. Rep. addition to conifer yield, are also higher. The use Brace, L.G. . Protecting understory white of these systems as a management strategy is spruce when harvesting aspen. In Northern important for ensuring long-term ecosystem Mixedwood ’. Proc. Symp. September – resiliency, wildlife habitat, biodiversity and , , Fort St. John, B.C. A. Shortreid landscape aesthetics, thereby addressing the (editor). For. Can. and B.C. Min. For. Victo- shortcomings of clearcutting. ria, B.C. FRDA Rep. No. . pp. -. Brace, L.G. and I.E. Bella. . Understanding Acknowledgements the understory: dilemma and opportunity. In Aspen Symposium ’. Proc. Symp. July – Funding for this work was provided by the , , Duluth, Minn. R.D. Adams (editor). Canada-Alberta Partnership Agreement in Forestry: U.S. Dep. Agric. For. Serv. North Cent. For. Projects A- and A-. Thanks are due to Exp. Sta., St. Paul, Minn. Gen. Tech. Rep. K.␣ Mitchell and K. Polsson, Forest Productivity NC-. pp. –.
44 Coutts, M.P. . Root architecture and tree sta- Lohmander, P. and F. Helles. . Windthrow prob- bility. Plant and Soil :–. ability as a function of stand characteristics and DeLong, C. . Dynamics of boreal mixedwood shelter. Scand. J. For. Res. :–. ecosystems. In Northern Mixedwood ’. McNaughton, K.G. . Micrometeorology of Proc. Symp. September –, , Fort St. shelter belts and forest edges. Phil. Trans. R. John, B.C. A. Shortreid (editor). For. Can. Soc. Lond. :–. and B.C. Min. For. Victoria, B.C. FRDA Rep. Navratil, S. []. Sustained aspen productivity No. . pp. –. on hardwood and mixedwood sites. In Ecol- Doucet, R. . Regeneration silviculture of ogy and management of British Columbia aspen. For. Chron. :–. hardwoods, December –, , Vancouver, Flesch, T.K. and J.D. Wilson. . Extreme value B.C. P.G. Comeau, G.J. Harper, M. Blache, analysis of wind gusts in Alberta. For. Can. J.O. Boateng, and K.D. Thomas (editors). and For. Lands Wildl. Alta. For. Serv. Ed- For. Can. and B.C. Min. For. Victoria, B.C. monton, Alta. Canada-Alberta Partnership FRDA Rep. In press. Agreement in For. Rep. .␣ . Minimizing wind damage in alter- Frohning, K. . Logging hardwoods to reduce native silviculture systems in boreal damage to white spruce understory. Environ. mixedwoods. For. Can. and For. Lands Wildl. Can., Can. For. Serv. Edmonton, Alta. Inf. Alta. For. Serv. Edmonton, Alta. Canada-Al- Rep. NOR-X-. berta Partnership Agreement in For. Rep. No. Jarvis, J.M., G.A. Steneker, R.M. Waldron, and J.C. Lees. . Review of silvicultural re- Navratil, S., L.G. Brace, E.A. Sauder, and S. Lux. search: white spruce and trembling aspen . Silvicultural and harvesting options to cover types, mixedwood forest section, boreal favour immature white spruce and aspen re- forest region, Alberta, Saskatchewan, Mani- generation in boreal mixedwoods. Nat. toba. Can. Dep. For. Rural Devel., For. Resour. Can., Can. For. Serv. Edmonton, Branch, Ottawa, Ont. Dep. Publ. No. . Alta. Inf. Rep. NOR-X-. Johnson, H.J. []. The release of white spruce Navratil, S., K. Branter, and J. Zasada, Jr. . from trembling aspen overstoreys: a review Regeneration in the mixedwoods. In North- of available information and silvicultural ern Mixedwood ’. Proc. Symp. September guidelines. Can. For. Serv. and Man. Dep. –, , Fort St. John, B.C. A. Shortreid Nat. Resour. Winnipeg, Man. Canada-Mani- (editor). For. Can. and B.C. Min. For. Victo- toba For. Renewal Agreement. Unpubl. rep. ria, B.C. FRDA Rep. No. . pp. –. Kabzems, R. []. Impacts of concentrated Oliver, C.D. and B.C. Larson. . Forest stand heavy equipment traffic on aeration porosity dynamics. McGraw-Hill Inc. New York, N.Y. an bulk density in an aspen ecosystem. In Samoil, J.K. (editor). . Management and uti- Ecology and management of British Colum- lization of northern mixedwoods. Proc. bia hardwoods, December –, , Vancou- Symp. April –, , Edmonton, Alta. ver, B.C. P.G. Comeau, G.J. Harper, M. Can. For. Serv. Edmonton, Alta. Inf. Rep. Blache, J.O. Boateng, and K.D. Thomas (edi- NOR-X-. tors). For. Can. and B.C. Min. For. Victoria, Sauder, E.A. . Timber-harvesting techniques B.C. FRDA Rep. In press. that protect conifer understory in Lieffers, V.J. and J.A. Beck, Jr. . A semi-natu- mixedwood stands: case studies. For. Can. ral approach to mixedwood management in the and For. Lands Wildl. Alta. For. Serv. Ed- prairie provinces. For. Chron. ():–. monton, Alta. Canada-Alberta Partnership Lieffers, V., A.G. Mugasha, and S.E. MacDonald. Agreement in For. Rep. No. . . Ecophysiology of shade needles of Picea Shepperd, W.D. . The effect of harvesting glauca saplings in relation to removal of activities on soil compaction, root damage competing hardwoods and degree of prior and suckering in Colorado aspen. West. J. shading. Tree Physiol. :–. Appl. For. :–.
45 Shortreid, A. (editor). . Northern Mixedwood Yang, R.C. . Growth response of white ’. Proc. Symp. September –, , Fort spruce to release from trembling aspen. For. St. John, B.C. For. Can. and B.C. Min. For. Can. Edmonton, Alta. Inf. Rep. NOR-X-. Victoria, B.C. FRDA Rep. No. . Youngblood, A. . Radial growth after a Stathers, R.J., T.P. Rollerson, and S.J. Mitchell. shelterwood seed cut in a mature white . Windthrow handbook for British Co- spruce stand in interior Alaska. Can. J. For. lumbia forests. B.C. Min. For. Victoria, B.C. Res. :–. Work. Pap. No. . .␣ . Structure and dynamics in␣ mixed Urban, S.T., V.J. Lieffers, and S.E. Macdonald. forest stands of interior Alaska. Ph.D. thesis. . Release in radial growth in the trunk Univ. Alaska-Fairbanks. Fairbanks, Alaska. and structural roots of white spruce as meas- Zasada, J. . Production, dispersal, and germi- ured by dendrochronology. Can. J. For. Res. nation of white spruce and first year seedling :–. establishment after the Rosie Creek fire. In Vicena, I., J. Parez, and J. Konopka. . [Pro- Early results of the Rosie Creek research tection of forest stands against wind dam- project - . G.P. Juday and C.T. Dryness age.] State Agricultural Publisher, Prague. (editors). School of Agric. and Land Res. p. (In Czech). Manage. Fairbanks, Alaska. Univ. Alaska Waldron, R.M. . Factors affecting white Misc. Publ. No. -. pp. –. spruce regeneration on prepared seedbeds at .␣ . Developing silvicultural alternatives the Riding Mountain Forest Experimental for the boreal forest: an Alaskan perspective Area, Man. Can. Dep. For. Rural Devel. For. on regeneration of white spruce. Fac. Agric. Branch, Ottawa, Ont. Dep. Publ. No. . For. Univ. Alta, Edmonton, Alta. For. Indus- .␣ []. Converting aspen stands to try Lecture No. . mixedwoods by underplanting and seeding Zasada, J.C. and H.M. Phips. . Populus white spruce, Manitoba, Canada. Can. For. balsamifera L. Balsam poplar. In Silvics of Serv. Winnipeg, Man. Manitoba Partnership North America. R.M. Burns and B.H. Agreement in For. Rep. Honkala (technical co-ordinators). U.S. Dep. Wurtz, T. and J. Zasada. . An exceptional Agric. For. Serv., Washington, D.C. Agric. case of natural regeneration of white spruce Handb. No. . pp. –. in interior Alaska. In Current topics in forest research: emphasis on contributions by women scientists. Proc. Natl. Symp. Gainesville, Fla. U.S. Dep. Agric. For. Serv., Ashville, N.C. Gen. Tech. Rep. SE-.
46 Stand Dynamics and Silviculture of Mixed Conifer-hardwood Stands in Southern New England
MATTHEW J. KELTY
Abstract colonization, and were also among the first to experience a succession of other uses. First, The mixed hardwood-conifer forests of southern widespread deforestation for agricultural New England have undergone a series of human development occurred in the eighteenth and early impacts that have significantly altered the relative nineteenth centuries, followed by natural proportions of tree species. Old-growth forests reforestation on abandoned farmlands in the late were comprised largely of eastern hemlock (Tsuga nineteenth century. Then, an industry developed canadensis [L.] Carr.) and many species of based on harvests of the second-growth stands. hardwoods, dominated by red oak (Quercus rubra Planned silvicultural treatments were subsequently L.) and chestnut (Castanea dentata [Marsh.] applied, together with associated scientific forestry Borkh.), with scattered white pine (Pinus strobus research in the early twentieth century. These L.) occurring as emergents above the level of the waves of human activity dramatically altered the main canopy. Most of these forests were cleared for relative proportions of tree species in the forests, agriculture in the eighteenth century. With the but did not permanently change the overall widespread abandonment of agriculture in the composition. Because of this relatively long history nineteenth century, fields regenerated to nearly of exploitation and silvicultural experimentation, pure white pine. Silvicultural research in the early lessons may be learned from this region that are of twentieth century attempted to design treatments value to forest managers from British Columbia or to harvest and regenerate pure pine stands, but other regions, where interest exists in managing these efforts generally failed. The management complex species mixtures. focus shifted to mixed stands, similar in structure to the original forests, and this focus has been Regional Forest Description largely maintained to the present time. However, white pine has proven difficult to maintain in This discussion focuses on forests covering a large mixture with hemlock and hardwoods. The species part of the states of Massachusetts, Connecticut, composition and average canopy position of each and Rhode Island, and the southern parts of New species in young, mixed stands is strongly Hampshire and Maine (Figure ). The region has influenced by the density and heights of the been repeatedly glaciated, and is neither truly advance regeneration initially present. For several mountainous nor level, but has a rolling topography of the most important species, the development of with broad flat-topped hills that rise to the same large advance regeneration is an important facet of elevation within a locale. The bedrock consists silviculture in this forest type. mainly of schist and gneiss, which are both resis- tant to glacial grinding. Consequently, the parent Introduction material for most soils is a thin layer of bouldery glacial till, with water-worked sand, gravel, and The forests of southern New England are a clay deposits occurring on a small percentage of complex mixture of conifer and broadleaf tree the land. Annual precipitation for the␣ region is species that often are composed of or more about mm, evenly distributed throughout the species per hectare. These forests were among the year. The large number of tree species that coexist first in North America to be heavily exploited for in these forests is directly related to the pattern of timber during the period of European uniform, abundant precipitation.
47 here, is comprised of both the “transition hardwoods- white pine-hemlock” and “central hardwoods- hemlock-white pine” types of Westveld (). The principal natural disturbance that affects these forests is wind damage associated with infrequent hurricanes and other more common, Maine but less severe, windstorms. Fires are uncommon in the region except on the droughtiest sites. Major fires can occur on average sites only after cata- Vermont strophic wind damage or widespread harvesting has left large amounts of woody fuels to dry. Precolonial Forests and Their Early Exploitation Only small patches of old-growth forest exist in southern New England today, and these occur mainly on steep inaccessible areas that are not New Hampshire typical of most sites. The last large remnants of the Range of hardwood- whitepine-hemlock original forest were either harvested or destroyed Massachusetts forest type by windstorms in the early part of the century. However, some of them survived long enough to Rhode 0 100 200 Island become the subjects of study by forest ecologists Connecticut (Nichols ; Cline and Spurr ). These studies Kilometres have been combined with reports from early travelers and reconstructions from windthrown stands (Henry and Swan ) to give a picture of The range of the hardwood-white pine-hemlock the original stand structure. Typical old stands forest type in southern New England. This forest were composed of widely scattered emergent white type includes both the “transition” and “central” pines above a main canopy of hardwoods (oaks, hardwood species (Westveld 1956), in which chestnut [Castanea dentata (Marsh.) Borkh.], northern red oak is the most common hardwood maples, and others), with hemlock forming an species of the main canopy. understorey canopy. Stands that had regenerated following overstorey destruction by windstorms The hardwood species include those of the would maintain their even-aged character for northern hardwood type, dominated by beech about years if no additional disturbance (Fagus grandifolia Ehrh.), yellow birch (Betula occurred. As these stands aged further and the alleghaniensis Britton), and sugar maple (Acer overstorey hardwoods began to die, the shade- saccharum Marsh.), and those of the central tolerant, long-lived hemlock in the lower canopy hardwood type to the south, dominated by oaks would be released and fill in the main canopy; (Quercus spp.) and hickories (Carya spp.). Stands hemlocks would also be the main regeneration composed of mixtures of northern and central occurring in small gaps (Figure ). Thus, hemlock species are thus referred to as “transition” would increasingly become dominant in all canopy hardwoods (Westveld ). The most common strata (except in the emergent position), and the main canopy hardwood species is northern red oak stands would become increasingly uneven-aged. In (Quercus rubra L.) in both the transition many stands, the conifers could form % or more hardwoods and the northernmost part of the of the total basal area; of the two, hemlock would central hardwoods. On most upland sites, eastern dominate in density and basal area, but white pine hemlock (Tsuga canadensis [L.] Carr.) and eastern would dominate in height. white pine (Pinus strobus L.) are the only conifer The emergent pines, rising to m or more in species of importance. Thus, the natural forest height, were the most important timber trees of vegetation is referred to as the “hardwood- these forests. They achieved considerable fame hemlock-white pine” forest type, and as discussed because of their value for ship masts during the
48 White pine
40 Hemlock
Hardwoods Height (m) 20
Typical structure of an old hardwood-white pine-hemlock stand. This stand is predominantly even-aged, and is just entering the “transition old-growth stage” as defined by Oliver and Larson (1990). colonial period. Disputes over the rights to cutting use continued for – years on much of the these trees led to the British Crown reserving all land, but farming was widely abandoned in the pines growing on unincorporated land in the New latter part of the nineteenth century. This occurred England colonies. White pine was also valuable for because of the development of transportation from other less strategic uses in timber-starved England; better-quality farmlands further west and the sawn pine lumber for clapboards was among the attraction of jobs in the New England cities. The first timber exported from New England (Perlin natural reforestation that occurred on abandoned ). White pine was then and continues to be fields was dominated by white pine. This pattern highly prized for products requiring soft, easily was so widespread that a wooden container worked wood. industry was established based on the plentiful As settlement of the region continued, hemlock supply of raw materials. The pines that grew on bark became important as a source of tannin for these old fields were not of the excellent quality treating leather, with the bark often being more found in the virgin forests. This was due partly to valuable than the wood of this species. As a result, the low density at which the pines became considerable harvesting of hemlock occurred. Oak, established, leading to the development of large chestnut, and other hardwoods were also harvested branches, and to the effects of the white pine for various products, but the initial exploitation weevil (Pissodes strobi Peck), a native insect that generally consisted of the removal of the conifer kills the terminals of pines. Pine seedlings that component of the original stands, leaving most grow in shaded conditions are generally not of␣ the hardwoods behind. This wave of conifer attacked, but the large diameters of the terminal removal occurred through most of the region and shoots of open-grown trees and the warm further throughout the northeastern United States. temperatures of the shoots in the direct sun create In some cases, the remaining hardwoods expanded optimum conditions for weevil activity. Thus, old- to fill the gaps left in the canopy, so that years field pines repeatedly have their terminals killed, later, it was not obvious that any species had with laterals assuming dominance. Although the been␣ removed. overall vigour of trees infested by weevils is not affected, multiple-stemmed, crooked trees result. The Era of Agriculture and the Second-growth Pine Still, the wood quality met the needs of the Industry As the population of the region grew packaging industry for producing boxes, barrels, during the eighteenth century, much of the forest and pails. Knotty wood could be used for many land was cleared for agriculture, reducing forests to purposes, and clear wood was needed only in short about % of the landscape. Most of the remaining lengths that could be cut from single knot-free forests were repeatedly cut either as farm woodlots internodes. This forest industry flourished from or for industrial charcoal production. Agricultural about to .
49 In old-field pine stands past the age of years, good pine seed year or shelterwood cutting, with understoreys composed of most native hardwood overstorey removal occurring about years later. In species began to develop. Hemlock was generally either case, the forest floor was disturbed during absent from these understoreys. The reasons for harvesting to expose mineral soil, and hardwood the slow return of hemlock compared to hardwood seedlings were cut back to ground level. species have not been clearly documented. Shelterwood cutting was found to be the most Hemlock has small wind-dispersed seeds that successful because the young pine seedlings were require exposed mineral soil or decomposing wood protected from excessive heating. However it for germination and overstorey shade for early became clear that the initial height growth of the seedling survival. It may be that this combination newly established pine seedlings was less than that of conditions were rarely found in the first- of the hardwood sprouts and also less than the generation pine stands. The widespread growth of new seedlings of cherry and birch. The clearcutting of pine stands for the boxboard key to successful pine regeneration was not in the industry was largely done without any silvicultural cutting method, but in the weedings, which were planning. As a result, the harvested pine stands accomplished by cutting back hardwood sprouts were replaced by a mix of hardwood species either and seedling competitors using machetes. But on a released from the understorey or newly established majority of sites, even repeated weedings at about after the harvest. It soon became evident that the -year intervals failed to release pines because of pine resource would be eliminated after the first the resprouting of cut hardwoods (Lutz and generation. The desire to maintain pines for this Cline , ). productive industry led to some of the earliest This early experience identified the importance applied silvicultural research in North America, of matching silvicultural goals and treatments to aimed at the regeneration and subsequent site conditions. The majority of the region’s management of white pine following the harvest of landscape is covered by thin glacial till deposits old-field stands. containing particles ranging in size from boulders to clays. The common texture of the upper soil Early Silvicultural Research horizon is sandy loam, and the moisture-holding capacity of such soils is good enough to allow Some of the early silvicultural research took on rapid growth of hardwoods. On these sites, many characteristics of “adaptive management” attempts to regenerate white pine stands were (Baskerville ; Walters )—a management generally abandoned entirely, and management style that is currently receiving attention as a shifted to the valuable hardwoods—principally red highly efficient model for research programs in oak and white ash (Fraxinus americana L.). On the natural resources. This occurred at the research droughtier sites underlain by glacial outwash sands forests of Yale and Harvard universities (Toumey and gravels, hardwoods were less dense and had ; Goodlett ) and the Massachusetts slower height growth, so one weeding was often Agricultural College (now the University of sufficient to release the young pines. These kinds of Massachusetts). The commercial harvesting of sites occupy only about % of the landscape on white pine was an important management average, and pine management was soon limited to component in these forests, but it was done so that these areas; even here, the objective was generally each operational-scale application of a silvicultural scaled back to that of creating a mixed pine- treatment became an experiment. Hypotheses were hardwood stand (Goodlett ). formed about the future development of stands It is interesting to reflect on the necessity of this following various treatments and these were course of action in the s. At that time, compared to actual results. The original hypotheses chemicals that could kill small hardwoods either were then modified and subsequent treatments during site preparation or in release operations did reflected this new knowledge. not exist. Also, there was no economical means to By about it was learned through this remove the hardwood understorey by intensive research that abundant white pine regeneration mechanical site preparation before pine estab- could be dependably established by two cutting lishment. Conifers are established on good soils in methods: block clearcutting carried out during a the face of aggressive hardwood competition in
50 many parts of the world today, and much money branches (Tarbox and Reed ). The same is true and effort is spent accomplishing now what could of red oak and other hardwood species that grow not be accomplished years ago. However, the above a dense understorey of hemlock. (This is the fact that it now can be done does not necessarily stand structure that was carefully developed to mean that it is always wise to do so. produce the famous high-quality oak [Quercus petraea] logs in the Spessart region of Germany, in Current Management in the Region which oak is grown above a dense European beech [Fagus sylvatica L.] understorey.) The early distinction between “hardwood sites” and There is also an interest in maintaining a “pine sites” in southern New England is still component of white pine within hardwood or recognized today. It is defined in current hardwood-hemlock stands. When it is knot-free, silvicultural guides by the height growth rate of white pine has high value for furniture and hardwoods (red oak or sugar maple), with an finishing wood. Although chemicals and approximate break point site index of . m at mechanical site preparation equipment are now base age of years (Lancaster and Leak ). available to control hardwoods, there is little Nearly all sites will revert to hardwoods following interest in using them to create pine-dominated overstorey removal, and nearly all can be made to stands on “hardwood” sites. The “ideal” stand grow pine with the technology now available. structure closely resembles that of the original However, because of the high cost of applying forest and does not differ much from the structure intensive treatments to control hardwoods on rich, desired by forest researchers in the s. moist sites, pure pine stands should only be established on sites with an index lower than Structure and Dynamics of Young Stands .␣ m; sites with an index of .–. m are favourable for mixed pine-hardwood stands, and Although important interactions occur throughout sites with an index greater than . m are best stand development, the most dynamic period is the managed for hardwoods (Lancaster and Leak ). earliest stage, during which a closed canopy is first Most forest land in southern New England is formed. This is the “stand initiation” stage of the owned by private nonindustrial landowners whose model described by Oliver and Larson ₍₎. The management objectives include a combination of structure and composition of the stand at the end aesthetic, wildlife habitat, and economic concerns. of this stage (usually at age to years in this Land in public ownership in this densely populated forest type) will determine a good deal about the region is managed for these same objectives, long-term pattern of development. At age , the with␣ watershed protection also being an structure is strongly influenced by the regeneration important␣ consideration. characteristics that exist just after the overstorey is On hardwood sites, red oak has been favoured cut or destroyed. The composition of the because of its high timber value. This has steadily regeneration will be affected by such factors as increased and the current stumpage price is several location of seed sources of each species, forest floor times that of white pine per unit volume (although conditions relative to germination requirements, volumes per acre are lower and rotation lengths are and presence of advance regeneration of some longer for oak than for white pine). It is also one species. The development of canopy structure is of the most important wildlife habitat tree species, then controlled by the inherent height growth because its acorns are a food source for many pattern of each species, the type of regeneration animal species. (sprout vs. seedling origin), and the spatial pattern Hemlock is also of interest, even though it has and density of each species, which can affect never had high value as a timber species. Its whether slower-growing species can escape being importance comes from its value as winter cover overtopped by faster-growing ones, simply because for deer and many other animal species, and for of their location (Oliver and Larson ). aesthetics and watershed protection. Hemlock also has timber-related value as a trainer tree: the best- “Pioneer” and “Gap-phase” Species To discern quality white pine is grown in mixture with a overall development patterns in complex species dense understorey of hemlock, which shades lower mixtures, it is useful to recognize groups (or
51 “guilds”) of species that have common charac- (Marshall ; Oliver and Stephens ). White teristics. However, this introduces the danger of pine is not as easy to classify, and its place within oversimplifying species life histories and therefore stand dynamics has been the subject of overlooking distinctions among the species, but is considerable interest and debate. Because of its still helpful if used with caution. The most basic prevalence and rapid height growth on exposed classification of species is into two groups sites following farm abandonment, it has (Whitmore ): sometimes been considered a pioneer. However, seedling growth becomes rapid only after an initial • “pioneer” species, with low shade tolerance, period of slow growth, lasting about years. rapid juvenile height growth, and the ability to Germination is best on exposed mineral soil or become established on exposed sites; and moss beds, but moderate shade benefits early • “gap-phase” or “advance-regeneration- establishment because of the small size of seedlings dependent” species, with intermediate to high during the first years. White pine has an levels of shade tolerance, slower juvenile height intermediate level of shade tolerance. Understorey growth, and the ability to become established pine saplings can survive with slow growth for at beneath an overstorey and advance to an upper least years and still respond quickly to release, canopy position after the overstorey is partially but the extent of this species’ behaviour as a gap- or completely removed or destroyed. phase species has not been well documented. Of the hardwood species in southern New Initial Regeneration Conditions The composition England, the pioneers consist mainly of birch and and structure of -year-old stands can be affected cherry species. Of these, paper birch (Betula by different initial conditions of regeneration. Four papyrifera Marsh.) and gray birch (Betula initial conditions are considered (Figure ). populifolia Marsh.) produce small, wind-dispersed seeds, whereas the seeds of black cherry (Prunus Following Severe Fire or on Abandoned Agricultural serotina Ehrh.) and pin cherry (Prunus pensylvanica Land On exposed, severely disturbed sites such as L.f.) are bird-dispersed, and remain dormant in the those left after abandonment of agriculture, the forest floor. Quaking aspen (Populus tremuloides colonizing species are limited to the wind- Michx.) and bigtooth aspen (Populus grandidentata dispersed pioneer hardwoods (mainly gray birch Michx.) also fit into this group, but they usually and paper birch), as well as white pine (Figure a). occur only in small numbers in these forests. The These species dominate following major fires as most important advance regeneration species well, but pin cherry and black cherry may also be include the oaks, maples, hickories, white ash, present, germinating from seed stored in the forest chestnut, and beech. These have a wide range in floor. In old-field stands, the presence of faster- shade tolerance, but all survive in the understorey growing birch species with their sparsely foliated and most can sprout vigorously from stumps (or crowns increases the early survival of pine by roots in the case of beech) after stems are cut or moderating microclimatic conditions, without broken off. The problems with grouping species are badly suppressing them. Free-ranging livestock illustrated by the characteristics of black birch selectively browsed on hardwoods in these (Betula lenta L.), which could be appropriately abandoned pasture lands, thereby increasing the placed in either group; it germinates on exposed dominance of pine in the young stands; large deer sites and grows rapidly in height, but is shade populations can cause the same effect. White pine tolerant and regenerates as an understorey has a greater average lifespan than the birch or seedling␣ as well, and produces sprouts from cherry species, so the dominance of pine would stumps␣ when␣ young. continue to increase with age as the other species Of the two conifer species, hemlock is fairly easy died. The young stand in Figure a is the same as to place within these two groups. It is a very shade- those that dominated the landscape years ago, tolerant, long-lived species with slow juvenile and developed into the high-volume but poor- height growth and depends on advance quality pine stands that fed the boxboard industry. regeneration. An understorey hemlock can respond Following Cutting, with Small Advance with increased growth when canopy gaps of all Regeneration Present Stands with greater species sizes are created repeatedly throughout its life
52 diversity develop following strip or patch 15 a clearcutting in a mixed-species stand, because a seed source for all species is near and advance regeneration of some species is usually already 10 present. If only very small seedlings are present for 5 species that cannot germinate after the cut, and if Height (m) sprouting from stumps is eliminated, a highly stratified stand structure develops by age 0 (Figure b). These specific conditions may not occur often, either in nature or in managed stands, but they have been incorporated in an experimen- 15 b tal design that efficiently examines the relative height growth of species growing in mixtures, with 10 all species starting from essentially an equal level as small seedlings (Smith and Ashton ). These 5 conditions produce a stand with an upper canopy Height (m) composed of pioneer birch and cherry, with advance regeneration species such as red oak, red 0 maple, and beech in a lower stratum. Hemlock and white pine occur in the lower stratum as well, often at the lowest heights. While finer distinctions in 15 c the pattern of stratification can be made (Smith and Ashton ), the distinction between these 10 two main canopy layers is of greatest importance. The long-term development of stands of this 5 structure is not well understood. As the short-lived Height (m) gray birch and pin cherry begin to die at about age 0 , the lower canopy oak and maple may be released to advance to the main canopy; however, black birch, paper birch, and black cherry are 15 d longer lived and may continue to dominate the upper canopy for many decades. Hemlock and 10 beech can tolerate deep shade and will likely persist with little growth for many decades. White pine 5 will not survive nearly as long and many will die Height (m) by age years; their survival would vary with the density and composition of the overstorey canopy. 0 Birch and cherry Advanced regeneration hardwoods Following Cutting, with Large Red Oak and Maple Hemlock RO – Red oak Advance Regeneration Present A more common White pine RM – Red maple B – Beech initial set of conditions is similar to those described for Figure b, except that red oak and Composition and structure of 20-year-old stands red␣ maple (and white ash on some sites) occur as in the hardwood-white pine-hemlock forest type, large seedling or sapling advance regeneration as influenced by initial regeneration conditions: (Figure␣ c). These species can respond quickly (a) following severe fire or on abandoned following overstorey removal, or can sprout from agricultural land; (b) following cutting, with only stumps if the stem has been cut or broken off. The small advance regeneration present; (c) rapid growth of these established hardwood following cutting, with large advance regener- seedlings or sprouts keeps their height equal to that ation of red oak and red maple present; (d) of the pioneer hardwoods. This was the common following cutting, with large advance regener- structure of young stands after the old-field white ation of red oak, red maple, white pine, and hemlock present.
53 pine had been clearcut from to Silvicultural Concepts about the Regeneration (McKinnon et␣ al. ), or after the overstorey was of Mixed Stands destroyed by winds such as the hurricane, which affected much of the region (Spurr ). The preceding discussion stressed the importance The long-term development of these stands, which of the size of advance regeneration in the early are the most common kind on the landscape today, development of stand structure in mixed stands. has been the focus of several studies (Oliver ; This concept is well accepted for regeneration of Hibbs ; Kelty ). The birch and cherry oaks; silvicultural guidelines recommend that oak species (but not the more shade-tolerant black advance regeneration be a minimum of . m tall birch) will generally die while still occupying before it can be considered safely established and overstorey positions. Red oak will increasingly likely to compete well with pioneer species dominate the overstorey canopy by age and following overstorey removal. older, with red maple and black birch falling to a The importance of large advance regeneration subordinate canopy layer. If hemlock is present at has not been stressed in silvicultural all, it will survive well and form the lowest stratum. recommendations for the conifer species. This is not surprising for hemlock, because it is often Following Cutting, with Large Red Oak and Maple, considered desirable to maintain hemlock in lower White Pine, and Hemlock Advance Regeneration canopy positions. However, there is ample evidence Present The initial conditions in this final example that small hemlock advance regeneration quickly differ from the one described for Figure c, in that falls to a lower canopy position and that larger some of the hemlock and white pine advance residuals can reach the main canopy following regeneration occurs as saplings or larger at the overstorey removal (Oliver and Stephens ; time of overstorey removal (Figure d). If sufficient Hibbs ; Kelty ). The situation with white in magnitude, this head start in height develop- pine is not as clear. Maintaining pine in mixture ment will prevent the conifer species from being with hardwoods in managed stands posed overtopped by the initial rapid growth of difficulties and led early researchers to study hardwoods. The pine and hemlock often do not natural stand structure to infer the developmental keep ahead of the hardwoods, but form part of the patterns. Their methods and ideas have impli- main canopy at approximately equal heights cations for mixed stand silviculture in general, and (Oliver and Stephens ; Hibbs ; Kelty ; are thus worthy of further consideration. Kelty and Entcheva ). The minimum initial After initial attempts at regenerating white pine size necessary for the conifers to maintain an upper in the s, researchers were faced with conflicting canopy positon will vary with site conditions; evidence about the competitive ability of pine on hemlock will likely need a greater initial size than moist sites. The existence of nearly pure pine white pine because of its slower height growth. stands on these sites clearly resulted from the These minimum sizes are not known, however. An particular condition found in abandoned fields. irregular size structure in conifer advance The next generation of pine seedlings rarely regeneration that included larger saplings may have survived and developed on these sites, even with been a common situation in old natural stands. It repeated weedings of the hardwood regeneration. is likely that canopy gaps would allow advance However, single large pines existed in or above the regeneration to develop, and when severe wind main canopy of mature hardwood stands, even on disturbances occurred would leave an irregular set the richest sites. These pines had developed of residuals to respond. The larger hemlocks that without the benefit of silvicultural treatment. fill this role may be older than is commonly Consideration of the potential developmental thought of as advance regeneration; they may be pathways for these stands led to the hypothesis of lower canopy trees of the same age as the destroyed group development of pines in hardwood- overstorey, but because they are still able to dominated stands (Cline and Lockard ). Each respond at advanced ages, they function as mature pine was thought to be the survivor of a advance␣ regeneration.
54 group of pine seedlings that had initially death of one species in a mixture arises when dominated the regeneration in a small area because regeneration is established at high densities; the of ground cover conditions, soil factors, or light species with faster initial growth will eliminate the conditions in canopy gaps before overstorey slower-growing one, unless the faster-growing one destruction (Figure ). By this hypothesis, the has sparse foliage, as happens with some pioneer group buffers the central pine from hardwood species. On the other hand, if the faster-growing competition during early stand development; the species occurs at low density, the other species may outside pines of the group are overtopped by be able to survive its period of slow juvenile height hardwoods and eventually die, but the central one growth and either grow for long periods as a lower escapes competition and advances to the main canopy tree or later advance into the main canopy. canopy. The sustained height growth of pines at This idea has been put forward for spruce- older ages would then eventually take them to dominated mixtures in Germany. Kenk ₍₎ emergent positions. suggested that establishing spruce at wider spacing New England researchers received support for would allow other species to develop without the this hypothesis from “several prominent European need to confine them into groups in a carefully foresters” (Cline and Lockard ). At that time, designed pattern. European foresters were shifting their focus from For New England stands, the importance of the Norway spruce (Picea abies [L.] Karst.) outer pines in groups sheltering the central pines monocultures to mixtures of spruce with other from hardwood competition is difficult to species, mainly European silver fir (Abies alba understand because hardwoods so quickly grow Mill.) and European beech. The technique widely taller than pines. The important factor appears to used to accomplish this, particularly in Germany, be that hardwoods have very low density or are was to incorporate fir and beech in groups missing entirely from a patch, because of initial comprising about % of the stand area, with pure soil␣ or ground cover conditions. (Pine seedlings spruce accounting for the remainder. Fir and beech initially grow so slowly that it is unlikely that their had slower juvenile growth than spruce, so those presence would prevent the initial establishment species were overtopped and often eliminated early of␣ the hardwoods). Thus, the patches represent in development. The group-wise establishment “uncontested” sites, where a stronger competitor is pattern for these species is still used in Germany. absent. It is likely that a pine establishing on such a This concept was generalized to a certain extent in site could advance to the hardwood canopy level, New England to explain the coexistence of species whether the site was occupied by a group of pines in mature, unmanaged stands. The conifer planta- or was devoid of other tree regeneration. This does tions established at the Harvard Forest had species not mean that group regeneration of pines does confined to monospecific blocks of a carefully not occur, but that the absence of high-density calculated size. Each block would then be reduced hardwood regeneration from a patch by itself is the through successive thinnings to one tree in the important factor, rather than the existence of a mature stand, although this was done only on a group of pines. limited experimental basis. This concept was more The hypotheses about white pine establishment importantly directed to the management of young and development can be summarized as follows. pine-hardwood stands that developed after the Pine occurs in nearly pure stands only on sites that harvest of old-field pines. The regeneration was are very droughty or that have been severely segregated into groups of pine and hardwoods by disturbed by fire or clearing for agriculture. It plays weeding hardwoods in patches of about .– the role of a pioneer species in these situations, . ha only where promising groups of pine even though it does not have all the life history seedlings had become established (Cline and characteristics generally associated with pioneers. Lockard ). Pine becomes established in mixed stands with The idea of grouping each species in natural hardwoods and hemlock after less severe distur- stands appears to be a rather artificial construct. bances (windthrow or overstorey cutting) in two ways: Much of the problem of the early suppression and Shortly before or after overstorey removal, pine
55 Red oak Red Paper Red oak White oak birch ash White pine
Hard Hard maple maple Hard maple
Age 70
Age 50
Age 30
A group of pines
Age 10
An illustration of the group development of white pine in hardwood stands. (Reprinted with permission from Cline and Lockard [1925].)
56 seedlings become established on patches where References hardwood regeneration is lacking or occurs at low density (i.e., uncontested sites); the pines Baskerville, G. . Adaptive management: wood may or may not regenerate in groups on availability and habitat availability. For. these␣ patches. Chron. ():–. Many years before overstorey removal or Cline, A.C. and C.R. Lockard. . Mixed white pine destruction, pines become established in the and hardwood. Harvard For. Bull. No. . p. understorey and grow slowly to sapling size; Cline, A.C. and S.H. Spurr. . The virgin up- these saplings can then respond to release and land forest of central New England. Harvard grow into overstorey positions. For. Bull. No. . p. In natural (unmanaged) stands, uncontested sites Goodlett, J.C. . The development of site con- are hypothetically more prevalent on drier “pine” cepts at the Harvard Forest and their impact sites because of the generally lower density of on management policy. Harvard For. Bull. hardwood establishment there. With higher No. . p. hardwood density on moister “hardwood” sites, Henry, J.D. and J.M.A. Swan. . Reconstruct- large pine advance regeneration may increase in ing forest history from live and dead plant importance as a means for pine to develop into the material: an approach to the study of forest main canopy. However, the behaviour of white succession in southwest New Hampshire. pine as an understorey species has not been Ecology :–. examined in silvicultural experiments, except in a Hibbs, D.E. . White pine in the transition very limited way (Kelty and Entcheva ). hardwood forest. Can. J. Bot. :–. Kelty, M.J. . Development patterns in two Conclusions hemlock-hardwood stands in southern New England. Can. J. For. Res. :–. Several of the concepts discussed in this review of Kelty, M.J. and P.K. Entcheva . Response of the management and silvicultural experimentation suppressed white pine saplings to release with the hardwood-white pine-hemlock forests in during shelterwood cutting. North. J. Appl. southern New England have more general For. ():–. application to silviculture in other forest types. Kenk, G.K. . Silviculture of mixed-species First, the study of stand development patterns in stands in Germany. In The ecology of mixed- unmanaged forests is an important part of species stands of trees. M.G.R. Cannell, D.C. silvicultural research. Designing silvicultural Malcolm, and P.A. Robertson (editors). treatments that mimic natural processes can lead Blackwell Scientific Publications, Oxford, to efficient management strategies. Second, U.K. pp.␣ –. knowledge of the composition and structure of Lancaster, K.F. and W.B. Leak. . A young stands, of an age just past the development silvicultural guide for white pine in the of a closed canopy, can lead to predictions of long- Northeast. U.S. Dep. Agric. For. Serv. Gen. term stand development. Understanding the Tech. Rep. NE-. p. dynamics that control early composition and Lutz, R.J. and A.C. Cline. . Results of the structure is therefore very important. Third, some first thirty years of experimentation in silvi- species have considerable flexibility in their culture in the Harvard Forest, –. characteristics, and may follow different develop- Part I. The conversion of stands of old field mental pathways depending on the nature of origin by various methods of cutting and disturbances, competing species, and site factors. subsequent cultural treatments. Harvard For. And finally, the role of the size and density of Bull. No. . p. advance regeneration is of great importance in the .␣ . Results of the first thirty years of early development of stands, even for species such experimentation in silviculture in the as white pine, which may behave as pioneers in Harvard Forest, –. Part II. Natural some circumstances. reproduction methods in white pine-hemlock stands on light, sandy soils. Harvard For. Bull. No. . p.
57 McKinnon, F.S., G.R. Hyde, and A.C. Cline. . Smith, D.M. and P.M.S. Ashton. . Early Cut-over old field white pine lands in central dominance of pioneer hardwood after New England: a regional study of the compo- clearcutting and removal of advanced regen- sition and stocking of the ensuing volunteer eration. North. J. Appl. For. ():–. stands. Harvard For. Bull. No. . p. Spurr, S.H. . Natural restocking of forests Marshall, R. . The growth of hemlock before following the hurricane in central New and after release from suppression. Harvard England. Ecology : -. For. Bull. No. . p. Tarbox, E.E., and P.M. Reed. . Quality and Nichols, G.E. . The vegetation of Connecti- growth of white pine as influenced by cut. II. Virgin forests. Torreya :–. density, site, and associated species. Harvard Oliver, C.D. . The development of northern For. Bull. No. . p. red oak in mixed stands in central New Toumey, J.W. . The Yale Demonstration and England. Yale Univ. Sch. of For. and Environ. Research Forest near Keene, New Hampshire. Studies. Bull. No. . p. Yale Univ. Sch. of For. Bull. No. . p. Oliver, C.D. and B.C. Larson. . Forest stand Walters, C. . Adaptive management of renew- dynamics. McGraw-Hill. New York, N.Y. p. able resources. Macmillan Publ. Co., New Oliver, C.D. and E.P. Stephens. . Reconstruc- York, N.Y. p. tion of a mixed-species forest in central New Westveld, M. . Natural forest vegetation England. Ecology :–. zones of New England. J. For. :–. Perlin, J. . A forest journey: the role of wood Whitmore, T.C. . Canopy gaps and the two in the development of civilization. W.W. major groups of forest trees. Ecology :–. Norton Co., New York, N.Y. p.
58 Managing Light and Understorey Vegetation in Boreal and Temperate Broadleaf-conifer Forests
CHRISTIAN MESSIER
Abstract Characteristics of the Forest Understorey Light Environment Overstorey tree canopies are often manipulated using different types of cutting regimes to create Manipulating the forest understorey light environ- light environments that favour the growth of the ment is one of the most powerful tools that forest desired understorey crop species. However, non- managers possess to influence the growing conditions, crop species often invade the understorey following succession, and future tree composition of a stand. the opening of the tree canopy. A review of studies The type and intensity of overstorey tree removal conducted in boreal and temperate stands showed determines the quantity, quality, and dynamics of that light quantity decreases linearly with an the light transmitted to the forest understorey increase in basal area for coniferous stands. Shade- (Ross et␣ al. ; Messier et Bellefleur ; Messier intolerant conifers appear to transmit more light et␣ al. ; Turnbull and Yates ; Messier and than shade-tolerant species for a similar basal area. Puttonen ; Messier et␣ al. ). Light quantity The presence of aspen or birch tends to increase measured in the wave band – nm plays a understorey light quantity. The understorey light major role in the growth, regeneration, and environment can be modified substantially by survival of forest plants because it directly affects manipulating stand structure and tree species photosynthesis. Light quantity is usually measured composition. The main factors affecting in units of watts/m, if the amount of energy understorey light environment are: total leaf area, received between and ␣ nm is of interest. If live crown height, spatial distribution of the trees, the amount of photons received (often referred to sun angle, sky conditions, and tree species. A new as Photosynthetic Photon Flux Density or PPFD) is classification of understorey shade into six types is of interest, then the units used are µmols · m- ·s-. proposed. These categories of shade have very For the rest of this paper, I will use the term light different light dynamics, and potentially very to mean the energy or the amount of photons different effects on understorey crop and non-crop received between and nm. vegetation. New methods are now available that In general, forest canopies greatly attenuate allow a rapid, accurate, and inexpensive the␣ amount of light received at ground level. characterization of the understorey light Tree␣ canopies have much less effect on the longer environment over a large area or for a particular far-red wavelengths (– nm) than shorter location under a stand. A good knowledge of the (– nm) wavelengths, causing the red (– physiological and morphological plasticity and ␣ nm) to far-red (– nm) ratio (R:FR) to ecological light requirements of both understorey diminish (Frankland and Letendre ; Smith crop and non-crop vegetation could allow the ; Ross et␣ al. ; Messier et␣ al. ). Both manager to create optimal conditions for crop angiosperm and gymnosperm species exhibit many species that will preclude the development of the modifications in development related to changes in aggressive non-crop vegetation. R:FR ratios (Frankland and Letendre ; Morgan
59 et␣ al. ; Pons, ; Kwesiga and Grace ). In this paper, light quality will refer to the changes in 1 the R:FR ratio induced by the vegetation. 2 In addition to changes in light quantity and 3 quality, forests present a complex, changing, and heterogeneous light environment. Light reaching a leaf in the forest understorey consists mainly of: • direct light from the solar disk coming unimpeded through gaps; 4 • diffuse light from the clouds coming unimpeded through gaps; • diffuse blue light from the sky coming unimpeded through gaps; 5 • leaf light being either reflected or transmitted from the leaves above; and • trunk light being reflected from the surrounding tree stems (Figure ) (Endler ). 1 – Diffuse light from blue sky 2 – Direct light from solar disk The shape, duration, amount, and quality of 3 – Diffuse light from clouds radiation in these sun and shade light patches are of great importance to plants. Under sunny sky 4 – Reflected and transmitted light from leaves conditions, patches of very bright light (i.e., 5 – Reflected light from trunk and branches sunflecks) of different sizes, duration, and periodicity are known to be essential for the growth and survival of many understorey species (Chazdon ; Pfitsch and Pearcy ). Many The five different types of light reaching the studies have shown that over the course of a season forest understorey. The direct light coming from more than % of the light reaching the forest the sun disk has a R:FR ratio close to 1. The diffuse light coming unimpeded from the clouds floor under clear sky conditions is in the form of and sky also has a R:FR ratio close to 1, but the sunflecks (e.g., Messier et␣ al. ; Messier and diffuse light coming from the blue sky has a Puttonen ). A sunfleck event is defined as any strong peak in blue. The light being reflected and patch on the forest floor that received a substantial transmitted from the leaves has a low R:FR ratio, amount of direct light (Chazdon ). The amount and a strong peak in green. The light being of light in a sunfleck varies between forest stands. reflected by the trunk has a high R:FR ratio µ - - The threshold can be as low as moles · m ·s (modified from Endler 1993). under very dense forests such as those found in tropical (Chazdon and Fetcher ) and temperate • spatial variation in leaf orientation angles, regions (Messier et␣ al. ). On the other hand, it • penumbral effect, µ - - can be as high as moles · m ·s under more • leaf movement (e.g., trembling aspen leaves), open forests such as pine stands growing on poor • cloud distribution and movement, sites (Messier and Puttonen ). This incredible •topography, light variability is associated with structural and • height of the canopy, environmental heterogeneity on various space and • seasonal trends in plant phenology, and time scales. The key factors that cause this hetero- • seasonal and diurnal movement of the sun geneity in the understorey light environment are: (Canham et␣ al. ; Baldocchi and • clumping of foliage, Collineau␣ ). • gaps in canopy crown resulting from treefall or cutting practices,
60 Understorey Light in Boreal and Temperate Québec. Figure presents the relationships found Broadleaf-conifer Forests between total basal area and light measured above the understorey vegetation in various temperate Comparison among Different Studies Relatively and boreal, pure and mixed, broadleaf and conifer few studies exist that allow us to compare light stands. Total basal area explained more than % transmission under pure boreal or mixed conifer- of the variation in percentage of light transmitted broadleaf forests. Futhermore, comparisons are for the pure coniferous stands. Broadleaf- difficult because many different methods have been dominated stands tended to transmit more light, used to estimate the percentage of light transmitted and among the pure coniferous stands, the most to the understorey. Table presents a summary of shade-intolerant species tended to transmit more existing data on the percentage of light transmitted light. Furthermore, the values obtained for boreal to both the forest floor and the understorey forest stands were much higher than those for vegetation under various boreal and temperate closed coastal conifer forests of the Pacific forests. Ross et␣ al. ₍₎ measured the percentage Northwest (Messier et␣ al. ; Canham et␣ al. ), of light in the understorey of eight different conifer closed tropical pine forests of Australia (Morgan stands that varied in age from to years and et␣ al. ), closed temperate deciduous forests of in species composition from pure Jack pine (Pinus North America (Messier and Bellefleur ; banksiana) to mixed conifer-broadleaf species. Canham et␣ al. ), and other tropical forests They determined that the mean percentage of light (Pearcy ; Chazdon and Fetcher ; Oberbauer would vary from . to .% under closed forests et␣ al. ; Lee ). Boreal forest stands (either of at m above the forest floor at mid-day on sunny pure conifer or mixed conifer-broadleaf species) days. Messier et␣ al. ₍₎ reported that percentage appear to cast a lighter shade than most other of understorey light transmission measured in types of closed forests previously investigated. successional boreal forests in Québec varied from % at the forest floor under a -year-old pure trembling aspen (Populus tremuloides) stand to % 50 under a late successional stand composed mainly of balsam fir (Abies balsamea) and eastern redcedar (Thuja occidentalis). The percentage of light 40 transmitted for the same stands and locations at ␣ m above the forest floor varied from % under the -year-old aspen stand to % under the late 30 successional stand. Such large vertical differences in light availability within forest communities, especially in the younger sites, could be important 20 in explaining the success or failure of understorey plant species (Grime and Jeffrey ). In these % of above-canopy PPFD forests, there could be some advantage for 10 understorey species to grow rapidly to overcome the deep shade cast by the dense herbs and low shrub cover. Hence, such species would be selected 0 0 10 20 30 40 50 60 70 for the life history characteristics that enable them Basal area (m2/ha) to grow quickly in height. In mixed broadleaf-conifer forests in Alberta, ₍₎ Relationships between basal area of dominant Lieffers and Stadt found no relationship trees and percentage of light transmitted to the between total basal area and percentage of light understorey for conifer-dominated stands (black transmitted. However, they noticed that light circles), mixed conifer-broadleaf species (stars), transmission tended to increase with an increase in and broadleaf-dominated stands (open squares). the proportion of broadleaf species in the canopy. The line represents the relationship between the These results concur with those of Messier et␣ al. basal area and conifer-dominated stands ₍₎ in mixedwood forests in southwestern (R2 = 0.423, P < 0.001) (data are from Table 1).
61 Ross et␣Ross 1986 al. a PPFD % of above-canopy % ofabove-canopy /ha) (#/ha) Location Treatment floor forest vegetation Reference 2 –– – –– – 1–8 – 4942 5–12 –1–20 128 7–17 – – 48 1948 13–19 –15–2517–21 – 24 – 1688 60 6080 15–21 20–25 – – 73680 1016 18–2370 Alberta Alberta 8–14 – Natural40 Natural43 – 856 –65 – – – 52 –95 Alberta 25(0.73) 33.9 42et␣ Ross 1986 (0.64) 12.6 al. 80 Natural – 45 et␣ Ross Alberta 1986 al. 90 – – – Natural 57 – – – Alberta 50 Alberta – 45 –(0.53) 11.6 Natural Alberta et␣ Ross 1986 al. Natural –(0.97) 60.4 Natural – Alberta et␣ Ross 1986 al. – – Alberta Natural – Alberta Natural 30 Natural – 15 and Stadt 1994 Lieffers 17 – and Stadt 1994 Lieffers – and Stadt 1994 Lieffers 11 and Stadt 1994 Lieffers 6 16 and Stadt 1994 Lieffers and Stadt 1994 Lieffers 38 9–14 – 924 Alberta Natural – 37.4 (0.83) 135 4–15 –120 – 7988 35–50 Alberta – Natural – Alberta Natural(0.62) 9.6 et␣ Ross 1986 al. – 30–40 and Stadt 1994 Lieffers 135 10–30 – 680 Alberta Natural –(0.61) 14.8 et␣ Ross 1986 al. Age Height Basal area Density Near understorey Above A summary of observations forests boreal temperate and under various of light environments
Pinus banksianaPinus Picea mariana tremuloides Populus Picea glauca Picea glauca Pinus banksianaPinus tremuloides Populus Picea glauca Picea mariana tremuloides Populus tremuloides Populus tremuloides Populus tremuloides Populus Picea glauca tremuloides Populus Picea glauca Picea glauca balsamea Abies Picea mariana tremuloides Populus tremuloides Populus Larix laricina Main tree speciesMain (yrs) banksianaPinus (m) (m TABLE
62 PPFD % of above-canopy % of above-canopy /ha) (#/ha) Location Treatment floor forest vegetation Reference 2 –– – – 3.0 600 –– 5.3 5.7 170 190 7 2.0 – 3,700 Finland Planted – 75 (1.0) 1995 Puttonen and Messier 9027 –47 1675 30 2275 30.950 26.5 29.150 15 – 2–13 1275 33.6 21 1350 6.9 5.2 Alberta 525 Quebec 25.460 Quebec 1525 375 Natural Natural50 12–23 Quebec 975 Natural 7–26 2.9 Natural – 370 10.3 Quebec 4 6–2080 75 Natural 4 300 8 8 21.1 20 8 1994 and Stadt Lieffers 611 et␣ Messier Quebec n.d. al. 900 8 8.620 et␣ Messier n.d. al. 5.0 Natural et␣ Messier n.d. al. 8.0 370 Quebec 9 16.0 5 24.0 Natural et␣ Messier n.d. al. Quebec 3,200 2,500 Natural 5 Finland 8 Finland Planted et␣ Messier – n.d. al. Natural 8 – et␣ Messier n.d. al. 10 – 18 (0.83) 1995b and Messier Parent 1995 and Puttonen Messier 8 (0.65) 1995 and Puttonen Messier 100121 6–20135 6–24130120 6–22 4 5.1 15 6–13 10.1150140 8.8 50 4.9 150 100 7–20 200 21 5–12140 350 4.1 175 Quebec 5–15 3.6 3.5 50 Natural 12.9 100 125 350 12 14 et␣ Messier n.d. al. Age Height Basal area Density Near understorey Above (Continued)
Populus tremuloides Populus tremuloides Populus Picea glauca Picea glauca Betula papyrifera balsamea Abies tremuloides Populus Picea glauca Thujaoccidentalis balsamea Abies Picea mariana Betula papyrifera sylvestrisPinus Populus tremuloides Populus tremuloides Populus Betula papyrifera Betula papyrifera balsamea Abies tremuloides Populus balsamea Abies tremuloides Populus Betula papyrifera Thujaoccidentalis Picea glauca sylvestrisPinus sylvestrisPinus Main tree speciesMain (yrs)Picea glauca (m) (m TABLE
63 PPFD % of above-canopy % ofabove-canopy /ha) (#/ha) Location Treatment floor forest vegetation Reference 2 –– – –– – –– – –– – –– – –– – 40 15.036 26.036 436 6.4 1,400 11.4 12.1 64.4 Finland 12,40043 51.0 20,000 Natural43 13.3 B.C. 6400 13.915 B.C. 46.1 –38 Natural 25.2 15 B.C.38 Natural 3256 13.9(0.40) 0.8 80 13 (0.69) 1096 14.780 Natural 0.8 (0.52) – 1995 and Puttonen Messier (0.85) 17.0 18.0 18.040 et␣ Messier 1989 al. B.C. 40.5 3.3 (0.65) 18.3 0.6 (0.58) B.C. 22.3 et␣ Messier 1470 680 1989 al. 20 Planted 1.2 (0.60) 1820 34 60% thinning et␣ Messier 1989 al. 760 52.6 Sweden N.C. – Sweden 1514 thinning Heavy – 1152 thinning No Sweden Planted – thinning Heavy Sweden 16.5 Light thinning – 2.3 Alaska – – 1994 and Mitchell Messier 1994 and Mitchell Messier – Natural 13 2 1987 Johanson 12 5 – 1987 Johanson 6 1987 Johanson 1982 and Knoerr Sinclair 1987 Johanson 1.5 1989 Alabeck and Tappeiner 100 22.5 32.5 420 Finland Natural – 17 (0.86) 1995 and Puttonen Messier Age Height Basal area Density Near understorey Above Old growthOld 30 185.0 490 B.C. Natural – 6–25 et␣ Messier 1989 al. (Concluded)
Values in parathenses are R:FR ratio in parathenses are Values Picea sitchensis a Tsuga heterophylla Tsuga heterophylla Tsuga heterophylla Tsuga heterophylla Tsuga Thujaplicata menziesii Pseudotsuga heterophyllaTsuga Old growth – 53.0 390 Alaska Natural – 4.0 1989 Alabeck and Tappeiner Pinus sylvestrisPinus Thuja plicata Thuja plicata Thujaplicata menziesii Pseudotsuga pine red Japanese taedaPinus –Picea abies Picea abies 10Picea abies Picea abies 16.0 heterophylla Tsuga Picea sitchensis 2000 Japan Planted – 1.5–10 1991 and Tang Washitani Main tree speciesMain (yrs) sylvestrisPinus (m) (m TABLE
64 These relatively high amounts of light, even in the overstorey maple species. It is therefore still densest aspen stands of Québec and Alberta, uncertain if a reduced R:FR ratio has any ecological presumably allow conifer seedlings to establish and significance for the growth and dynamics of eventually replace the shade-intolerant deciduous understorey trees. trees. Such open canopy conditions might also be Seasonal Changes in Light Seasonal variations in necessary for the conifer understorey and sub- light transmission are strongly dependent on canopy tree species to reach the overstorey because changes in solar position. In conifer-dominated of the limited resources and short growing season stands, light transmission tends to increase as the associated with boreal forest ecosystems. sun approaches its zenith at the summer solstice. A fairly strong relationship exists between the This is because the amount of foliage that a sun ray percentage of light transmitted and the R:FR ratio must penetrate to reach the understorey decreases under various temperate and boreal pure and (i.e., the mean gap frequency increases) as the sun mixed broadleaf-conifer stands (Figure ). In these moves from the horizon to its zenith (Anderson cases, the R:FR ratio never dropped lower than .. ; Messier and Puttonen ). In broadleaved Good relationships have been reported between forests, maximum radiation penetrates the forest in percentage of light transmitted and R:FR ratio early spring before leaf expansion. Light trans- values under broadleaf canopies for both overcast mission then declines rapidly as leaves reach their and clear sky conditions (Messier and Bellefleur full expansion, but increases slightly at the summer ; Lee ; Turnbull and Yates ). Although solstice when the sun is the highest in the sky. A many studies report that very low R:FR ratios reduction in solar angles after the summer solstice (i.e.,␣ < .) may affect many plant growth functions lessens light transmission until the trees start (Frankland and Letendre ; Smith ; Kwesiga dropping their leaves. Light transmission increases and Grace ; Schmitt and Wulff ), Lei ₍₎ again in the fall. In mixed conifer-broadleaf forests, failed to find any effect with a decrease in R:FR these seasonal variations are important because ratio from . to . for several understorey and conifer regeneration growing under broadleaved species can benefit from the maximum light 1.5 transmission in spring and autumn, assuming that soil and air temperature are high enough to allow for photosynthesis. While this could be significant in any boreal and temperate region, it is especially beneficial for conifer saplings growing under 1.0 broadleaved forests in coastal northwestern North America where the winters are relatively warm.
A Classification of Understorey Light Habitats R:FR ratio ₍₎ 0.5 Messier has divided the forest shade into six major light habitat classes, based on a suggestion put forward by Endler ₍₎. These light habitats are characterized by different light quantity, quality, and dynamics, all of which are known to 0.0 potentially influence plant growth (Chazdon ; 0 10 20 30 40 50 60 70 80 Schmitt and Wulff ; Wayne and Bazzaz ). % of above-canopy PPFD These characteristics of light will vary according to the geometry of the forest (mainly tree height, tree Relationships between percentage of light spacing, and gap size), weather (sunny or cloudy transmitted and R:FR ratio for boreal and conditions), time of day, and period of the growing temperate stands. The line represents a linear season (Canham et␣ al. ; Rich et␣ al. ; regression with R2 = 0.765 and P < 0.001 (data Messier and Puttonen ). are from Table 1).
65 Some uncertainty still exists about the irradiance Under overcast sky conditions, the R:FR ratio will regime and its influence on the growth and sun- even be higher, nearing .. Any broadleaf shade responses of tree seedlings. Total amount of vegetation above the habitat type microsite will irradiance, magnitude of daily peaks, diurnal time tend to reduce the ratio. of peaks, or overall variability of irradiance may all Small Gap Shade The second shade type is found be responsible. Because species in understorey on the southern side of a small gap, and I call it communities vary in their ability to use transient the small gap shade type. The light reaching this light, changes in light availability over time may type is composed of transmitted and reflected light play a significant role in the composition of from the foliage situated above the microsite, as understorey communities (Chazdon ). Wayne well as some diffuse light coming directly from the and Bazzaz ₍₎ have shown that some daily sky or clouds without being modified by variability in light will affect birch seedling vegetation. Again, no major periods of sunflecks performance. Thus, adjacent microsites within occur. The light quantity will tend to be higher forests, which might be considered equivalent in than in the forest shade type. The R:FR ratio is also total or average light, may still provide different higher because of the diffuse light coming regeneration niches if certain other light charac- unimpended from the sky and clouds. teristics differ. The complexity of the natural light Small Gap Opening The third habitat type is found environment is often ignored in traditional studies on the northern side of small gaps, and I call it the of photosynthetic and growth acclimation in arti- small gap opening type. The gap is less than one ficial environments (e.g., Loach ; Logan ). and one-half times the height of the surrounding dominant trees. The light reaching this type is the Six Shade Types A better understanding of the same as for the small gap shade type, but different types of light habitat created by forestry depending on the size of the gap, the height of the practices is essential. Figure illustrates the six edge trees, and the solar elevation, it can receive light habitat types that can be found in forests. direct sunlight under clear sky conditions. This Different light habitats are created by the location type might receive direct sunlight when the trees within and around canopy gaps, the size of the are m tall, but none when the trees are m tall. gaps, the height of edge trees, the sun angle, and Gap Edge Shade The fourth type is found on the sky conditions. These five factors affect the amount southern side of a large gap, and is termed the gap of direct and diffuse light reaching each habitat. Of edge shade type. The light reaching this type is the course, the quantity, quality, and dynamics of light same as for the small gap shade type, but the are specific to each forest community, but knowing amount of diffuse light reaching the microsite how these vary within a community allows forest unimpeded is much larger. managers to understand the different light Large Gap Opening The fifth is the large gap environments created by cutting practices. opening type. It is found on the northern side of a Futhermore, new and efficient methods for large gap. The light reaching this type is similar to estimating light under forest canopies are available, the small gap opening type, but it receives direct and these methods could be used to obtain the sunlight for a longer period of the day. basic information needed to estimate light Forest Edge Opening Finally, the sixth type is availability among the different light habitats. found under a closed forest on the northern side of Forest Shade The first light habitat is the forest a large gap, and is termed the forest edge opening shade type (Endler ). The light reaching this type. The amount of diffuse light reaching the habitat type is mainly transmitted or reflected by microsite can be fairly substantial if no the foliage of the vegetation situated above the undergrowth vegetation has developed in response microsite. It receives sunflecks that are of low to the large opening. It differs from the large gap intensity and short duration. This habitat type is opening and gap edge shade types because it is not influenced by any large gaps and is the basic technically not in a gap but under a forest. understorey shade found in most closed forests. If A small gap could be created by the fall of one the vegetation above the habitat type microsite is or a few dominant trees or by the removal of some composed mainly of conifer trees, then the R:FR trees by partial cutting. A large gap could be the ratio will ususally tend to be higher than .. result of a large windthrow event, a wildfire, the
66 South North (a) Diffuse skylight – High sun angle – 15 m tall forest – Clear sky conditions Direct sunlight
Reflected and transmitted light
1 2 3 4 5 6 Understorey shade types
(b) – Low sun angle – 15 m tall forest – Clear sky conditions
1 2 3 4 5 6
(c) – High or low sun angles – 15 m tall forest – Overcast sky conditions
1 2 3 4 5 6
(d)
– High sun angle – 30 m tall forest – Clear sky conditions
1 2 3 4 5 6
Effects of sun angle, tree height, gap size, location within gaps, and sky conditions on the amount of direct and diffuse light reaching the forest understorey. The numbers from 1 to 6 represent the six different types of shade found in the forest understorey: (1) forest shade, (2) small gap shade, (3) small gap opening, (4) gap edge shade, (5) large gap opening, and (6) forest edge opening.
67 killing of many trees by insects, or the clearcutting always fairly low on the horizon. To increase direct of large patches. Along with differences in the light light penetration, the gap must therefore be larger quantity, quality, and dynamics among these six than in a similar forest at lower latitude. The light habitats are the associated changes in air and effects of latitude (i.e., sun angle) and stand height soil temperature, evapotranspiration potential, on direct and diffuse light penetration in the six mineralization rates, and habitats for a host light habitats are shown graphically in Figure . At of␣ organisms. low latitude (i.e., high sun angle), the small gap The total amount of light reaching the different opening type does receive direct light (Figure a), light habitats should increase from type through but at high latitude (i.e., low sun angle) it does not to type , and then to type . Type receives the greatest amount of light. As the light increases so
) Open should the presence and vigour of the more shade- -1 2000 • s intolerant plant species. Such classification is a first -2
step toward a better understanding of the different PPDF
mol • m 0 light environments found naturally or created by µ ( 0600 2000 the different forestry practices. Similar approaches Beginning of the ) are being used by horticulturists to determine the -1 2000 Large gap sunlight direct sunlight effect • s
best location within the heterogeneity of light -2
habitats of a garden for a particular plant species. PPDF Type 5
The daily distribution of light for each light mol • m 0 µ
( 0600 2000 habitat under clear sky conditions is shown in
) Figure . While these graphs show a smoothed -1 2000 Large gap skylight • s
representation of the daily light distribution we -2
usually observed many peaks and troughs resulting PPDF Type 4 from short pulses of direct sunlight that penetrated mol • m 0 µ
( 0600 2000 the small gaps in the canopy. Sipe and Bazzaz ₍₎ showed that the mean daily amount of light ) measured on the southern side of small gaps was -1 2000
• s Edge skylight two to three times lower than on the northern side -2 under clear sky conditions. It was not different PPDF Type 6
mol • m 0 µ under overcast sky conditions. These results ( 0600 2000 illustrate the effects of sky conditions and location ) on the light environment within a gap. -1 2000 Small gap sunlight • s Messier and Puttonen ₍₎ observed very little -2 difference in the mean percentage of light PPDF Type 3
mol • m 0 µ transmitted daily among locations near the bole of ( 0600 2000 a tree, between trees, and in small gaps that ) represented only one-third of the height of the -1 2000 • s Small gap skylight dominant trees in –-year-old Scots pine stands -2 under both overcast and clear sky conditions. PPDF Type 2
mol • m 0 µ Although in closed pine stands at high latitude ( 0600 2000 (i.e., °) the mean percentage of light varies a lot ) for any given period of the day under clear sky -1 2000 • s Forest shade conditions, it does not vary much spatially when -2 PPDF measured over a full day under both overcast and Type 1
mol • m 0 µ clear sky conditions. ( 0600 2000 Canham et␣ al. ₍₎ found that latitude, canopy height, and gap size were the main factors Smoothed representation of daily variation in influencing gap effects on the understorey light light for each of the six types of light habitats environment. At high latitudes (> °), the sun is under clear sky conditions.
68 (Figure b). Similarly, no direct light reaches the many studies characterize the understorey light small gap opening type when the forest is m tall environment based on a few instantaneous (Figure d) compared to m (Figure a). measurements made at midday during completely cloudless sky conditions (Ross et␣ al. ; Lee ; Methods for Estimating Understorey Light Johansson ; Smith ; Lieffers and Stadt Environment ), during completely overcast sky conditions (Lawton ; Alaback and Tappeiner ), or The light environment below closed forest canopies both (Morgan et␣ al. ; Messier et␣ al. ; consists of a low-level background diffuse radiation Tappeiner and Alaback ; DeLong ; Carter regime that is punctuated with patches of sunflecks and Klinka ; Klinka et␣ al. ). The advantage under sunny sky conditions. These sunfleck patches of measuring light at mid-day under sunny sky vary in size, intensity, duration, and periodicity. conditions is that the total light quantity does not The dimension of sunflecks can vary because of the vary much. Therefore, above-canopy light can be position, number, size, and orientation of overhead estimated once before or after the understorey leaves, size of gaps, and the location of the sun. measures. During clear sky conditions, however, Below closed canopies, sunflecks of small size and the sun acts effectively as a point source, and local low duration are most frequent, but those with variations in canopy structure can have a major moderate size and duration contain the most effect on light penetration. Usually, the percentage energy and may have the most effect on the photo- of light transmitted to the understorey tends to be synthetic gain in understorey plants. Because of higher at mid-day under clear sky conditions this highly variable light environment, sampling because the actual mean gap frequency increases as light in forest understorey is a difficult task the sun moves from the horizon to its zenith (Anderson ; Gay et␣ al. ; Norman and Jarvis (Anderson ; Messier and Puttonen ). ; Baldocchi and Collineau ). Reifsnyder Therefore, an instantaneous measure of light et␣ al. ₍₎ calculated that between and sensors transmission made at mid-day under clear sky are needed to measure the instantaneous field conditions is not representative of the daily radiation in deciduous and conifer forests under percentage for that particular point. Reifsnyder clear sky conditions. However, as few as two sensors et␣ al. ₍₎ pointed out that several hundred might be sufficient to measure the more uniform sensors would be required to characterize the mean diffuse light environment on overcast days. light transmission for a stand. Baldocchi and Collineau ₍₎ estimated that the Hemispherical Photographs Recently, many number of sensors needed to characterize the light researchers (Chazdon ; Oberbauer et␣ al. ; environment under a certain forest canopy increases Rich et␣ al. ; King ; Olesen ) have used from to as the spatial␣ coefficient of variation the hemispherical canopy photography technique of the population increases from to %. to characterize the understorey light environment. Reifsnyder et␣ al. ₍₎ calculated that only This technique uses digitized images of the between and sensors are needed to evaluate the overstorey canopy to determine the movement of daily average light environment in deciduous and the solar disk throughout the day and the season coniferous forests. As explained by Baldocchi and and the position of the gaps. From this information, Collineau ₍₎, fewer sensors are needed because a computer program can calculate the amount of the coefficient of variation for the flux density of diffuse light coming from all directions at different light at the forest floor decreases as the averaging intervals, and the amount of light coming directly interval of the measurements increases. Therefore, from the sun through the gaps. The technique to measure short-term radiation below a forest requires calibration with actual light measurements canopy requires many sensors, whereas only a few to give absolute values. It gives fairly accurate sensors are needed to measure the average light estimates of the daily light regime for any environment over several hours or over a day. particular point in the understorey (Canham ; Instantaneous Measures under Clear Sky Rich et␣ al. ), but is expensive and can become Conditions Because of equipment limitations, fairly time-consuming.
69 A New, Simple and Efficient Method Washitani and Unlike the hemispherical photographic method, Tang ₍₎, Tang et␣ al. ₍₎, and Messier and this method does not provide information about Puttonen ₍₎ all suggest that the total percentage the temporal variation of the light regime over a of light transmitted through plant canopies can be day or a season. Futhermore, the method will not accurately estimated by measuring the amount of characterize differences between different locations full sunlight under diffuse light conditions. In within canopy gaps (i.e., north vs. south side of the effect, several studies indicate that understorey gap). Research is under way to evaluate the effects light is much less variable both spatially and of gap sizes, position within gaps, topography, and temporally under overcast sky conditions monthly variation during the growing season on (Anderson ; Reifsnyder et␣ al. ; Sheehy and the accuracy of the proposed method. This method Chapas ; Smolander and Lappi ). Messier has been used successfully by the research group at and Puttonen ₍₎ found that the percentage of the University of Québec in Montreal to evaluate light transmitted varies little on a daily basis under the relationship between light availability and overcast sky conditions, but it varies a lot under understorey plant growth and acclimation cloudless sky conditions. This is because the light (Beaudet ; Parent and Messier a; Ricard is diffused more uniformly from all directions and Messier ; Beaudet and Messier n.d.; throughout the day during overcast conditions. Chengeou and Messier, in prep.; Hilbert and Messier and Puttonen ₍₎ and Parent and Messier, in prep.; Messier and Nikinmaa, in prep.; Messier ₍b₎ found that the percentage of mean Williams and Messier, in prep.). daily light transmission measured during different P. Comeau (B.C. Ministry of Forests, pers. months at any particular point in the understorey comm., ) suggested that the LICOR LAI- was very similar under both clear and overcast sky plant canopy analyzer may also be useful for conditions for different stands located at different obtaining an estimate of canopy light conditions. latitudes. Based on these findings, Messier and This instrument can provide measurements of Puttonen ₍₎ suggested that the percentage of diffuse transmittance and overtopping leaf area light transmitted during overcast sky conditions index for the whole sky and for selected sectors of could be used as a quick and efficient index of the the sky. Studies are currently under way in British mean daily light received by any location in the Columbia to evaluate the use of this instrument for understorey of a closed forest. Parent and Messier characterizing gap and understorey light conditions ₍b₎ demonstrated that one instantenous in broadleaved stands (P. Comeau, B.C. Ministry of measure of the percentage of light transmitted Forests, pers. comm., ). under completely overcast sky conditions is enough Modelling Several models exist that attempt to to accurately estimate the mean daily percent light simulate light penetration within forest canopies transmission for any particular point under a forest using geometrical and statistical approaches canopy (Figure ). (Norman and Welles ; Lang and Xiang ; For such an estimate to be accurate, it must be Grace et␣ al. ; Wang and Jarvis ; Pukkala measured under completely overcast sky conditions et␣ al. ; Pukkala et␣ al. ). at exactly the same time both above and below the Many models use the Beer-Lambert law to tree canopy. This is achieved by linking a quantum describe radiation transmitted through forest sensor to a data recording device for continuous canopies (Nel and Wessman ). FORECAST and above-tree canopy measurements (often done in an other models use this approach (Kimmins et␣ al. adjacent clearing), and a hand-held quantum sensor ). This simple relationship incorrectly assumes for below-canopy measurements. In this way, more that the total leaf area within a stand is randomly than forest understorey microsites could be distributed. Several other factors such as leaf characterized over just a few hours. The percentage inclination angle distributions, site latitude, and of light transmitted could be calculated for any season also influence light penetration (Oker- particular location and then directly related to the bloom et␣ al. ). mean daily percentage of light transmitted to that Most geometrical radiation transfer models location under both clear and overcast neglect factors such as penumbra, clumping of sky␣ conditions.
70 70 70 (a) (b) Measure 3 60 60 Measure 2 Measure 1 50 50
40 40
30 30
20 20
10 10 Mean daily % PPFD on an overcast day Mean daily % PPFD on an overcast Instantaneous % PPFD on overcast days Instantaneous % PPFD on overcast
0 0 0 10 20 30 40 50 60 70 0 10 20 30 40 50 60 70 Mean daily % PPFD on a cloudless day Mean daily % PPFD on a cloudless day
70 (b) Measure 3 60 Measure 2 Measure 1 50
40
30
20
10 Instantaneous % PPFD on overcast days Instantaneous % PPFD on overcast
0 0 10 20 30 40 50 60 70 Mean daily % PPFD on an overcast day
Relationships between (a) the mean daily percentage of light transmission measured continuously under overcast and clear sky conditions, (b) the mean daily percentage of light transmission measured continuously under overcast sky conditions and three instantaneous measures done on overcast days, and (c) the mean daily percent light transmission measured continuously under clear sky conditions and three instantaneous measures done on overcast days in a balsam fir mixed forest in Abitibi, Québec (after Parent and Messier 1995a).
71 foliage, the content of woody biomass, and accept light as the main factor affecting the scattering of light (Pukkala et␣ al. ). Wang and development of understorey plants (Pacala et␣ al. Jarvis ₍₎ suggested that information on total ), then plant properties that affect light leaf area and its distribution within the crown of a interception and transformation should have conifer forest is more important than information significant acclimatization value. While previous on leaf inclination and crown shape. research on plant response to variations in forest Although progress has been made in developing light regimes focused on the photosynthetic accurate models, a recent model proposed by responses of leaves (e.g., Chazdon ), recent Pukkala et␣ al. ₍₎ was able to predict only % of studies also include a wide range of architectural the spatial variation in both direct and diffuse and morphological traits that determine the radiation under a structurally simple -year-old pattern of light interception by leaves (Canham Scots pine stand in Finland. Two major reasons ; Koyhama ; Wayne and Bazzaz ). were given to explain this low level of prediction: Givnish ₍₎ discussed the need to evaluate plant 1 Many of the parameters used to calibrate the responses to shade with a whole-plant perspective. model were simulated by other models. Walters et␣ al. ₍a, b₎ used such a perspective and 2 At each point under the stand the daily light found that patterns of carbon allocation and environment was described as the mean of only respiratory costs incurred from structural and four instantaneous measurements. maintenance functions of stem and root tissues are A large portion of the unexplained variation may critical factors in explaining the success of many have been due to errors in parameter estimation. tree species. Many tree species are capable of More work is required before a simple and reliable surviving under a closed tree canopy for many model can be produced that estimates the spatial years, but information about which functional heterogeneity of the forest understorey light traits confer such shade persistence is lacking. environment, especially in heterogeneous stands Furthermore, Givnish ₍₎ suggested that the support like those usually found in Canada. costs rapidly increase with tree size, so that the light requirements of seedlings should increase as they grow. Adaptation and Acclimation of Understorey Many factors account for the wide variety of Vegetation light requirements in plants, and many species are able to acclimate to a wide variety of light Leaf versus Plant Level Photosynthesis provides conditions. The main factors responsible for such acclimation are changes in photosynthetic capacity green plants with almost all of their chemical energy, and is central to their ability to compete (e.g., Björkman ; Beaudet ), branch orientation (Steingraeber et␣ al. ), and leaf and reproduce. Comparative studies of the photosynthetic response and leaf characteristics of versus stem allocation (Beaudet ; King ). plants grown under high and low levels of Such phenotypic plasticity enables many forest irradiance provide valuable insights into the trees to both tolerate shade and respond vigorously significance of several leaf-level traits that are seen to sudden openings in the canopy. Yet, there appear to be clear tradeoffs between shade- and in plants adapted to sunny or shady conditions (Björkman ). However, whole-plant growth sun-adapted species (Table ). Most studies and competitive ability depend not only on the investigating the light requirements of understorey trees have used a static approach (e.g., Clark et␣ al. photosynthetic rate of individual leaves, but also on the geometry and dynamics of a plant’s canopy ) in which growth and architecture are related and the pattern of energy allocation among all to the current micro-environmental conditions. organs (Kohyama ; Givnish ; King ; The light environment in the forest understorey is Küppers ). The ability of understorey plant very dynamic and changes continuously over time species to grow and develop in shade is the product as the stand develops. Therefore, a plant’s leaf and of complex combinations of whole-seedling crown morphology may not be optimal for the responses, including biomass allocation, light environment currently experienced, but could morphological changes, leaf orientation, and leaf- be optimal over the many different light level photosynthetic changes (Givnish ). If we environments experienced by a tree growing from seedling to mature tree. Because of this, trees have
72 Differences in traits between plants adapted or a␣ much lower crown morphological plasticity in acclimated to open versus closed canopy relation to light availability than other more shade- conditions (adapted from Givnish 1988) tolerant species such as balsam fir, Douglas-fir or western hemlock (Kohyama ; Carter and Traits Open Closed Klinka ; Parent and Messier a). Partial cutting could improve the understorey light environment and therefore the potential of under- Leaf level storey conifers to respond to a final harvest that Photosynthesis Photosynthetic rate removes the entire overstorey canopy. For many at saturation high low shade-tolerant conifer species, above-canopy light Light compensation point high low should increase to values greater than – % for Light saturation point high low height growth to exceed lateral growth (Kohyama ; Carter and Klinka ; Parent and Messier Leaf morphology a). Values greater than % are needed to Leaf thickness high low achieve optimal growth for several species (Table ). Leaf orientation erect horizontal Stomatal density high low Ecological Light Compensation Point Several Leaf mass per area high low studies indicate that shade tolerance is not related to high growth rates at low light levels (Beaudet Canopy level and Messier n.d.). Actually, the reverse appears to Leaf area index high low be true. A recent study shows tree seedling Twig orientation erect horizontal mortality is higher for tree species that have high growth rates in low light conditions (Kitajima Plant level ). Futhermore, growth rate appears to be Proportion allocated strongly related to leaf area ratio (i.e., the ratio of to leaf low high total leaf area of a seedling to its total biomass or Proportion allocated LAR). Both Beaudet ₍₎ and Kitajima ₍₎ have to root high low found growth rate to be positively related to LAR. Proportion allocated Figure 7 illustrates the relationships between to reproduction high low the␣ minimal percentage of light transmitted above different conifer and broadleaf species and the corresponding height growth for shade-tolerant, mid-tolerant, and intolerant species reported in evolved high acclimation potential and phenotypic the␣ literature. The trend is clear despite the small plasticity (Bazzaz ). number of measurements: very shade-tolerant In boreal forests, several studies indicate that species are found at lower light levels and have understorey conifer seedlings and saplings with lower height growth than the shade-intolerant well-developed crowns are best able to respond to species. This light level represents the ecological sudden openings of the overstorey canopy (Hatcher light compensation point. ; Herring and McMinn ). Parent and The leaf-level compensation point and plant- ₍ ₎ Messier a showed that many attributes of level ecological light compensation point must be balsam fir crown morphology are strongly differentiated. Jarvis ₍₎ conducted a series of correlated with understorey light conditions. experiments on the relationships between light and Several other studies present similar results for the growth of Quercus petraea. He found that the other conifer species (Kohyama ; Carter and light compensation point at the leaf level was % Klinka ; O’Connell and Kelty ). For of full sunlight, but that the effective light example, studies of white pine (O’Connell and compensation point at the plant level was %. Kelty ; Chengeou and Messier, in prep.) and The␣ difference results from calculating the cost of lodgepole pine (Williams and Messier, in prep.) producing a leaf and maintaining the above- and show that shade-intolerant conifer species have below-ground woody structure.
73 Minimum and optimum percentage of above-canopy light and growth measured in the field for tree saplings, shrubs, and herbs in the understorey of boreal and temperate forests. The values represent means. The percentage of above-canopy light was measured using different techniques, so the values are not completely comparable.
% of above-canopy light Growth (cm)
Species Min. Opt. Min. Opt. Reference
Trees Pseudotsuga menziesii 1.0 – 1.0 – Atzet and Waring 1970 P. menziesii (dry sites) 2.0 90 2.0 20 Carter and Klinka 1992 P. menziesii 3.0 – 4.0 – Williams and Messier n.d. Abies concolor 0.5 – 7.0 – Atzet and Waring 1970 Pinus ponderosa 8.0 – 10.0 – Atzet and Waring 1970 Quercus petraea 8.0 – – – Jarvis 1964 Quercus serrata 1.0 – – – Washitani and Tang 1991 Abies balsamea 2.5 50 0.5 35 Parent and Messier 1995a Abies amabilis 0.1 90 0.5 76 Klinka et␣ al. 1992 Abies lasiocarpa 1.0 90 0.1 45 Klinka et␣ al. 1992 Tsuga heterophylla (dry sites) 1.0 80 2.0 20 Carter and Klinka 1992 Thuja plicata (dry sites) 1.0 20 4.0 15 Carter and Klinka 1992 Thuja plicata (fresh sites) 2.0 100 2.0 55 Carter and Klinka 1992 Pinus contorta 4.0 – 4.0 – Williams and Messier n.d. Picea glauca 8.0 40 5.0 25 Lieffers and Stadt 1994 Betula pubescens 5.0 100 3.0 35 Messier and Puttonen 1995 Betula pendula 5.0 100 2.8 35 Messier and Puttonen 1995 Pinus strobus 5.0 100 5.0 40 O’Connell and Kelty 1994 Acer saccharum 1.0202.0 20 Beaudet and Messier n.d. Betula alleghaniensis 2.0405.0 40 Beaudet and Messier n.d. Fagus grandifolia 1.0202.0 20 Beaudet and Messier n.d.
Understorey shrubs Gaultheria shallon 1.0 40 – – Messier et␣ al. 1989 Rubus idaeusa 7.0 100 30.0 >130 Ricard and Messier 1996 Rubus pedatusb 1.5 100 2.0 160 Tappeiner and Alaback 1989
Understorey herbs Calamagrostis canadensisa 8.0 100 42.0 >101 Lieffers and Stadt 1994 Epilobium angustifoliuma 8.0 100 30.0 >125 Lieffers and Stadt 1994 Cornus canadensisb 1.5 100 3.0 130 Tappeiner and Alaback 1989
a The growth is the total height growth after one growing season. b The growth is the mean annual length growth of rhizome.
74 15 decline with size was found for similar low light levels (Figure 8). These results suggest that shade tolerance should decline fairly drastically with increasing size. Therefore it is possible that following a partial cutting, many understorey 10 tree␣ and shrub species will improve their growth significantly, but as their size increases and as the overstorey canopy closes slowly, they will reduce their growth or even be eliminated. This hypothesis is now being tested for understorey 5 Height growth (cm) aspen and balsam fir regeneration in south- western␣ Québec. One of the problems associated with partial cutting is the effect it has on some undesirable competitors. A successful partial cut system could 0 increase the amount of light above the ecological 0 5 10 15 % of above-canopy PPFD light compensation point for the crop species, while keeping it below that of the undesirable non- crop species. To achieve this goal, the light Relationships between the ecological light requirements of both crop and non-crop species requirements and height growth for different conifer and broadleaf shade tolerant (black must be known and the effects of stand mani- circles), mid-tolerant (stars), and intolerant pulations on understorey light. Calamagrostis (open squares) tree species of the boreal forest canadensis and Epilobium angustifolium (Lieffers (data are from Table 3). and Stadt ) and Rubus idaeus (Ricard and Messier ) are always absent under the forest Givnish ₍₎ calculated the effective (or canopy when light transmission is lower than %, ecological) light compensation point for Liriodendron increase in abundance and vigour when light tulipifera, a fairly shade-intolerant broadleaf transmission is between to %, and reach their species of eastern North America. It increases: greatest abundance in completely open sites. The exact basal area needed to obtain these light values •from %, if only the instantaneous leaf depends on several of the factors discussed above. respiration is␣ considered; These three species are considered fairly shade •to %, if night leaf respiration and the intolerant. More shade-tolerant shrubs often have construction cost of the leaf are added; lower light requirements. Messier et␣ al. ₍₎ •to %, if the construction cost of the above- estimated the minimum light requirement of and below-ground woody tissues for a -m tree shade-tolerant salal (Gaultheria shallon) at between is added; and % of above-canopy light. Under such low •to %, if the construction cost for a -m tree light levels, however, the growth is very limited, is added; and especially below-ground, and the detrimental •to %, if the construction cost for a -m tree effects to the overstorey canopy trees appear to be is added. minimal (Messier ). Messier and Mitchell These calculations suggest that the ecological light ₍₎ found that thinning, which increased light compensation increases with size. Messier and levels from . to .% of above-canopy light, Nikinmaa (in prep.) tested this idea in the field for increased the production of salal leaves and sugar maple saplings in Québec, and Williams and rhizomes. A good knowledge on the effects of Messier (in prep.) for Douglas-fir and lodgepole canopy openings on the understorey vegetation pine saplings in a conifer stand in British Columbia. growth and allocation strategy is therefore helpful In both cases, the absolute and relative growth to select the appropriate levels of canopy opening.
75 Lodgepole pine Douglas-fir 0.3 0.3 Large (between 301 and 600 cm) Large (between 301 and 600 cm) Medium (between 101 and 300 cm) Medium (between 101 and 300 cm) 0.2 Small (less than 100 cm) 0.2 Small (less than 100 cm)
0.1 0.1 Relative height growth Relative height growth
0 0 15 30 45 15 30 45 % PPFD % PPFD
Effects of three sapling sizes and three light environments on the relative height growth for Douglas-fir and lodgepole pine growing under a natural forest near Williams Lake.
Conclusion that the ecological light requirements increase with␣ plant size. This has important silvicultural As forest managers, we have the potential to implications, since, after partial cutting, the influence the spatial and temporal understorey understorey vegetation often increases in size light environment in our managed mixed conifer- quickly and the understorey light level broadleaf forests. A good knowledge of the effects decreases␣ slowly. of stand structure, species composition, latitude, and sky conditions on the quality, quantity, and References variability of the understorey light environment are essential to successfully implement partial cutting Alaback, P.B. and J.C. Tappeiner. . Response practices. In this paper, I have described a simple of western hemlock (Tsuga heterophylla) and classification of the understorey light habitats. This early huckleberry (Vaccinium ovalifolium) classification has six broad types that consider the seedlings to forest windthrow. Can. J. For. spatial and temporal variability in the understorey Res. :–. light environment. Such a classification could be Anderson, M.C. . Stand structure and light used for the mapping of the different light habitats penetration. II. A theoretical analysis. J. that have been created by cutting practices. Appl. Ecol. :–. Simple and efficient methods are available to .␣ . Interpreting the fraction of solar characterize the average understorey light radiation available in forests. Agric. environment. This information could increase our Meteorol. :–. understanding of the ecological light requirements Atzet, T. and R.H. Waring. . Selective filter- of our most common crop and non-crop species. ing of light by coniferous forests and mini- To design optimal cutting practices, we also mum light energy requirements for need to understand how understorey vegetation regeneration. Can. J. Bot. :–. responds to a sudden removal of parts of the Baldocchi, D. and S. Collineau. . The physi- overstorey canopy. Information on physiological cal nature of solar radiation in heterogenous and morphological differences between shade- canopies: Spatial and temporal attributes. In tolerant and intolerant species in open and closed Exploitation of environmental heterogeneity habitats is available for most species. These by plants: ecophysiological processes above- differences could be used to predict how and belowground. M.M. Caldwell and R.W. understorey vegetation responds to sudden full or Pearcy (editors). Academic Press. San Diego, partial canopy openings. Recent studies suggest Calif. pp. –.
76 Bazzaz, F.A. . The physiological ecology of Chengeou, M. and C. Messier. [n.d.] Effects of plant succession. Ann. Rev. Ecol. Syst. :– overstorey deciduous canopy on understory . white pine growth and crown morphology. Beaudet, M. . Effet de l’ouverture du couvert In preparation. forestier sur la croissance, la morphologie et Clark, D.B., D.A. Clark, and P.M. Rich. . la réponse photosynthétique de semie de Comparative analysis of microhabitat utiliza- bouleau jaune, d’érable à sucre et de hêtre à tion by saplings of nine tree species in grandes feuilles. M.Sc. thesis. Université du neotropical rain forest. Biotropica :–. Québec à Montréal. Montréal, Que. Delong, S.G. . The light interception index: a Beaudet, M. and C. Messier. [n.d.] Growth and potential tool for assisting in vegetation morphological plasticity in understory seed- management decisions. Can. J. For. Res. lings of three northern hardwood tree species :–. across a light gradient. Ann. Bot. Submitted. Endler, J.A. . The color of light in forests and Björkman, O. . Responses to different quan- its implications. Ecolog. Monogr. :–. tum flux densities. In Encyclopedia of plant Fairbairn, W.A. and S.A. Neustein. . Study of physiology. O.L. Lange, P.S. Nobel, C.B. response of certain coniferous species to light Osmond, and H. Ziegler (editors). Springer intensity. Forestry :–. Verlag. Berlin, Germany. New Ser., Vol. A. Frankland, B. and R.J. Letendre. . pp. –. Phytochrome and effects of shading on Canham, C.D. . An index for understory growth and woodland plants. Photochem. light levels in and around canopy gaps. Ecol- and Photobiol. :–. ogy :–. Gay, L.W., K.R. Knoerr, and M.O. Braaten. . Canham, C.D., J.S. Denslow, W.J. Platt, J.R. Solar radiation variability on the floor of a Runkle, T.A. Spies, and P.S. White. . pine plantation. Agric. Meteorol. :–. Light regimes beneath closed canopies and Givnish, T.J. . Adaptation to sun and shade: tree-fall gaps in temperate and tropical for- a whole-plant perspective. Aust. J. Plant ests. Can. J. For. Res. :–. Physiol. :–. Canham, C.D., A.C. Finzi, S.W. Pacala, and D.H. Grace, J.C., P.G. Jarvis, and J.M. Norman. . Burbank. . Causes and consequences of Modelling the interception of solar radiant resource heterogeneity in forests: interspecific energy in intensively managed stands. N.Z. J. variation in light transmission by canopy For. Sci. :–. trees. Can. J. For. Res. :–. Grime, J.P. and D.W. Jeffrey. . Seedling estab- Carter, R.E. and K. Klinka. . Variation in lishment in vertical gradients of sunlight. J. shade tolerance of Douglas fir, western hem- Ecol. :–. lock, and western red cedar in coastal British Hatcher, R.J. . Balsam fir advance growth af- Columbia. For. Ecol. Manage. :–. ter cutting in Québec. For. Chron. :–. Chazdon, R.L. . Sunflecks and their impor- Herring, L.J. and R.G. McMinn. . Natural tance to forest understory plants. Adv. Ecol. and advance regeneration of Engelmann Res. :–. spruce (Picea engelmannii) and subalpine fir .␣ . Photosynthetic plasticity of two (Abies lasiocarpa) compared years after site rain forest shrubs across natural gap treatment. For. Chron. ():–. transects. Oecologia :–. Hilbert, D. and C. Messier. [n.d.] Physical simu- Chazdon, R.L. and C.B. Field. . Photographic lation of trees used to study the effects of estimation of photosynthetically active radia- light environment, branch characteristics, tion: evaluation of a computerized technique. and branch spacing on light interception by Oecologia :–. sugar maple. (In preparation). Chazdon, R.L. and N. Fetcher. . Photosyn- Jarvis, P.G. . The adaptability to light inten- thetic light environments in a lowland tropi- sity of seedlings of Quercus petraea (Matt.) cal rain forest in Costa Rica. J. Ecol. Liebl. J. Appl. Ecol. :–. :–.
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81 Ecology and Dynamics of Mixed Red Alder–Conifer Stands
KLAUS J. PUETTMANN AND DAVID E. HIBBS
Abstract California (Figure ). It is usually found in areas below m , but in the southern limit of its range Interest in mixed red alder–conifer stands is it has been found at an elevation of m increasing in the Pacific Northwest. This paper (Harrington ). Even though isolated reviews the current knowledge about the ecology populations exist east of the Cascades and Sierra and dynamics of such mixtures, including the Nevada, the most common distribution of red silvics and distribution of red alder and associated alder is within km of the Pacific coast conifer species. Nitrogen fixation can be a major (Harrington et␣ al. ). objective for growing red alder in mixed species Red alder is found as a monoculture along river stands since it improves site productivity and bottoms or on the lower parts of the slopes and is sustainability on low-nitrogen sites. Also, mixing the major component of several other forest cover red alder into conifer stands seems to improve types (Eyre ). Usual associates are Douglas-fir conditions for a variety of mammal and bird (Pseudotsuga menziesii [Mirb.] Franco), Sitka species. Because of its N-fixing ability, red alder spruce (Picea sitchensis [Bong.] Carr), western can actually improve the growth of associated hemlock (Tsuga heterophylla [Raf.] Sarg.), grand fir conifers on low-nitrogen sites, but in other cases it (Abies grandis [Dougl. ex Don] Lindl.), and may lead to reduced growth compared with pure western redcedar (Thuja plicata Don. ex D. Don). conifer stands. Chambers’ () empirical yield The distributions of these species overlap tables for red alder predict red alder yields significantly and with red alder’s distribution adequately in mixed stands with a red alder (Burns and Honkala ). Other associated proportion of % or greater. Factors that need to species include a variety of hardwoods such as be considered when choosing an associated species black cottonwood (Populus trichocarpa Torr. and include soil nitrogen levels and the flood and Gray), bigleaf maple (Acer macrophyllum Pursh), drought potential of the site. Stand density, as well and willow (Salix spp.). as mixture proportions, are discussed. Spatial Red alder is a pioneer species and invades aspects of mixtures include single tree, row, and recently disturbed sites where mineral soil is group mixtures. Temporal considerations such as exposed. Because of its relatively short life span underplanting in red alder stands, simultaneous compared with the long-lived conifers, its presence plantings of both species, and delayed plantings of and dominance are directly linked to disturbance red alder are presented. Finally, technical aspects systems and patterns. such as specific needs for site preparation, Historically, the distribution of red alder was harvesting machinery, and separation of species at limited to lower parts of valleys and it was mainly the landing conclude the paper. dependent on erosion on stream banks (Heusser ). Only after fire or other disturbances does it Silvics and Distribution of Red Alder invade uphill areas. Harvest of conifer stands has led to an expansion of red alder forests (Davis Red alder (Alnus rubra Bong.) is a common species ). Considering this fairly recent expansion is in lowlands of western Oregon, western important, since it indicates that red alder is Washington, and western British Columbia. It growing on sites that are suitable to support and, generally ranges from southwest Alaska to northern until recently, have supported significant conifer
82 ° 65 ° ° ° 160 ° 150 140 130 Nitrogen Fixation Red alder is a unique tree ° 170 species in the Pacific Northwest because of its 60 ° association with Frankia, a nitrogen-fixing actinomycete. This association allows red alder to grow and thrive on sites with low nitrogen supply. It also creates special opportunities and problems in management of mixed red alder–conifer stands. Red alder can benefit associated conifers by providing nitrogen, but at the same time it 55 ° competes with these conifers for light, water, and other resources. While very little research has explored the specific resources and conditions that affect nitrogen fixation (Binkley et␣ al. ), it seems that 50 ° all the conditions that favour red alder growth also increase nodule biomass and thus nitrogen fixation (Dixon and Wheeler ). This relationship seems to hold on a stand- (e.g., temperature, soil, water levels) and individual-tree level. The nodule biomass 45 ° of individual trees seems to be related to growth and therefore to its competitive position in the canopy (Heilman and Stettler ). While clear trends between nitrogen fixation have not been 40 ° found, it seems to be reduced at high nitrogen levels (Lipp ). This has a direct implication in selecting sites where red alder can be used to in- crease fertility (nitrogen level) for conifer growth. 35 ° Alder will contribute little to stand productivity on␣ sites that have an abundant supply of available␣ nitrogen. In monocultures, red alder nitrogen fixation has 30 ° been measured in a variety of conditions (stand ages and site fertilities). Fixation rates are usually between and kg · ha- ·yr- (Binkley et␣ al. 0 100 200 300 400 miles ). Nitrogen fixation in mixed species stands 0 200 400 600 kilometres has been investigated mainly for those with Douglas-fir. The “classic” study is the Wind River The distribution of red alder (from Harrington mixed alder–Douglas-fir plantation (Tarrant ; 1990). Tarrant and Miller ; Miller and Murray ) in which red alder produced an estimated forests. Successional trends on these sites also show ␣kg·ha- ·yr- of fixed nitrogen. Other long-term this fact. In most mixed red alder–conifer stands, studies in the Cascade Head Experimental Forest red alder will be outlived and replaced by the coni- suggest a fixation rate of – kg · ha- ·yr- of fers (Burns and Honkala ). In some situations, nitrogen for the first five decades (Binkley et␣ al. when conifers are absent, red alder stands can be ). The fixation rate at age was lower than replaced by shrub communities with dense cover of in␣ earlier decades, suggesting that fixation may salmonberry and other species (Newton et␣ al. ; decline because of accumulation of nitrogen in the Carlton ; Tappeiner et␣ al. ). soil, changes in soil properties, or declining vigour
83 of alder at this age (Binkley and Greene ; the associated species can germinate and colonize Puettmann et␣ al. ; Binkley et␣ al. ). On a site recently disturbed sites, they invade after a that was not nitrogen deficient, a young mixed disturbance that exposes mineral soil (Burns and plantation was no different in soil nitrogen level Honkala ). Thus, stands with significant red than a monoculture Douglas-fir plot after years alder components are usually even-aged (Ruth and (Cole and Newton ). The only fixation Harms ; Carlton ). Because of the measurement with another of the associated disturbance frequency and the short life span of conifers was done in Scotland where increased soil red alder, these stands do not develop into uneven- nitrogen levels in a mixed alder–Sitka spruce stand aged stands until other species have replaced red indicated a nitrogen fixation rate of kg · ha- ·yr- alder. While red alder establishes in Oregon in a (Malcolm et␣ al. ). very short time span (Carlton ), studies on red While red alder was the same age as the alder–conifer stands in Washington indicate that associated conifer in all of the studies described this establishment period can last up to years above, Berg and Doerksen ₍₎ investigated a (Stubblefield and Oliver ). However, in a single stratified mixed stand. Red alder had invaded the plot of red alder, ages were within years, understorey of a Douglas-fir stand that had been indicating that secondary disturbances might be heavily thinned years earlier. Over the stand’s responsible for longer establishment periods. In life span, nitrogen levels indicate fixation levels stands considered mixed species by Stubblefield averaged and kg · ha- ·yr- in areas with a␣ low and Oliver ₍₎, red alder had invaded after and high red alder component, respectively. western hemlock and western redcedar became Because the red alder had slowed in growth and established. In stands where western hemlock was showed increased mortality at the time of – years older, the conifers outgrew red alder in measurement, rates were probably higher during height. In mixed red alder–Douglas-fir stands, earlier phases when red alder was growing Douglas-fir had at least a -year head start before more␣ vigorously. red alder got established. The time of establishment is critical because of Associated Conifer Species The main conifer the difference in growth pattern between red alder species associated with red alder in the coastal and associated conifer species. Under even-aged fogbelt are western hemlock, Sitka spruce, grand conditions, red alder’s fast initial height growth fir, western redcedar, and, in the southwest, allows it to overtop all of its associated conifers on Oregon Port Orford cedar. Farther inland, most sites (Harrington and Curtis ) (Figure ). Douglas-fir is one main species growing in mixture During this period, conifers can coexist in red with red alder. The characteristics that influence alder stands either in the understorey or in gaps their presence in mixed stands are based on their where red alder is missing or is at an extremely low silvics. These species have a medium (Douglas-fir), density. The shade tolerance of the conifers allows high (Sitka spruce, grand fir), or very high them to grow slowly, but to survive in the (western hemlock, western redcedar) shade understorey for several years (Miller and Murray tolerance. In addition, all these species have a ; Stubblefield and Oliver ; Murray and slower initial growth but are longer lived than red Miller ; Shainsky and Radosevich ). alder (Burns and Honkala ). Most of these Eventually, dense red alder thickets result in species either prefer or tolerate mineral soil as mortality of some or all conifers, leading to a pure seedbed. Western hemlock is an exception; it also red alder stand (Newton and Cole ). However, germinates and grows on stumps or rotten logs. if the density of red alder is variable, the conifers Western redcedar frequently regenerates through can probably survive through the first two decades layering (Minore ). Some species are more in the understorey. For instance, western hemlock adapted to summer drought (e.g., Douglas-fir) seedlings that germinate on stumps—a seedbed and␣ less adapted to extended flooding periods. that is unsuitable for red alder—have a height advantage. Other successful examples of mixed- Stand Structure and Dynamics species stands are the result of a selective reduction The above-mentioned silvics influence spatial and of red alder height growth because of frost (Tarrant temporal aspects of mixed species stands. Since all ; Miller and Murray ) or preferential
84 40 Douglas-fir potential to continue growing vigorously after red 35 alder drops out of the stand (Puettmann et␣ al. ; 30 Puettmann, unpubl. data). This two-stage development of alder-conifer stands is similar to 25 Red alder the management scenario suggested by Hibbs ₍₎ 20 for red alder monocultures. During the initial 15 decades, the understorey conifers experience high density, resulting in small branches that self-prune 10 Dominant height (m) effectively. After the conifers expand their crowns 5 above the hardwood canopy, their crown and 0 branch size increases (Stubblefield and Oliver 10 20 30 40 50 60 70 80 ), leading to associated gains in vigour and Total age (years) growth, but also to a reduction in wood quality.
Ecosystem Functions Height-growth curves for red alder (SI20 =15m) and Douglas-fir (SI50 = 30 m). Growth curves are based on equations from Harrington and Red alder is valued because of its positive effect on Curtis (1986) and King (1966). ecosystem productivity and long-term sustainability (Bormann et␣ al. ). This status is browsing (Newton and Cole ). In areas in due to its nitrogen-fixing capacity and its effect on which red alder eliminates the conifer component, various soil characteristics. the understorey may become dominated by Soil fertility is one factor used to assess the salmonberry, further reducing the chance for a feasibility of mixed red alder–conifer stands conifer invasion (Tappeiner et␣ al. ). (Atkinson et␣ al. ; Tarrant et␣ al. ). When understorey Douglas-fir is present at very Nitrification can lead to cation leaching and high densities (up to /ha), it may reduce the lowering of pH in red alder stands (Van Miegroet growth of overstorey alder, possibly through and Cole ; Cole et␣ al. ). However, this competition for water (Shainsky and Radosevich effect seems to be expressed more strongly after ). If the understorey conifers survive, they can multiple rotations of red alder (Bormann et␣ al. act as “training trees,” shading lower parts of the ) and therefore should not be that prevalent red alder crowns and causing natural pruning, in␣ mixtures, especially if red alder is not growing which alters the canopy structure (Stubblefield and in␣ the same location in mixed stands in Oliver ). subsequent␣ rotations. After two decades, red alder height growth and Red alder will increase the annual litterfall in crown expansion slows dramatically (Harrington mixed stands (Waring and Schlesinger ). and Curtis ). The slower crown expansion and Because of the increased amount and the higher the increased time required to close canopy gaps nutrient concentration in red alder litter may allow conifers to maintain or improve their (Zavitkowski and Newton ; Gessel and Turner vigour and take advantage of a better height ; Cole et␣ al. ), red alder will accelerate growth potential during these decades (Stubblefield nutrient cycling in mixed stands (Bormann et␣ al. and Oliver ; Cole and Newton ). The ). Red alder litter, especially woody debris, associated conifers, especially Douglas-fir, can then initially decomposes faster than associated conifers grow above the red alder canopy and rapidly (Neal et␣ al. ). The decomposition rates of other accelerate their crown expansion and growth, litter mixed with red alder litter are also improved leading to a two-storeyed stand. During this (Bormann and Sidle ). However, after the first transition phase, the appearance of a forest can year, litter decomposition rates slow down to rates change significantly from an alder-dominated similar to conifer litter (Edmonds ). Thus, red forest with a few weak conifers growing in the alder mixed with conifers can accelerate and understorey to a forest with red alder in the lower improve the nutrient distribution within the soil canopy layers and substantial amounts of conifer profile and may also increase nutrient availability volume (Williamson ). The conifers have the for the plants.
85 At the same time, red alder increases organic losses in managed stands, especially if red alder is matter content in the soil in mixed-species stands at a low density (Newton and Cole ). (Bormann et␣ al. ). This increase improves soil Mixed deciduous (mainly red alder) and aggregation and decreases soil bulk density coniferous forests are predicted to support more (Wild ), and also improves the soil water- wildlife species than either pure conifer or pure red holding capacity (Crocker and Dickson ). In alder stands (McComb ). Most studies that addition, organic matter content is positively compare the abundance of wildlife in red alder or related to the cation exchange capacity of soils mixed red alder–conifer stands with pure conifer (Bormann et␣ al. ). stands cannot separate the effects of location from Mixtures of red alder also affect the microbial those of species composition. Frequently, red alder and other animal life in the soil. For example, red is found in riparian zones and is compared with alder leaves are a preferred food source for upland, conifer-dominated forests. The con- earthworms (Graham and Wood ), which help founding of location, dominant tree species, and incorporate the organic matter into the soil. The age may explain why McGarigal and McComb increased amount of high-quality litter should lead ₍₎ found more breeding bird species and small to increased populations of detritivores that in turn mammals in a -year-old upland conifer forest provide a better food source for other insects (–% red alder) than in younger, mixed-species and␣ birds. riparian forests (–% red alder). All these factors have been directly related to eco- While direct research into the effects of red alder system productivity (Powers ) and lead to the in monoculture and as a component of mixed conclusion that mixing red alder into conifer stands species forests is very limited, it seems to be a improves ecosystem productivity and sustainability. critical component of many ecosystems. The presence and abundance of a variety of species Wildlife Aspects show the benefits of alder-conifer mixtures for some mammals and birds. Red alder is one of the few deciduous trees in a conifer-dominated region (Tesch ). Its effect Aesthetic Aspects on wildlife can be separated into effects directly related to the presence of red alder and effects due Hardwood mixtures bring variety to a conifer- to adding diversity into a “homogenous” landscape. dominated landscape. Visual quality indices, the Bruce et␣ al. ₍₎ measured direct effects by Scenic Beauty Index (Schroeder and Daniels ) estimating the number of species that use red alder for example, include the number of species in a for food ( species) or reproduction ( species). stand as an important measure of its aesthetic value. Current research shows that this is a conservative Thus, mixtures of red alder with other species estimate (McComb ). Other species found in improve the visual aspects in the coastal conifer higher numbers in red alder than conifer forests belt, although red alder does not turn a bright include frogs, salamanders, shrews, and newts colour in fall. (Aubry and Hall ; Corn and Bury ). Red alder cover is correlated to the frequency of several Management Considerations flycatcher species (Gilbert and Allwine ), and it is an important food source for deer and various Because of the various benefits of mixed red alder– bird species (Leslie et␣ al. ). Beavers use red conifer stands, the objectives of mixed-species alder both as food and as material to build dams management must be identified before making any (Brunner ). Deer and elk use red alder as recommendations and decisions. Reasons for the browse (Newton and Cole ), but because of its interest in mixed red alder–conifer stands include fast initial growth the leaves are within browsing the use of red alder to increase biodiversity or range for only ‒ years. Young alder saplings can improve the growing conditions of conifers (e.g., also be damaged by antler rubbing. While natural replace artificial fertilization with nitrogen fixation red alder stands in these stages are usually dense on low-nitrogen sites), or the value of both species enough that damage is negligible, it can lead to as crop trees.
86 Growth and Yield Currently, most red alder yield determine the red alder component in mixed estimates in monocultures are based on the Normal red alder–Douglas-fir stands. Yield Tables for Red Alder (Worthington et␣ al. ), The Stand Projection System cannot be the Empirical Yield Table for Predominantly Alder recommended for red alder stands because of Stands in Western Washington (Chambers ), inflated mortality estimates. In mixed stands, the and the Stand Projection System ₍SPS₎ (Arney ). diameter growth is also overestimated. All three prediction tools are derived from data All three tools estimated trees per acre and collected in unmanaged, monoculture stands. mortality with less accuracy than diameter To investigate the use of these tools in mixed red estimates. Because spacing and thinning activities alder–Douglas-fir stands, Puettmann and Hibbs (rather than natural mortality) will determine ₍₎ compared the yield table and growth model stand density in managed stands, growth and yield predictions with growth records from actual estimation for managed stands can be simplified stands. The stand growth data were obtained from and improved by assuming that controlled patterns research and inventory plots in natural stands in of mortality exist. However, further research is southwestern British Columbia, western necessary to help understand the dynamics and Washington, and northwestern Oregon. This data predict the performance of mixed red alder–conifer set included measurements in mixed red alder– stands. Further research is also needed to Douglas-fir stands. Only directly measured investigate the conifer yield in mixed stands. variables such as a quadratic mean diameter (Dq), While yield in mixed species stands is usually trees per acre, and basal area were compared to intermediate between the yields of monoculture avoid the confounding effects of volume equations. stands of each of the associated species, red alder To test the application of these tools to mixed can actually increase the yield in mixed species species stands, different versions and adjustments stands on low-nitrogen sites. In the Wind River to the basal area proportions were used. Despite study, Douglas-fir height growth was improved by these corrections, the Normal Yield Tables for Red % and its yield in mixture was approximately Alder (Worthington et␣ al. ) did not provide % higher than in monoculture (Miller and accurate predictions for mixed species stands. To Murray ). In addition, because red alder was of use the Empirical Yield Table for Predominantly sawtimber size, the volume of the mixed stands was Alder Stands in Western Washington (Chambers double that of pure stands. While this is probably ), the percent normal basal area was calculated one of the best possible scenarios, size-density only from the red alder proportion of mixed relationships (Puettmann et␣ al. ) indicate that stands. While the average predicted values were the yield in mixed species stands might be lower very close, at lower red alder proportions (less than than in Douglas-fir monocultures (Newton and %), diameters and trees per acre of red alder Cole ). Another simulation study using were overestimated. A stand diameter table for FORCYTE- suggests that the maximum Douglas-fir each species was used for growth projections with yield was % higher in mixture with red alder the Stand Projection System. Even in short-term than in monoculture on a low-quality site predictions (average years), mortality was greatly (Comeau and Sachs ). Studies to investigate underestimated. In combination with the these aspects have been established by several state, overestimation of diameter growth, this also federal, and private landowners throughout resulted in severe overestimation of red alder basal western Oregon, Washington, and British Columbia area in mixed stands. This problem increased with as part of the Hardwood Silviculture Research the length of projection. Cooperative program at Oregon State University. The normal yield table is therefore not Other studies have been established by Newton and sufficiently accurate for yield estimates of red alder co-workers (Oregon State University), and by in mixed red alder–Douglas-fir stands, especially in Comeau (British Columbia Ministry of Forests). stands below or more than years of age. When measuring basal area the empirical yield Species Choice It is primarily site characteristics table can be used. With a red alder proportion and potential markets that determine the selection of␣ % or greater, its use is recommended to of species. The species that currently occur on the
87 site of interest and similar sites nearby are an specific requirement of the species under con- obvious first choice. This suggestion is especially sideration. Because extremely low site qualities are useful for species mixed with red alder because red usually associated with more than one growth- alder currently occupies sites that conifers occupied limiting factor, lower-quality sites are generally not until relatively recently. The general location as suitable for mixed species management (Hibbs (coastal fogbelt and Coast Range) will provide and DeBell ). the␣ first selection. However, specific site factors, Alternatively, a mismatch of site conditions and like nitrogen content, and potential for drought species requirements can probably be used to offset or␣ flooding should also be considered or enhance differential growth patterns (Murray (Ahrens et␣ al. ). and Miller ). For example, the mixed stand at Ideally, a mixed stand should contain tree Wind River (Miller and Murray ) may have species with compatible height growth patterns. been successful because the red alder was from an However, any of the associated conifers will have a off-site, coastal seed source. slower initial growth than red alder. The shade Density Considerations Hibbs and DeBell ₍₎ tolerance of the conifers partially offsets the slower discussed several scenarios for red alder–conifer growth (i.e., by allowing them to survive under a mixtures that assumed equal spacing throughout red alder canopy). Since the degree of shade the stand. They suggested that density should be tolerance indicates the need for release, Douglas-fir determined by the suggested monoculture density needs more space (i.e., it requires more attention for the more light-demanding species, in this case for release and thinnings) than Sitka spruce, grand red alder. Examples of a red alder monoculture fir, and western hemlock and redcedar. density diagram and examples of density scenarios can also be found in Puettmann et␣ al. ₍₎. These Site Selection Mixed species require more intensive scenarios are easiest to implement when both management, and the return on investment is species are planted. In a simulation study, Comeau generally larger on higher-quality sites. The site and Sachs ₍₎ suggested that the optimal density requirements for red alder and the associated for red alder in mixture with Douglas-fir per conifers overlap significantly (Harrington ). A hectare should be – trees per hectare on good red alder site is frequently also a good conifer low-quality sites and less than trees per hectare site (Harrington and Curtis ; Puettmann et␣ al. on higher-quality sites. ). However, a more detailed look at the site Alternatively, the density throughout a stand can requirements indicates that nitrogen availability, vary depending on the species proportions at the water availability (drought and flooding), and specific locality. Except in the individual tree microclimate factors may affect the species mixture at a / proportion, varying the density differently (Newton et␣ al. ; Harrington ). would allow the spacing requirements for all species to be accommodated by ensuring that the The ability to fix nitrogen allows red alder to light-demanding red alder have more room while grow vigorously on sites that, because of the keeping the shade-tolerant conifers at a higher limited nitrogen supply, have a low site index for density. This could be very useful in clumped conifers. Alder’s nitrogen-fixing ability can make it mixtures, where intraspecific competition is insensitive to soil nitrogen levels (Harrington dominant within clumps. The size-density surface ). However, when grown in various nitrogen developed by Puettmann et␣ al. ₍₎ allows solutions, red alder grows slowly under low calculation of relative densities for mixed red nitrogen levels (Lipp ), possibly because of the alder–conifer stands based on average diameter, energy expended for nitrogen fixation. On the basal area proportion, and total stand density. other hand, some conifers (e.g., Douglas-fir) are However, relative density can only be evaluated not as affected by drought as red alder, but are when the change in proportion can be predicted. more affected by flooding (Harrington ; Depending on the trajectory, a . relative density Hermann and Lavender ). To determine the has a different interpretation and leads to different site quality for both species, the specific factors management suggestions (Figure ). Further that limit the growth potential of each species on research is needed to determine how other factors the site must be determined and linked to the
88 such as spatial arrangement effect these density maintained in the stand for wildlife or timber issues (Puettmann, in prep.). interests, the proportion required may be different and may result in greater reduction in conifer yield. Mixture Proportions The management objective The amount of nitrogen fixed by red alder is not will determine the optimal proportion of species in a direct function of the number of trees, but is mixtures. This objective will not necessarily be more related to total red alder leaf area (Binkley constant for the life of a stand. If nitrogen fixation et␣ al. ). A few vigorous red alder will fix drives the interest in mixtures, the red alder pro- proportionally more nitrogen than many red alder portion should be adequate to provide sufficient in high-density, competitive environments. Thus, nitrogen, while keeping competitive effects on the the optimal proportion will vary directly with the conifer to a minimum. Alternatively, if red alder is amount of nitrogen fixation needed to provide adequate growing conditions for the conifers. A minimum number suggested by Miller and Murray ₍₎ and Comeau and Sachs ₍₎ is – trees per hectare. With a total density of or trees per hectare, respectively, this would mean a (a) –% proportion of red alder. If the timber value of red alder is assumed to be 2 High 3 close to that of Douglas-fir, or if the other benefits that red alder provide are more important, this proportion would increase. The tradeoffs between 1 volume or other benefits and mixture programs are likely to be nonlinear. Mixtures with more than % red alder probably behave similarly to pure red alder stands. When red alder comprises less than % of the stand there is a curvilinear Proportion (%) 100 Species x 0 increase toward Douglas-fir levels (Figure ). The 0 Species y 100 main benefits of mixing red alder with conifer Low High stands may accrue by increasing alder at very low densities. However, these benefits soon reach a plateau beyond which further increases in red alder High make few additional contributions. 2 3 (b) Spatial Considerations Spatial patterns are of special concern in red alder–conifer mixtures 1 Quadratic mean diameter (log scale) because the positive interaction (e.g., nitrogen fixation) as well as the negative interactions (e.g., competition) are spatially explicit processes. The range of spatial options includes single tree mix- tures, row mixtures, and mixtures of monoculture clumps of various sizes (Hibbs and DeBell ). The optimal spatial mixture changes with management objectives and over time. Inter- Low High mediate treatments, such as release or thinning, Number of trees (log scale) might be necessary to adjust for the dynamics of mixtures; that is, the optimal spatial setup is also (a) Conceptual self-thinning surface for a two- dependent on the ability to enter the stand. species mixture. Plotted on the surface are three A single tree mixture provides the maximum different scenarios for developing stands. (b) amount of interaction between the species (and Self-thinning lines (from Figure 3a) plotted on a minimum within species). This means the two-dimensional self-thinning graph (from competition between the species will be most Puettmann et␣ al. 1992).
89 of “buffer trees” is not detrimental if the “final crop trees” are growing vigorously. Another benefit is that microsite differences are of a scale that will allow plantings of groups of trees. For example, a depression in a Douglas-fir plantation that has a temporarily high water table could be planted into␣ red alder. When supplementing natural 102 regeneration, spots that did not seed in by one 50 species could be seeded or planted with another species. As well, brushing, clearing, and spraying activities can be organized to leave clumps of 10 0 100 broadleaves. By letting the microsite or natural 5 A. rubra agents define the clumpiness, the patch structure should reflect the growing conditions on the site. 100 Proportion P. menziesii (%) Clumped mixtures need to be laid out to allow 1 0 2 access to the various clumps without damaging Quadratic mean diameter using log scale (cm) 10 5 x 102 Number of trees per hectare103 (log scale) intermediate areas. The patch structure should 3 5 x 10 104 allow for a layout of skid trails that reduce damage to the residual stand. Ongoing research is investigating how far a neighbouring Douglas-fir patch influences growth Size-density surface for mixed red alder–Douglas- of a red alder patch and vice versa (Puettmann, in fir stands (from Puettmann et␣ al. 1992). prep.). This project also tries to quantify how these relationships change over time. The results of this intense. The benefit from nitrogen fixation, project might provide better recommendations for however, is dispersed throughout the stand. Miller a spatial layout of mixed species stands. and Murray ₍₎ estimated the zone of influence to be up to m. They suggested that red alder be Temporal Considerations Depending on spaced regularly at trees per hectare, with management objectives, several timing options are intermediate spots occupied by conifers. However, available to establish and manage mixed species because of the intensity of contact between the stands. These options are based on the principle species, competitive relationships seem to become that time of establishment can offset or enhance very important in a single tree mixture. To avoid differential growth patterns and significantly affect unnecessary growth losses and mortality in the the success of a species (Mack and Harper ). conifers, these mixtures must be monitored very We will discuss three scenarios that assume that closely and may need to be entered frequently to two or more species must grow together for part of maintain a desired mixture proportion. the rotation to qualify as mixed-species stands. Row mixtures take less planning and supervision Scenario 1: Underplanting Underplanting a conifer and still ensure that each tree has trees of a with an established red alder stand simulates (and different species as a neighbour. Row mixtures will accelerates) succession toward conifers. also need to be monitored. However, they can be Underplanting or seeding ensures that conifers are regulated or altered by row thinning that allows available to succeed the red alder when it matures more efficient use of machinery in future thinning and dies or is removed during a harvest operation. operations. Management considerations include the need for The third alternative is a clumped mixture. If site preparation and weed control. The need to one species is more competitive, such as red alder control brush competition will increase after the during the first few decades, this option allows stands go through the stem exclusion phase and conifer neighbours to buffer the competitive effects openings are filled with salmonberry (Rubus (Hibbs ). With this approach, the manager can spectabilis Pursh) and other shrubs (Tappeiner take advantage of the higher value of final crop et␣ al. ). When underplanted during the earlier trees compared with thinning. Thus, a growth loss phases, weed control may not be necessary. Species
90 vary in their ability to survive under a canopy. The competition, and red alder numbers should be on greater the shade tolerance, the better the survival the lower end of the spectrum compared to a and growth of the seedlings. For example, hemlock mixture with more shade-tolerant species (e.g., survived better than Douglas-fir when under- western hemlock or western redcedar). The red planted with a -year-old red alder stand alder can be removed at around age . By this (Emmingham et␣ al. ). time, the contribution through nitrogen fixation is Underplanting conifers implies a conversion reduced (Zavitkowski and Newton ; Binkley during the next rotation, and plans for harvest and et␣ al. ). Red alder can provide income for the removal need to be developed. Even shade-tolerant landowner because its removal can be combined species can survive under a canopy for only a with commercial thinning of the conifer limited time. Then, either thinning or stand component (Miller and Murray ). If the removal must open the canopy to give them room remaining red alder is left beyond age , it will be to grow. During this harvest process, damage to overtopped and die. This provides snags and conifer regeneration must be avoided. Harvest woody debris, and creates gaps that will increase should be during the winter. Species with a thin the benefit to wildlife. bark (e.g., western hemlock and western redcedar) Scenario 3: Delayed Establishment The third option need to be protected from logging damage. is to delay the establishment of red alder until – Scenario 2: Regeneration The second option is to ␣ years after the conifer establishment. This is based regenerate a site by establishing both red alder and on the assumption that conifers do not occupy the the associated conifers at the same time. The fast site fully during the establishment phase and after initial height growth of red alder will result in thinning operations later in the␣ rotation. conifer seedlings being overtopped. With density Delaying establishment of red alder was first ranges of up to trees per hectare, the Douglas- suggested by Newton et␣ al. ₍₎. Ongoing studies fir and other conifers seem able to survive for – indicate that on coastal Oregon sites a -year delay years (Newton et␣ al. ; Murray and Miller ). does not allow red alder to catch up with Douglas- If red alder seeded-in or was planted at densities fir height growth (Hibbs, unpubl. data). On these within the prescribed range, both species will high-quality sites, a – year delay should allow coexist for up to a decade. When the main both species to survive. For lower-quality sites in objective of the mixture is improving the site’s southwestern and northwestern Washington, mixed nitrogen status, managers can either reduce or stands survive a delay of – years with Douglas- eliminate the red alder before it reduces the vigour fir (Tarrant and Miller ), – years with of the conifer species (Newton et␣ al. ; Murray western hemlock, and – years with western and Miller ). To help the conifers to adapt to redcedar (Stubblefield and Oliver ). These the change in the microclimate, red alder should delays vary with site factors (e.g., soil moisture, not be cut and dropped to the ground, but nitrogen availability) (Figure ). For example, in a chemically thinned or girdled. This allows the simulation study Comeau and Sachs ₍₎ found standing dead stems to provide some shade. only a slight increase in Douglas-fir (.%) and When the objective is to produce harvestable red total (%) biomass yield when red alder planting alder, the density of red alder should be reduced was delayed five years after the Douglas-fir on a during a thinning operation. One approach low-quality site. This scenario requires additional includes a pre-commercial thinning at age that weed control and a thinning that ensures survival reduces the red alder to trees per hectare of all desired species. If the setup is successful, the (Miller and Murray ). The thinning operation delayed establishment allows both species to mainly focuses on red alder and should release the survive until they have a reasonable size (Miller conifers in heavy red alder thickets, ensuring their and Murray ; Stubblefield and Oliver ) and survival and growth. A second thinning at age permits simultaneous harvesting. reduces the red alder to between to trees per Intensive thinning operations free up resources hectare. While these numbers are based on limited that understorey vegetation can use (Smith ). research, the more shade-intolerant conifers (e.g., Underplanting red alder after a thinning operation Douglas-fir) will be more sensitive to the allows red alder to take advantage of the available
91 germinate on rotten stumps or logs. This is 10 Cascades 10 Coast A B especially important when both species are to be 8 Wet 8 regenerated at the same time. If the natural Wet conditions are inadequate, a site preparation 6 6 method that provides good germination conditions
Years Years for more than one species or a two-stage site 4 4 Nonwet preparation must be employed. One species may 2 Nonwet 2 find suitable conditions (e.g., red alder will seed-in naturally), while the second species (e.g., Sitka 0 0 spruce or western hemlock) needs to be planted or 0 10 20 30 0 10 20 30 seeded. In these cases, site preparation can focus Slope (%) Slope (%) on one species, but must allow the already established species to survive and grow through the Number of years to delay red alder establishment operation. Frequently, preparing planting or seeding as affected by site factors (from Newton 1968). spots may be preferable to broadcast treatments. During any intermediate treatment such as pre- resources and grow until the overstorey closes. The commercial thinning, the remaining trees must be objective of this option is to add nitrogen to the protected. The degree of caution required depends site and does not include any harvesting of mer- on the species sensitivity. For example, western chantable red alder. As with other nitrogen-fixing hemlock is very susceptible to bark damage and plants, the conditions needed by red alder to grow associated rot (Packee ), while older Douglas- and fix significant amounts of nitrogen may fir is less sensitive (Hermann and Lavender ). necessitate a heavy thinning operation (e.g., This will affect the spatial layout of mixtures. Skid thinning to Douglas-fir per hectare at age ) trails that take advantage of the more damage- (Berg and Doerksen ). This is a heavier resistant species must be designed. Also, the thinning than practised under traditional conifer harvesting equipment and method must consider management. However, new options aimed at these species sensitivity issues (e.g., short-wood recreating old-growth structures recommend early harvesting may reduce damage for the intensive thinning in conifer stands (Newton and residual␣ trees). Cole ), which may permit red alder to develop In addition, the density and timing of in an understorey. establishment may not account for the different growing patterns, and the optimal time for thin- Technical Aspects ning and final harvest could vary with species. This would require more frequent entries and greater Most management techniques, tools, and machinery cost. Alternatively, the differential growth patterns have been developed for operation in even-aged and species longevity may allow the final harvest of monocultures. Before using these techniques and one species (e.g., red alder) to be combined with a tools in mixed species stands, we should determine thinning operation of associated conifers. whether modifications or alternative tools are When more than one species is harvested (e.g., necessary. Most of these technical aspects must be if red alder is harvested at age and Sitka spruce considered in any mixed species management and is thinned at the same time), the size range of are not specific to red alder–conifer mixtures. harvestable trees is usually wider than in even-aged First, the establishment requirements for various monocultures, and the machinery must be flexible species are different. The preferred germination enough to accommodate the range of sizes. Red conditions must be provided for each species. For alder and the associated conifers are usually sold to example, red alder as well as Sitka spruce and different buyers and the species must be separated, Douglas-fir prefer a mineral soil seedbed for creating another expense for the loggers. germination, while western hemlock will also
92 Summary Binkley, D., P. Sollins, R. Bell, D. Sachs, and D. Myrold. . Biogeochemistry of adjacent Growing mixtures of red alder and conifers can conifer and alder/conifer stands. Ecology provide benefits for biodiversity, ecosystem :–. functions, productivity, and sustainability. Research Bormann, B.T., K. Cromack, Jr., and W.O. into the temporal or spatial aspects of mixture Russell III. . Influences of red alder on management and appropriate densities is needed. soils and long-term ecosystem productivity. Better predictions of the growth and yield of In The biology and management of red alder. mixed-species stands is also required. Recommen- D.E. Hibbs, D.S. DeBell, and R.F. Tarrant dations are currently based on a limited number of (editors). Oreg. State Univ. Press, Corvallis, studies that focus primarily on mixtures of red Oreg. pp. –. alder and Douglas-fir stands. Bormann, B.T., and R.C. Sidle. . Changes in productivity and distribution of nutrients in References a chronosequence at Glacier Bay National Park, Alaska. J. Ecol. :–. Ahrens, G.R., A. Dobkowski, and D.E. Hibbs. Bruce, C., D. Edwards, K. Mellen, A. McMillan, . Red alder guidelines for successful re- T. Owens, and H. Sturgis. . Wildlife rela- generation. For. Res. Lab. Oreg. State Univ., tionships to plant communities and stand Corvallis, Oreg. Spec. Publ. No. . conditions. In Management of wildlife and Arney, J.D. . SPS: Stand projection system for fish habitats in forests of western Oregon mini- and micro-computers. J. For. :. and Washington. E.R. Brown (editor). U.S. Atkinson, W.A., B.T. Bormann, and D.S. DeBell. Dep. Agric. For. Serv. Publ. No. R-F&WL- . Crop rotation of Douglas-fir and red -. pp. –. alder: a preliminary biological and economic Brunner, K. L. . Effects of beaver on streams, assessment. Bot. Gaz. (Suppl.) :–. streamside habitat, and coho salmon fry Aubry, K.B. and P.A. Hall. . Terrestrial am- populations in two coastal Oregon streams. phibian communities in the southern Wash- M.Sc. thesis. Oreg. State Univ., Corvallis, Oreg. ington Cascade Range. In Wildlife and Burns, R.G. and B.H. Honkala (technical edi- vegetation of unmanaged Douglas-fir forests. tors). . Silvics of North America. U.S. L.F. Ruggiero, K.B. Aubry, A.B. Carey, and Dep. Agric. For. Serv., Washington, D.C. M.H. Huff (editors). U.S. Dep. Agric. For. Agric. Handb. No. . Vol. Serv. Gen. Tech. Rep. PNW-. pp. –. Carlton, G.C. . The structure and dynamics Berg, A., and A. Doerksen. . Natural fertiliza- of red alder communities in the central tion of a heavily thinned Douglas-fir stand Coast Range of western Oregon. M.Sc. thesis. by understory red alder. For. Res. Lab. Oreg. Oreg. State Univ., Corvallis, Oreg. State Univ., Corvallis, Oreg. Res. Note No. . Chambers, C.J. . Empirical yield tables for Binkley, D., K. Cromack, Jr., and D.D. Baker. predominantly alder stands in western Wash- . Nitrogen fixation by red alder: Biology, ington. th print. Wash. State Dep. Nat. rates, and controls. In The biology and man- Resour., Olympia, Wash. DNR Rep. No. . agement of red alder. D.E. Hibbs, D.S. Cole, D.W., J. Compton, H.V. Miegroet, and P. DeBell, and R.F. Tarrant (editors). Oreg. Homann. . Changes in soil properties State Univ. Press, Corvallis, Oreg. pp. –. and site productivity caused by red alder. Binkley, D. and S. Greene. . Production in Paper presented at IUFRO Symp. on Manage- mixtures of conifers and red alder: the im- ment of nutrition in forests under stress, portance of site fertility and stand age. In September –. Frieberg, Germany. IUFRO symp. on forest site and continuous Cole, D.W., S.P. Gessel, and J. Turner. . Com- productivity. R. Ballard and S.P. Gessel (edi- parative mineral cycling in red alder and tors). U.S. Dep. Agric. For. Serv., Portland, Douglas-fir. In Utilization and management Oreg. Gen. Tech. Rep. PNW-. pp. –. of alder. D.G. Briggs, D.S. DeBell, and W.A. Atkinson (compilers). U.S. Dep. Agric. For. Serv. Gen. Tech. Rep. PNW-. pp. –.
93 Cole, E. and M. Newton. . Nutrient, mois- Gilbert, F.F. and R. Allwine. . Spring bird ture and light relations in -year-old Doug- communities in the Oregon Coast Range. In las-fir plantations under variable Wildlife and vegetation of unmanaged Doug- competition. Can. J. For. Res. :–. las-fir forests. L.F. Ruggiero, K.B. Aubry, A.B. Comeau, P.G. and D. Sachs. . Simulation of Carey, and M.H. Huff (editors). U.S. Dep. the consequences of red alder management Agric. For. Serv. Gen. Tech. Rep. PNW-. on the growth of Douglas-fir using FORCYTE- pp. –. . B.C. Min. For. and For. Can., Victoria, Graham, R.C. and H.B. Wood. . Morphologic B.C. FRDA Rep. No. . development and clay redistribution in Corn, P.S. and R.B. Bury. . Terrestrial am- lysimeter soils under chaparral and pine. Soil phibian communities in the Oregon Coast Sci. Soc. Am. J. :–. Range. In Wildlife and vegetation of unman- Harrington, C.A. . A method of site quality aged Douglas-fir forests. L.F. Ruggiero, K.B. evaluation for red alder. U.S. Dep. Agric. For. Aubry, A.B. Carey, and M.H. Huff (editors). Serv. Gen. Tech. Rep. PNW-. U.S. Dep. Agric. For. Serv. Gen. Tech. Rep. .␣ . Alnus rubra Bong.: red alder. In PNW-. pp. –. Silvics of North America. R.G. Burns and Crocker, R.L. and B.A. Dickson. . Soil devel- B.H. Honkala (technical editors). U.S. Dep. opment on the recessional moraines of the Agric. For. Serv., Washington, D.C. Agric. Herbert and Mendehall glaciers south-eastern Handb. No. . Vol. . Alaska. J. Ecol. :–. Harrington, C.A. and R.O. Curtis. . Height Davis, M.B. . Pollen evidence of changing growth and site index curves for red alder. U.S. land use around the shores of Lake Washing- Dep. Agric. For. Serv. Res. Pap. PNW-. ton. Northwest Sci. ():–. Harrington, C.A., J.C. Zasada, and E.A. Allen. Dixon, R.O.D. and C.T. Wheeler. . Biochemi- . Biology of red alder (Alnus rubra cal, physiological and environmental aspects Bong.). In The biology and management of of symbiotic nitrogen fixation. In Biological red alder. D.E. Hibbs, D.S. DeBell, and R.F. nitrogen fixation in forest ecosystems: foun- Tarrant (editors). Oreg. State Univ. Press, dations and applications. J.C. Gordon and Corvallis, Oreg. pp. –. C.T. Wheeler (editors). Martinus Nijhoff/D. Heilman, P., and R.F. Stettler. . Mixed, short- W. Junk, The Hague, Netherlands. pp. -. rotation culture of red alder and black Edmonds, R.L. . Litter decomposition and cottonwood: growth, coppicing, nitrogen nutrient release in Douglas-fir, red alder, fixation, and allelopathy. For. Sci. :–. western hemlock, and Pacific silver fir eco- Hermann, R.K. and D.P. Lavender. . systems in western Washington. Can. J. For. Pseudotsuga menziesii (Mirb.) Franco: Doug- Res. :–. las-fir. In Silvics of North America. R.G. Emmingham, W.H., M. Bondi, and D.E. Hibbs. Burns and B.H. Honkala (technical editors). . Underplanting western hemlock in a U.S. Dep. Agric. For. Serv., Washington, D.C. red alder thinning: early survival, growth and Agric. Handb. No. . Vol. . damage. New Forests :–. Heusser, C.J. . Palynology of four bog sec- Eyre, F.H. (editor). . Forest cover types of tions from the western Olympic Peninsula, the United States and Canada. Soc. Am. For- Washington. Ecology :–. esters. Washington, D.C. Hibbs, D.E. . Managing hardwood stands for Gessel, S.P. and J. Turner. . Litter production timber production. Oreg. State Univ. Ext. by red alder in western Washington. For. Sci. Serv., Corvallis, Oreg. Ext. Circ. No. . :–. .␣ . White pine in the transition hard- wood forest. Can. J. Bot. :–.
94 Hibbs, D.E. and D.S. DeBell. . Management Murray, M.D. and R.E. Miller. . Early sur- to young red alder. In The biology and vival and growth of planted Douglas-fir with management of red alder. D.E. Hibbs, D.S. red alder in four mixture regimes. U.S. Dep. DeBell, and R.F. Tarrant (editors). Oreg.␣ State Agric. For. Serv. Pac. NW For. Range Exp. Univ. Press, Corvallis, Oreg. pp. –. Sta. Res. Pap. PNW-. King, J.E. . Site index curves for Douglas-fir Neal, J.L., W.B. Bollen, and K.C. Lu. . Influ- in the Pacific Northwest. Weyerhaeuser For. ence of particle size on decomposition of red Res. Cent., Centralia, Wash. Weyerhaeuser alder and Douglas-fir sawdust in soil. Nature For. Pap. No. . :–. Leslie, D.M. Jr., E.E. Starkey, and M. Vaura. . Newton, M. . Herbicides in alder manage- Elk and deer diets in old-growth forests in ment and control. In Utilization and man- western Washington. J. Wildl. Manage. agement of alder. D.G. Briggs, D.S. DeBell, ():–. and W.A. Atkinson (compilers). U.S. Dep. Lipp, C.C. . The effect of nitrogen and phos- Agric. For. Serv. Gen. Tech. Rep. PNW-. phorus on growth, carbon allocation and pp. –. nitrogen fixation of red alder seedings. M.Sc. Newton, M. and E.C. Cole. . A sustained- thesis. Oreg. State Univ., Corvallis, Oreg. yield strategy for old-growth Douglas-fir. McComb, W.C. . Red alder: interactions with West. J. Appl. For. ():–. wildlife. In The biology and management of .␣ . Stand development and succes- red alder. D.E. Hibbs, D.S. DeBell, and R.F. sional implications: Pure and mixed stands. Tarrant (editors). Oreg. State Univ. Press, In The biology and management of red alder. Corvallis, Oreg. pp. –. D.E. Hibbs, D.S. DeBell, and R.F. Tarrant McGarigal, K. and W.C. McComb. . (editors). Oreg. State Univ. Press, Corvallis, Streamside versus upslope breeding bird Oreg. pp. –. communities in the central Oregon Coast Newton, M., B. A. El Hassan, and J. Zavitkovski. Range. J. Wildl. Manage. :–. . Role of red alder in western Oregon Mack, R.N. and J.L. Harper. . Interference in forest succession. In Biology of alder. J.M. dune annuals: Spatial pattern and neighbour- Trappe, J.F. Franklin, R.F. Tarrant, and G.M. hood effects. J. Ecol. :–. Hansen (editors). U.S. Dep. Agric. For. Serv., Malcolm, D., J.E. Hooker, and C.T. Wheeler. . Portland, Oreg. pp. –. Frankia symbiosis as a source of nitrogen in Packee, E.C. . Tsuga heterophyla (Raf.) Sarg.: forestry: a case study of symbiotic nitrogen- western hemlock. In Silvics of North fixation in a mixed Alnus-Picea plantation in America. R.G. Burns and B.H. Honkala Scotland. Proc. R. Soc. Edinburgh B:–. (technical editors). U.S. Dep. Agric. For. Miller, R.E. and M.D. Murray. . The effects Serv., Washington, D.C. Agric. Handb. No. of red alder on the growth of Doulgas-fir. In . Vol. . Utilization and management of alder. D.G. Powers, R. . Are we maintaining the produc- Briggs, D.S. DeBell, and W.A. Atkinson tivity of forest lands? Establishing guidelines (compilers). U.S. Dep. Agric. For. Serv. Gen. through a network of long-term studies. In Tech. Rep. PNW-. pp. –. proc. management and productivity of west- Minore, D. . Thuja plicata Donu. ex D. Don: ern-montane forest soils. A.E. Harvey and western redcedar. In Silvics of North L.P. Neuenschwander (editors). U.S. Dep. America. R.G. Burns and B.H. Honkala Agric. For. Serv. Gen. Tech. Rep. INT-. (technical editors). U.S. Dep. Agric. For. pp. –. Serv., Washington, D.C. Agric. Handb. No. Puettmann, K.J. [n.d.]. Spatial competition dy- . Vol. . namics in mixed red alder/Douglas-fir stands. In preparation.
95 Puettmann, K.J., D.S. DeBell, and D.E. Hibbs. Tarrant, R.F. . Stand development and soil . Density management guide for red al- fertility in a Douglas-fir-red alder plantation. der. For. Res. Lab., Oreg. State Univ., For. Sci. :–. Corvallis, Oreg. Res. Contrib. No. . Tarrant, R.F., B.T. Bormann, D.S. DeBell, and Puettmann, K.J. and D.E. Hibbs. . Compari- W.A. Atkinson. . Managing red alder in son of stand characteristics from long-term the Douglas-fir region: some possibilities. J. remeasured plots of red alder with yield table For. ():–. and growth model prediction. Rep. to the Tarrant, R.F. and R.E. Miller. Accumulation Hardwood Silviculture Co-operative. of organic matter and nitrogen under a plan- Puettmann, K.J., D.E. Hibbs, and D.W. Hann. tation of red alder and Douglas-fir. Soil Sci. . Extending the size-density analysis to Soc. Am. Proc. :–. mixed species populations and investigation Tesch, S. D. . The Pacific Northwest region. of underlying dynamics using red alder and In Regional silviculture of the United States. Douglas-fir stands. J. Ecol. :–. rd ed. J.W. Barrett (editor). John Wiley and Ruth, R.H. and A.S. Harms. . Management Sons, Inc., New York, N.Y. pp. –. of western hemlock–Sitka spruce forests for Van Miegroet, H. and D.W. Cole. . The im- timber production. U.S. Dep. Agric.␣ For.␣ Serv., pact of nitrification on soil acidification and Portland, Oreg. Gen. Tech. Rep. PNW-. cation leaching in a red alder ecosystem. J. Schroeder, H. and T.C. Daniel. . Progress in Environ. Qual. :–. predicting the perceived scenic beauty of for- Waring, R.H. and W.H. Schlesinger. . Forest est landscapes. For. Sci. :–. ecosystems: concepts and management. Aca- Shainsky, L.J. and S.R. Radosevich. . Mecha- demic Press, San Diego, Calif. nisms of competition between Douglas-fir Wild, A. . Russell’s soil condition and plant and red alder seedlings. Ecology ():–. growth. Longman Scientific and Technical, Smith, J.H.G. . Growth and yield of red alder Essex, England. in British Columbia. In The biology and Williamson, R.L. . Productivity of red alder management of red alder. D.E. Hibbs, D.S. in western Oregon and Washington. In Biol- DeBell, and R.F. Tarrant (editors). Oreg. ogy of alder. J.M. Trappe, J.F. Franklin, R.F. State Univ. Press, Corvallis, Oreg. pp. – Tarrant, and G.M. Hansen (editors). U.S. . Dep. Agric. For. Serv., Portland, Oreg. pp. Stubblefield, G. and C.D. Oliver. . –. Silvicultural implications of the reconstruc- Worthington, N.P., F.A. Johnon, G.R. Staebler, tion of mixed alder/conifer stands. In Utiliza- and W.S. Lloyd. . Normal yield tables for tion and management of alder. D.G. Briggs, red alder. U.S. Dep. Agric. For. Serv. Pac. D.S. DeBell, and W.A. Atkinson (compilers). NW For. Range Exp. Sta. Res. Pap. No. . U.S. Dep. Agric. For. Serv. Gen. Tech. Rep. Zavitkovski, J., and M. Newton. . Effect of PNW-. pp. –. organic matter and combined nitrogen on Tappeiner, J., J. Zasada, P. Ryan, and M. Newton. nodulation and nitrogen fixation in red al- . Salmonberry clonal and population der. In Biology of alder. J.M. Trappe, J.F. structure: the basis for a persistent cover. Franklin, R.F. Tarrant, and G.M. Hansen Ecology ():–. (editors). U.S. Dep. Agric. For. Serv., Port- land, Oreg. pp. –.
96 Mixedwood Management Research and Practice in Québec
MARCEL PRÉVOST
Abstract that maintaining a mixed composition is a possible management objective, and related silvicultural In Québec, % of the productive forest lands are treatments are now eligible as payment of stum- classified as mixedwood forests. As a whole, they page dues (Ministère des Ressources naturelles du cover nearly km and represent a large Québec ). proportion of the wood supply in some regions. We are aware that we must develop silvicultural Recently, maintaining a mixed species composition systems that are adapted to mixed stands because has been recognized as a sound management they are increasing in area and represent a large objective for mixedwood stands, and research on proportion of the wood supply in some regions. the subject has been initiated. This paper describes The benefits of forest diversity are numerous and, the present status of management practices and because pesticide use is very restricted in Québec, research on mixedwood forests in Québec. environmentally sensitive management methods must be developed. Even though we still have few Introduction results, we are establishing research programs at different levels and can foresee that research efforts This paper provides an overview of the extent and will increase over the next few years. characteristics of mixedwood forests in Québec. The silviculture systems, and regeneration and Importance of Mixedwood Forests stand-tending practices being used to manage mixed-species forests in this province are also The importance of mixedwood forests in Québec discussed. Our knowledge is limited because until can be illustrated by the following data (Parent now we have not been intentionally managing ). The area of the province is . million km, these stands as mixed-species stands. In the past, of which % is occupied by forest. Productive we simply managed them as pure softwood or forests occupy % (. million km ), south of hardwood stands, depending on the dominant ° N (Figure ). Excluding the nonstocked areas, species. Managing mixedwood stands was then conifer stands, dominated by black spruce (Picea often limited to harvesting only the softwood or mariana [Mill.] B.S.P.) and balsam fir (Abies hardwood species. Obviously, these practices balsamea [L.] Mill.), cover approximately % of resulted in degraded residual stands and this area, while broadleaved stands cover about established weed species that suppressed the %. The remaining % of productive forests is preferred ones, especially on the best sites. classified as mixedwood stands, which represent Integrated logging, which is the cutting of all nearly km. Mixedwood forests are merchantable species at the same time, was concentrated in an east-west strip between ° and introduced in the early s. Its application varied ° N, where they cover nearly % of the forest widely between regions because it was dependent area. On the map of ecological regions of southern on existing markets for these species. Even though Québec (Thibault ), this area roughly waste wood was reduced, this system was too corresponds to the balsam fir–white birch and simple for the complex environment of mixed- balsam fir–yellow birch associations, which occur species forests. Recently, changes to forest in transition between coniferous forests to the management policy in Québec clearly recognize north and broadleaved forests to the south.
97 Autecology of the Main Component Species
Balsam fir is a very shade-tolerant species; it is present in almost all stands, especially in eastern Québec where precipitation is more abundant. It Artic tundra can grow on a variety of soil conditions, is usually Forested tundra abundant as advance growth, and is known to Taiga respond well to release. Trees older than years Forest are subject to decay and are susceptible to the spruce budworm (Choristoneura fumiferana Clem.). White spruce is also widely distributed in our mixedwood stands, but it is less subject to decay and to foliage losses caused by spruce budworm. It can live more than years and is frequently encountered as veteran stems. White spruce is intermediate in shade tolerance, and may fail to establish under cover. Another coniferous species that is common in the southeast of the province is red spruce, which seems to be adapted to edaphic conditions that are unfavourable to other species. This species is very Québec’s biogeographical regions. shade tolerant and it can respond well to release even after a long period of suppression. Climatic conditions of the mixedwood forests are Jack pine, black spruce, and tamarack are also characterized by – degree-days during the found, the two latter on wet sites. Except for growing season and an average annual balsam fir, which can germinate on several types of precipitation of about mm. The mixedwood seedbed, these conifers require good seedbeds such forest corresponds with a rather heavily populated as mixed soil, mineral soil, or sphagnum moss. area (by Canadian standards). One-third of the Paper birch is the most common shade- forested area is in private ownership. intolerant broadleaved species encountered in Mixedwood stands represent nearly billion m mixture with conifers. Often considered as a weed of gross merchantable volume, which is evenly species, paper birch has rapid early growth and can distributed between hardwood and softwood invade a site immediately after logging by natural species. Balsam fir, white spruce (Picea glauca seeding and sprouting. This species can live for [Moench] Voss), black spruce, paper birch (Betula ␣ years, but crown dieback is often observed after papyrifera Marsh.), yellow birch (Betula years of age. alleghaniensis Britton.), and aspen (Populus Aspen is also widespread, especially in the tremuloides Michx) together account for % of western part of the province. Because of its this volume, but other species such as tamarack aggressive root suckering, aspen may increase in (Larix laricina [Du Roi] K. Koch), jack pine (Pinus proportion after logging. This species is very shade banksiana Lamb.), red pine (Pinus resinosa Ait.), intolerant and survival of the suckers largely white pine (Pinus strobus L.), eastern white cedar depends on light intensity. With a maturity around (Thuja occidentalis L.), eastern hemlock (Tsuga years old, it is considered as a potential canadensis [L.] Carr), red spruce (Picea rubens competitor for many years. Aspen cover often Sarg.), sugar maple (Acer saccharum Marsh.), and produces the shade that enables tolerant species, red maple (Acer rubrum L.) are also common. which will follow in the succession, to establish. These species all have their own ecological Yellow birch is commonly found on better sites. requirements and a given stand may contain It is intermediate in shade tolerance and can live several of them, greatly differing in age and size, so up to years. The tallest of our birches, this that mixedwood forests are highly variable in time species regenerates well from seeds and by and space. sprouting. It tends to produce epicormic branches
98 when the forest cover is opened, so stand density is Shade-tolerant Hardwood-Conifer Stands critical for stem quality.
Sugar maple is very shade tolerant and regener- Even-aged Uneven-aged ates well in the understorey. It can live for years and is known to respond well to release, even though Adequate Shelterwood Selection cutting cutting frost and radiation cracks are potential problems. advance growth: 20–35% of Finally, red maple has a very large ecological careful logging basal area range and is a common species in mixedwood Fill planting stands. It reproduces vigorously by both seeds and sprouting. Its shade tolerance and longevity of Pre-commercial thinning about years, make it a persistent competitor. Silviculture of shade-tolerant hardwood-conifer Management and Research stands.
When we consider mixedwood stands, two man- selection cutting and also group-selection cutting agement groups are usually recognized according to adapt this method to the mosaic of mixedwood to their broadleaved species: () the shade-tolerant forests. As these forests are often a mixture of hardwood-conifer stands, and () the shade- homogeneous clumps, group-selection cutting may intolerant hardwood-conifer stands. The shade- be more appropriate than other methods. Early tolerant hardwood-conifer stands are composed results indicate that it can be useful if a good seed primarily of yellow birch, maples, balsam fir, and supply and receptive seedbeds are available. In the white spruce and comprise about % of Québec summer of , the Canadian Forest Service in the mixedwoods. They contain our most valuable Québec Region (Dr. Denis Ouellet) established a species and occupy the most productive sites of the study on regeneration of mixedwood forests by mixedwood forests. Past clearcuts of these stands shelterwood cutting. Different cutting intensities have shown that competition from mountain are combined with soil preparation to create maple (Acer spicatum Lam.), pin cherry (Prunus favourable seedbeds and limit weed development. pensylvanica L.f.), squashberry (Viburnum spp.), The effects of treatment on natural succession are and raspberry (Rubus idaeus L.) is a frequently being examined along with site factors. observed problem. To reduce the development of The shade-intolerant hardwood-conifer stands, these competing species, the opening of the stand which make up nearly % of our mixedwood must be limited as much as possible. Depending on forests, include the paper birch–conifer association the stand structure, three different scenarios are that represents % of all Québec’s mixedwood possible (Figure ). In the case of even-aged stands, stands, the aspen-conifer stands that cover %, the management policy is to apply either shelter- and a combination of aspen, birch, and various wood cutting or careful clearcut logging, depending softwoods that cover %. For these three on the presence and height of advance growth. In associations, a study indicated that stocking of uneven-aged stands, selection cutting with the softwood advance growth was % or more in removal of –% of the basal area is proposed approximately % of the mature stands (Doucet to␣ maintain a mixed structure and to allow full ). On this basis, the forest management policy growth of long-lived merchantable species. In all recommends careful logging to protect conifer cases, regeneration stocking of the harvested advance growth. It is anticipated that the stand will species must reach at least their pre-harvest levels become a mixedwood stand as the hardwoods within years after the last cut, otherwise fill regrow (Figure␣ ). Both regeneration standards and planting is mandatory. During the first – years stand tending must follow the same rules as after logging, pre-commercial thinning is also mixedwoods with tolerant hardwoods. Results of proposed to adjust spacing and to maintain a these practices are still unknown, but it is expected mixed stand composition. that the lack of advance growth will be a limiting At the Ministry of Natural Resources, Dr. Zoran factor in several cases. Depending on its height at Macjen has been studying yellow birch–balsam fir the time of logging, advance growth may also be stands for the last years. He tried single-tree overtopped by fast-growing species such as aspen
99 Shade-intolerant Hardwood-Conifer Stands through partial cutting (Hittenrauch ; Jones ). Where aspen is increasingly being used, Careful logging shelterwood cutting can promote growth of residual stems and the development of a softwood Fill planting understorey. This is the basis of a research project that the Québec Ministry of Natural Resources is Pre-commercial thinning conducting. The aim is to define the optimal ? – Lack of advanced growth canopy opening to improve the establishment and – Height of advance growth development of conifer regeneration and con- – Species composition currently limit aspen growth. In the summer of , we established an experiment on shelterwood Silviculture of shade-intolerant hardwood-conifer cutting near Québec City. The design consisted of stands. four replications of five cutting intensities– , , , , and % of the canopy. In the summer of and paper birch. The requirement that , each treatment will be applied on a × m regeneration must be composed of the harvested area and sixteen -m plots will be established in species is also expected to be frequently unfulfilled. the inner × m (Figure ). To assess the However, we consider that this policy is a good influence of seedbeds on seedling establishment, beginning, which will prompt research in this area. half of these plots will be scarified. Seedbeds, In the paper birch–conifer stands, a team from regeneration, and competition were estimated in Laval University (Dr. Marius Pineau) is these plots last summer to gather information on experimenting with shelterwood cuttings. The pre-treatment conditions and we expect to collect experiment consists of a regeneration cut, to be the same kind of information each year for the followed by a final careful logging when the next few years. The experiment will also include regeneration is established. The study compares micro-environmental measurements in each different methods of shelterwood cutting in balsam treatment to improve our understanding of fir stands, where competition from hardwoods is a vegetation dynamics. Photosynthetically active potential problem. Effects of seedbed and radiation will be monitored at one fixed point in environmental conditions on seedling each treatment and periodic point samplings will establishment are also examined. The productivity of mixedwood forests is also under study. Mixed stands with aspen create management – Growth of residual stems – Net precipitation problems because of the aggressiveness of this – Soil moisture species following logging. Even stands classified as – Soil temperature softwood, but with a small proportion of aspen, may become pure aspen after logging (Perala ; Doucet ), as was often the case historically in Québec forests. Given the structure of our industry, it is important to maintain the softwood 20 m component of our forests. At the other extreme, 50 m stands classified as pure aspen may contain a small – Seedbed type – Vegetation heterogeneity – Regeneration – (overstorey vs understorey) proportion of softwood in the main canopy, by – Competition – Range of densities definition up to % of the basal area. The poten- – Light measurements – Potential windthrow tial exists to increase the softwood component, but methods must be found to accomplish this. Layout of sampling plots within an experimental Studies demonstrated that it is possible to unit in the shelterwood cutting project for aspen- reduce suckering by controlling light intensity conifer stand regeneration.
100 be made at each sampling plot. Net precipitation, Acknowledgements soil moisture, and soil temperature will also be monitored in the different treatments. I would like to thank the Province of British Columbia At this point, we realize what a challenge for giving me the opportunity to participate at this establishing experiments in mixedwood stands can workshop and to benefit from this event. The be. Our pre-treatment data indicate that vegetation advise of René Doucet, David Pothier, Zoran heterogeneity makes it difficult to assign Macjen, and Pierre Grondin during the preparation treatments so that experimental error is kept to a of the manuscript is gratefully acknowledged. I also minimum. For example, experimental units that thank M. Jean-Pierre Lapointe for his help in are comparable for the overstorey often strongly preparing this paper for the conference. differ for the understorey. We also have questions about the feasibility of protecting understorey References across a range of stand densities, and are aware of the potential for windthrow. Doucet, R. . Méthodes de coupe et de Research is also being conducted in the Abitibi préparation de terrain pour favoriser la region, by the University of Québec in Montreal régénération naturelle de quelques tremblaies (Dr. Yves Bergeron) and the University of Québec de l’est du Québec. For. Chron. :–. in Abitibi–Temiscamingue. Their studies are .␣ . La régénération préétablie dans les focusing mainly on the natural dynamics of the peuplements forestiers naturels au Québec. boreal balsam fir stands, where aspen and paper For. Chron. :–. birch are important pioneer species. Stand Hittenrauch, H.R. . Response of aspen to productivity, soil fertility, and susceptibility to the various harvest techniques. In Utilization and spruce budworm are being examined on a marketing as tools for aspen in the Rocky successional basis. Mountains. U.S. Dep. Agric. For. Serv. Gen. Tech. Rep. RM-. pp. –. Conclusion Jones, J.R. . Aspen harvesting and reproduc- tion. In Utilization and marketing as tools There is a need to improve our knowledge of for aspen in the Rocky Mountains. U.S. Dep. mixedwood management practices in Québec. The Agric. For. Serv. Gen. Tech. Rep. RM-. new forest protection strategy will give priority to pp.␣ –. an environmentally sensitive silviculture aimed at Ministère des Ressources naturelles du Québec. limiting insect, disease, and competition problems . Manuel d’aménagement forestier. without using pesticides. Vegetation diversity is Ministère des Ressources naturelles du known to reduce the risks of disease and insect Québec, Direction des programmes forestiers, damage. In our province, where the spruce Québec, Que. p. budworm periodically causes severe damage to fir Perala, D.A. . Manager’s handbook for aspen and spruce stands, mixedwood stand management in the north-central states. U.S. Dep. Agric. must be considered as an alternative to pesticide For. Serv. Gen. Tech. Rep. NC-. p. use. Furthermore, technological advances have Parent, B. . Ressource et industrie forestières: created increased demands for hardwood species. Portrait statistique, Édition . Ministère This is another point that favours increased des Ressources naturelles du Québec, Direc- research of mixedwood stands. Research on tion des politiques forestières, Québec, Qué. mixedwood management will certainly be an p. important issue for the next few years and only Thibault, M. . Les régions écologiques du time will tell us how foresters can adapt to this Québec méridional. Carte au / . new␣ challenge. Ministère des Ressources naturelles du Québec, Direction de la recherche forestière, Québec, Qué.
101 Mixedwood Management Research and Practice in Ontario
G. BLAKE MACDONALD
Abstract definitions are important to help resource managers determine which sites are good The Boreal Mixedwood Forest in Ontario is vast candidates for proactive mixedwood prescriptions. and productive, but has been underutilized and The following definitions have been developed for poorly managed. Increased interest in maintaining Ontario conditions (MacDonald and Weingartner diversity is revealing shortfalls in knowledge, ), but the concepts apply wherever mixed- policies, and operating procedures for Ontario’s woods are managed. The characteristic mixedwood mixedwood forests. Current inventories, manage- species in northern Ontario are listed in Tables ment guidelines, and silvicultural standards are and . inadequate for mixedwood forests. An opportunity exists to advance integrated resource management The defining boreal mixedwood tree species in Ontario through improved mixedwood silviculture. Preliminary research results support Common name Scientific name the ecological and economic feasibility of modified harvesting to perpetuate productive mixed-species White spruce Picea glauca (Moench) Voss crops. Progress in mixedwood management will Black spruce Picea mariana (Mill) B.S.P. depend on accelerated programs of research, demonstration, training, and information sharing Balsam fir Abies balsamea (L.) Mill. between jurisdictions. Trembling aspen Populus tremuloides Michx. White birch Betula papyrifera Marsh. Introduction
The extensive mixedwood forests in northern Ontario have not been actively managed or The associated boreal mixedwood tree species utilized, leaving their potential largely untapped. Advances in mixedwood management in other jurisdictions and the inherent advantages of species Common name Scientific name mixtures are encouraging resource managers in the province to embrace the opportunities and address Jack pine Pinus banksiana Lamb. the challenges of mixedwoods. White pine Pinus strobus L. This paper describes the Ontario Boreal Red pine Pinus resinosa Ait. Mixedwood Forest, discusses the advantages and Eastern white cedar Thuja occidentalis L. current status of mixedwood management in Tamarack Larix laricina (Du Roi) K. Koch Ontario, and presents some opportunities for Largetooth aspen Populus grandidentata Michx. management and research in the near future. Balsam poplar Populus balsamifera L. Definitions White elm Ulmus americana L. Black ash Fraxinus nigra Marsh. The structure and composition of Ontario Black willow Salix nigra Marsh. mixedwood stands are complex, and clear
102 Boreal Mixedwood Sites A boreal mixedwood site Boreal Mixedwood Stands A boreal mixedwood is an area with climatic, topographic, and edaphic stand is a tree community on a boreal mixedwood conditions that favour the production of closed site in which no single species exceeds % of the canopies dominated by trembling aspen or white basal area. All of the defining and associated tree birch in early successional stages, black spruce or species (Tables and ) qualify as canopy white spruce in mid-successional stages, and components. The definition imposes no constraint balsam fir in late successional stages. on the proportions of hardwood or softwood Disturbances on boreal mixedwood sites in species for a stand to be classified as mixedwood. It Ontario create conditions suitable for the establish- accommodates stands composed of two or more ment of shade-intolerant species. Early successional hardwood species or two or more conifer species, trees such as trembling aspen and white birch provided the stands are established on boreal usually become established with pioneer shrub mixedwood sites. The implication is that the stand species. As the hardwood trees close their canopy, is a candidate for mixedwood management the abundance of shrubs in the understorey prescriptions because the site has potential to declines. Shade-intolerant pines may become increase its diversity of tree species in the future. established with the early successional hardwoods. Stand composition at any point in time depends The diffuse canopy provides a suitable regeneration on the successional stage, which is controlled by environment for moderately shade-tolerant the disturbance type and availability of seed or conifers such as black spruce and white spruce, vegetative propagules. although significant mineral soil exposure is Boreal Mixedwood Forests A boreal mixedwood required to ensure establishment of spruce forest is the aggregate of all boreal mixedwood sites germinants. Balsam fir is the characteristic late- in any distinct area. Specifically, the Boreal Mixed- successional species on boreal mixedwood sites, as wood Forest of Ontario is the aggregate of all it tolerates the increasingly dense shade and boreal mixedwood sites in the province. Boreal establishes well on the undisturbed accumulations mixedwood forests may contain mixtures of several of litter and humus. Spruce-fir mixedwoods species within each stand, or mosaics of small seldom endure as climax formations because fire, single-species stands. The emphasis on site windthrow, spruce budworm epidemics, or other provides geographical stability to these definitions, disturbances soon return these forests to an earlier making them useful for inventory and successional stage. management␣ planning. The successional pattern of mixedwood sites is a key component of the definition, and it should Relationship to Forest Ecosystem Classification guide management prescriptions. The definition Terminology The forest ecosystem classification assumes that adequate supplies of seed or ₍FEC₎ systems in northern Ontario identify vegetative propagules are available to produce the approximately boreal mixedwood vegetation characteristic dominant species at each stage. The types (Jones et␣ al. ; Sims et␣ al. ), which are abundance, diversity, and relative position of analogous to boreal mixedwood stands as defined associated species at each successional stage depend in this paper. A drawback to using vegetation types on the disturbance type and pre-disturbance for management planning is that they change on a stand␣ composition. given site through natural forest succession. The use of characteristic site types provides a The FEC soil types are simpler, more stable, and stable frame of reference to define these complex more relevant for identifying sites where long-term and dynamic forests. Boreal mixedwood sites mixedwood management would be appropriate. typically have well-drained, fertile soils on mid- Thus, the concept of soil type is compatible with slope positions, and exclude wet lowlands, dry sand the definition of boreal mixedwood site. The plains, and shallow soils on bedrock outcrops shallow soils and organic soils in the FEC (McClain ). Deep soils, medium to fine descriptions can generally be excluded from the textures, and unrestricted drainage are essential definition of mixedwood sites. elements of a mixedwood site.
103 Extent of Mixedwoods in Northern Ontario ). Nutrient replacement times following disturbances such as harvesting are more rapid in The Boreal Mixedwood Forest covers approx- mixedwood stands than in spruce monocultures imately % of Ontario’s productive forest area (Gordon ). (Armson ; Spectranalysis, Inc. ). The greatest concentration of mixedwood sites occurs Economic Considerations Mixedwoods that were north of Lake Superior, between ° and ° N formerly by-passed in the search for pure conifer latitude. The area of mixedwood cover types in stands offer an attractive source of high-quality northern Ontario is increasing as harvested conifer fibre close to mills. Markets are rapidly expanding stands regenerate to a variety of species and the for these previously ignored species, especially spruce component is overtaken by balsam fir and aspen. Technological developments allow the use of trembling aspen (Yang and Fry ). Fire control hardwood species for paper, oriented strandboard, has permitted the expansion of mixedwood forests and other wood products. Furthermore, as on sites normally dominated by monocultures of utilization practices come under greater public early successional species such as jack pine, black scrutiny, companies seek to harvest a wider range spruce, or aspen. of species from their limits. Mixedwood forests A regeneration audit (Hearnden et␣ al. ) provide stability for industry because hardwoods revealed that the stocking of young stands in and softwoods focus on different products Ontario is acceptable, but the species composition and␣ markets. is shifting decisively toward mixedwoods and Mixedwood stands in northern Ontario occur hardwoods. This trend persists despite attempts to on upland sites, which can be harvested during the establish conifer stands after harvesting. The frost-free season. This provides a flexibility in expansion of mixedwoods emphasizes the urgency harvest scheduling not available for lowland conifer of developing silvicultural guidelines for these stands, which must be harvested during the winter forest types. to minimize site damage. Operational productivity is high in mixedwood stands because stem sizes are Why Manage for Mixedwoods? larger, compared to spruce stands. Furthermore, access roads can be built more easily in upland Ecological Sustainability Biological and physical mixedwood areas than in lowland conifer areas. diversity ensure healthy and productive forest Harvest yields are increased when both the ecosystems (Suffling et␣ al. ; Brooks and Grant hardwoods and softwoods are used, and mixed- ). Interactions among tree species maintain site species stands often have higher total growth rates productivity and resistance to infestation and than pure stands. Mixedwoods in north-central disease. Mixed-species ecosystems are resilient and Ontario produce about m /ha, compared forgiving of management errors, providing to␣ m /ha for average black spruce stands insurance against failure of one of the component (Opper␣ ). species (Debyle ). Companion species differ in The high cost of establishing spruce stands is their limiting environmental factors, growth unjustified on mixedwood sites that produce habits;,and physiological processes (Chan et␣ al. abundant natural regeneration. However, successful ; Schuler and Smith ) permitting more mixedwood stands cannot be produced without biological activity per unit area in mixtures than silvicultural investment. For example, mid-rotation in␣ monocultures. tending and modified harvesting are required to In Ontario, the nutrient cycling capabilities of maintain productive densities, protect advance soil may deteriorate under conifers because of raw regeneration, and reduce the susceptibility of fir humus formation. Hardwoods, particularly birch, and aspen to insect and disease losses. Also, site prevent this, and the best-quality conifers are often preparation techniques must be modified to found in mixedwood stands (Bedell ). enhance conifer establishment while preventing Impermeable iron pans may develop under black damage to the root systems of the complementary spruce stands, resulting in wet, unproductive sites. hardwood crop (Smith ; Lieffers and Beck ). This process is minimized by growing black spruce Mixedwoods can also improve non-timber in combination with hardwood species (Bedell economic values. For example, the maintenance of
104 species mixtures creates aesthetically varied Ontario lags behind Alberta by about years, landscapes, providing opportunities for recreation regarding interest in boreal mixedwood manage- and tourism. The diverse succession of vegetation ment. However, social, economic, and ecological supports many wildlife species and protects the realities are causing resource managers in Ontario quality of watersheds. to consider mixedwood management more seriously. What about Balsam Fir? The conifer supply short- Policy Concerns age in Ontario could be addressed by increasing the use of balsam fir. This species is the least- Ontario’s forest policies and practices are increas- preferred mixedwood conifer in the province, ingly influenced by public attitudes. A number of although it is as satisfactory as spruce for most emerging issues affected by public input relate to products. For example, fir wood pulp is in some mixedwoods. These include ecosystem-based manage- ways superior to spruce pulp (Bedell ). ment, maintenance of biodiversity, enhancement of Mixedwood management strategies must recognize non-timber forest values, and cost reduction␣ (e.g., the role of fir, which is replacing spruce through shifting away from artificial regeneration toward natural succession on many sites in northern natural regeneration). The Crown Forest Sustain- Ontario (Yang and Fry ). ability Act, introduced by the Ontario government Natural fir regeneration is easily obtained. A in , is a response to public attitudes. It parallels faster growth rate and more vigorous root the B.C. Forest Practices Act, indirectly supporting development allow fir seedlings to become mixedwood silviculture on appropriate sites by established on thicker layers of litter and survive emphasizing integrated resource management and longer periods of drought, compared to spruce. systems that promote biological diversity. Unmanaged fir is more susceptible to insect and The environmental assessment for timber disease damage, but such problems could be greatly management in Ontario (Koven and Martel ) reduced by applying management techniques thoroughly examined all aspects of timber suited to the silvics of the species, such as thinning management policy and operations in the Ontario young stands and lowering the rotation age. In Ministry of Natural Resources. The Ontario Ontario, fir is managed on the same rotation age as government is legally bound to implement spruce (approximately years). However, fir is ␣ terms and conditions arising from this invest- faster-growing and susceptible to decay and spruce igation. Many of the terms address mixedwood budworm. Thus, it should be managed on a much issues such as the effects of management practices shorter rotation (e.g., years). on biodiversity, ecosystem processes, and non- The Global Perspective Mixedwood silviculture is timber values such as fish and wildlife habitat. The well accepted in Scandinavia, where concern exists Ministry of Natural Resources is specifically about extensive clearcutting and heavy site ordered to produce a mixedwood silvicultural preparation (Norokorpi ). Careful logging of guide to be used in the timber management aspen to protect understorey spruce has been planning process. The environmental assessment applied widely in Alberta and to a more limited compels the Ministry of Natural Resources to extent in Saskatchewan and Manitoba (Lieffers and consider more active mixedwood management and Beck ). In western Canada, increased use of to address the large information gap regarding hardwoods and a public demand to maintain mixedwood ecology and silviculture. mixedwoods for non-timber uses are challenging The Ontario Ministry of Natural Resources is the traditional softwood bias in mixedwood developing a new business relationship with the management (Brace Forest Services ). Forest forest industry, in which companies would assume management to promote mixed-species crops is responsibility for funding and conducting all also gaining wide acceptance in the United States silvicultural operations on their limits. A portion (Baker ; Irland and Maass ). of stumpage fees (e.g., $/m for spruce or pine) Mixedwood management is the most important would be deposited into a trust fund dedicated to research priority in Alberta (Forest Research supporting silviculture on the limits of the contri- Advisory Council of Canada ), reflecting the butor. On the positive side, this arrangement rapid increase in aspen utilization in that province. creates stability of silvicultural funding and merges
105 harvesting and silviculture under a single operator. expensive and seldom completely successful. However, the main intention of the arrangement is Silvicultural prescriptions should specify the stand to fund conifer regeneration, and from a mixed- structures and species compositions desired at wood perspective the scope must be expanded to critical points in the rotation, and indicate the include all species and silvicultural operations modified harvesting, regeneration, and tending throughout the rotation. systems required to achieve these stand conditions. Ecosystem dynamics should be given as much An overriding constraint in northern Ontario is attention as mill requirements in formulating the low unit value of mixedwood tree species, silvicultural guidelines. Policies should encourage which limits the amount of intensive silviculture multi-rotation mixedwood management, that can be conducted. understorey protection during harvesting, wider Most forest companies in northern Ontario use species utilization, mill integration, and movement only softwoods or hardwoods. This presents a of raw material between different sectors of the problem in mixedwood stands, where hardwoods wood-processing industry. Stocking standards and and softwoods have different rotation ages. During free-to-grow assessments must be revised to allow harvesting of the preferred species, a secondary for natural regeneration of mixed species. species is often damaged. If protection of the Furthermore, the provincial inventory system, secondary species is prescribed, the harvesting which classifies a stand according to its single operation is slower and more expensive. Although dominant species and ignores the understorey, recent revisions to Ontario’s forest management must be revised to accommodate mixedwoods. planning system advocate diversity and multiple The diversity and productivity of mixedwood values, mechanisms to resolve these operational forests make them ideal candidates for integrated concerns are not well developed. resource management. The wide range of resource The mixedwood logging systems being used in values demands improved co-operation between western Canada to preserve advance conifer agencies and disciplines. In this regard, recent growth␣ are cost-effective and ecologically sensible changes in the Ontario Ministry of Natural (Lieffers␣ and Beck ). Interest in these systems is Resources are encouraging: forest management growing␣ in Ontario, and large-scale research and planning is now conducted by interdisciplinary development trials have recently been established. teams, public participation in the process is However, cost factors and the lack of applied mandatory, and funding for research depends on guidelines are impeding widespread acceptance the degree of collaboration. by␣ industry. A dual rotation system could be applied in Management Guidelines Ontario boreal mixedwood stands. An initial harvest of the unmanaged forest would remove Management guidelines for Ontario’s northern mature or overmature aspen, birch, and jack pine. mixedwoods are being developed by drawing on Younger merchantable fir and spruce should also experience in other jurisdictions, consolidating the be cut. The operation would release advance spruce results of past field trials in the province, and in the understorey and stimulate aspen suckering. establishing operational-scale research and demon- A two-stage shelterwood, with the removal cut stration projects. A mixedwood silvicultural guide following the initial cut by – years, would is being produced, as legally required by the recent provide residual cover for the acclimation of large environmental assessment (Koven and Martel advance growth. Thinning operations should be ). The guide will present the most current scheduled within years of harvesting to obtain knowledge in a concise, updatable format the required density of aspen. A clearcut or seed- for␣ practitioners. tree cut years after the initial harvest would Pre-harvest silvicultural prescriptions for remove aspen, birch, and pine aged – years, mixedwood sites must adapt to successional trends, and spruce and fir aged – years. Pre-cut site rather than resist them. Multiple rotations of a preparation or post-cut underplanting of the aspen single species lead to a depletion of site suckers may be needed to enhance the spruce productivity and biological diversity. Attempts to component. Fire should be used for site perpetuate conifer crops on mixedwood sites are preparation if control of balsam fir is desired and
106 advance spruce regeneration is absent. The cycle planted conifers, and can also maintain the value would be repeated with careful shelterwood of the stand for wildlife habitat (Weingartner ). logging of the overstorey after a subsequent Balsam fir and jack pine are other mixedwood ␣ years. Timing adjustments would be advisable if species that may require pre-commercial thinning. all mixedwood species were present because on Although thinning is often neglected in Ontario, it most sites fir should be harvested at – years, is one element of an entire-rotation management and pine at – years. system that must be applied to achieve regulated, Modified cutting techniques that promote productive, and healthy mixedwood forests. natural regeneration are the foundation of Large areas of northern Ontario are dominated mixedwood management. One of the greatest by mixedwood stands that have become degraded opportunities for advancing mixedwood forestry from a commercial standpoint. On many and obtaining industry co-operation lies in the mixedwood sites in eastern Canada, natural refinement of these harvesting techniques. Other succession produces deteriorating stands of aspen, good candidate areas for immediate attention are birch, and jack pine, with dense thickets of balsam species-site relationships, site preparation fir and shrubs in canopy gaps. The lack of mineral techniques that mimic natural disturbances, soil exposure prevents the establishment of spruce modified artificial regeneration (e.g.,␣ under- seedlings. Similar conditions result after planting), stand rehabilitation, and thinning. All commercial clearcutting and high-grading for the silvicultural guidelines must be evaluated from an spruce and pine components of mixedwood stands. ecological and an economic perspective, to obtain These stands are often regarded as obstacles to operational acceptance. The primary opportunities traditional silvicultural practices. In some cases, for progress through cross-discipline linkages the stands can be used for wildlife habitat or include the ecosystem effects of alternative recreation without any silvicultural rehabilitation. silviculture, biodiversity, and species interactions, However, they represent a valuable opportunity for and the enhancement of non-timber values, forest- timber and other uses because of their location on level wood supply, and mixed-species, variable- fertile sites close to mills and population centres. density yield tables. Research is required to determine the most Mixedwood management could be greatly effective approaches for regaining commercial improved in northern Ontario by the development productivity of such stands. of appropriate equipment for harvesting, Harvesting and silvicultural operations that forwarding, and site preparation. Because existing imitate natural disturbances must be encouraged to machinery is designed for large-scale clearcuts, it is maintain the species diversity and commercial unsuitable for protecting advance regeneration and productivity of the Boreal Mixedwood Forest. For site quality. Machines should be produced example, reduced soil disturbance resulting from specifically for the mixedwood forest environment, lighter equipment and improved fire protection combining strength and manoeuvrability. will decrease the viable populations of white spruce Scandinavian equipment designed for partial on mixedwood sites unless counteracted by cutting is generally too light to handle the large- silvicultural prescriptions (Bedell ). diameter aspen in northern Ontario mixedwood Herbicides should be used selectively in stands. Collaboration among the Forest mixedwood stands to supplement the manual and Engineering Research Institute of Canada, forest mechanical control of broadleaved species where companies, resource managers, and silviculture conifer maintenance or enrichment is prescribed. researchers will be essential for significant progress The use of prescribed burning should be expanded in this area. Equipment development must be because the Boreal Mixedwood Forest is a fire- paralleled by initiatives to train and retrain dependent ecosystem that would lose its character, skilled␣ operators. vigour, and diversity in the absence of fire Pre-commercial thinning or spacing of aspen (Alexander and Euler ). Fire could assist the may be required to maintain or enhance stand rehabilitation of unproductive mixedwood stands, vigour early in the rotation. In boreal mixedwoods, reduce undesirable species in naturally regenerated this early release can provide an increase in the mixedwood stands, and prepare the site for the coniferous component or accelerate the growth of establishment of natural spruce regeneration.
107 Most mixedwood stands arise from natural tolerant balsam fir. Thus, rigid stocking standards regeneration, but productive mixtures of species are less appropriate where multiple species are can be established artificially. Successful mixtures prescribed than in monocultures. The main combine faster-growing shade-intolerant species function of the standards would be to ensure that above slower-growing shade-tolerant species. the prescribed proportion of spruce is achieved. Underplanting a mid-rotation aspen stand with Stocking standards are essential for degraded spruce could be used to maintain stocking of late-successional mixedwood stands. The overmature desirable conifers. Planting patterns could be hardwoods do not provide adequate stocking of adopted that facilitate the protection of the conifer suckers, and the heavy layers of litter and humus understorey during subsequent removal of the limit the establishment of natural spruce aspen overstorey. regeneration. Shrubs and herbaceous vegetation dominate stand openings until a major disturbance Silvicultural Standards creates conditions suitable for early-successional tree species. Large silvicultural inputs are usually Close monitoring of the effectiveness of required to restore such stands to production, and silvicultural operations is required under recent accurate stocking standards are␣ important to legislation in Ontario. However, standards for evaluate the effectiveness of the␣ treatments. assessing the effectiveness of regeneration and Stocking assessments must be timed to suit the other silvicultural operations in mixedwood stands dynamics of the mixedwood stand being evaluated. are lacking. These standards cannot be imported A fifth-year assessment will be more accurate than from other jurisdictions that have different a second-year assessment for determining whether ecological, social, and economic conditions. The advance conifer growth will be overtaken by standards will be more complex for mixedwood hardwood suckers. However, earlier assessments stands than for even-aged monocultures. For allow more time for remedial action. example, a northern Ontario mixedwood stand All silvicultural standards must relate to the could contain three or four crop species, each prescriptions made for the future stand conditions. requiring different assessment criteria. Mixedwood The forest manager’s plan for the long-term silvicultural standards must also recognize the silvicultural system on each mixedwood site will importance of non-timber values. promote a distinct succession of species. Standards Traditional stocking standards can be applied for stocking and free-to-grow assessment will be where mixedwoods occur as mosaics of small pure different at each silvicultural entry during the stands. However, a lowering of the acceptable cycle. For example, an initial harvest may remove stocking levels is appropriate where regeneration is the dominant hardwoods and protect the conifer comprised of species mixtures with variable understorey, with the objective of increasing the vertical stand structure. Adjustments to standards merchantable conifer basal area to %. The can be derived from the target basal area of each corresponding stocking standards would favour component crop species and the current size of spruce and fir, based on the difficulty of promoting advance growth. However, much research is conifers on mixedwood sites. required to determine the optimum stocking of advance growth by species composition and site Some Preliminary Research Results type. Prescriptions such as spot scarification, prescribed burning, seeding, interplanting, and The strategy of the Mixedwood Silviculture underplanting would be considered where pre-cut Program at the Ontario Forest Research Institute is conifer stocking is inadequate. to form a silvicultural co-operative as a framework Full stocking of aspen regeneration can be for operational-scale adaptive research addressing assumed for Ontario boreal mixedwood stands that all aspects of mixedwood management. The supported a minimum of aspen trees per prescriptions will be based in part on the results of hectare before harvest. White birch regeneration is modified harvesting trials established during the also plentiful from stump sprouts and seedlings on last years. Some of the initial results are exposed soil. If white spruce stocking suffers under presented here to indicate the feasibility of hardwoods, it may be replaced by the more modified mixedwood management in Ontario.
108 Careful winter logging of mature mixedwood The advance regeneration sustained more wind stands removed either % or % of the damage in the clearcut than in the shelterwood cut hardwood overstorey, while protecting understorey (Table ). Mechanical damage was low in both spruce and fir. The original basal area averaged cases. The protective effect of the residual m/ha. A feller-buncher and grapple skidder overstorey is clear from Table , which reveals a worked from -m wide skid trails spaced m major increase in windthrow damage above the apart. Only the feller-buncher was permitted to cm diameter class only in clearcut stands. leave the skid trails while operating in the stands. These first-year results also reveal response There was no difference in productivity between differences in white spruce that was underplanted the treatments, averaging m/hr for harvesting or interplanted in the residual stands during the and m/hr for skidding. While this productivity spring immediately following the harvest (Table ). was commercially acceptable, it is recommended Survival, growth, and quality of spruce seedlings that the distance between skid trails be reduced to are high under both harvesting systems, but are – m to eliminate the need for off-trail significantly lower in the uncut stands. This operation by the harvester. emphasizes the benefit of controlled crown
Damage to advance conifer regeneration, recorded 1 year after different winter logging intensities in northern Ontario aspen-spruce-fir stands
Proportion of trees damaged on each treatment (%)
Damage Clearcut Shelterwood Uncut
Location Stem 33.8 23.8 17.1 Crown 1.7 0.5 0.3 Type Wind 17.4 6.5 0.7 Mechanical 2.0 2.8 0.0 Other 16.1 15.0 16.7 Extent Major 22.3 12.7 5.3 Minor 13.2 11.6 12.1
Diameter distribution of windthrown advance First-year performance of white spruce planted conifer growth, recorded 1 year after different during the spring after different winter logging winter logging intensities in northern Ontario intensities in northern Ontario aspen-spruce-fir aspen-spruce-fir stands stands
% trees windthrown Response Clearcut Shelterwood Uncut Diametera (cm) Clearcut Shelterwood Survival (%) 97.3 97.9 83.2 0.00–4.00 5.06 1.45 Height growth (cm) 10.8 9.6 7.6 4.01–8.00 9.09 12.00 Quality index 60.5 59.3 44.8 8.01–12.00 30.38 15.28 12.01–16.00 27.87 18.80 16.01 + 72.73 23.33 a Diameters measured at . m above ground level.
109 opening for conifer establishment. It is anticipated by considering mixedwoods on a landscape scale. that the shelterwood system will create the There is an urgent need for predictive tools to optimum regeneration environment by providing assist mixedwood resource managers (Burkhart and increased seedling growth, while reducing frost Tham ). Models of growth and yield for damage and moderating hardwood suckering. mixed-species stands are essential for making Based on these early results, a two-stage sound management decisions. The infinite variety shelterwood is recommended to enhance conifer of possible species mixtures, coupled with the regeneration in northern Ontario mixedwood range of environmental conditions under which stands. The entries should be separated by years mixtures might be grown, necessitates a modelling to permit the establishment of seeded or under- approach. Models supporting ecosystem-based planted conifers and the wind acclimation of silviculture require information on the structure advance conifer regeneration. and natural development of mixed forests. There is also a need for detailed site classifica- High-priority Information Needs tion and mapping to assist resource managers in selecting sites for mixedwood prescriptions. In A formal survey of mixedwood stakeholders in Ontario, groundwork for this information base has Ontario revealed important issues for management been completed through the forest ecosystem and research (Weingartner and MacDonald ). classifications (Jones et␣ al. ; Sims et␣ al. ). The following areas rated highly regarding the need Subsequent initiatives (Uhlig and Baker ) will for acquiring and applying knowledge: refine and supplement this knowledge. • modified cutting to obtain natural regeneration • impacts of harvesting Information Transfer and Training • ecosystem structure and function Mixedwood management in Ontario will be • ecologically sensitive site preparation techniques successful if a proactive mixedwood approach is • wider species utilization adopted by policy makers and appropriate • mixed-species, variable-density yield tables management techniques are available to resource • adaptive artificial regeneration managers. Thus, the development and transfer of (e.g.,␣ underplanting) applied knowledge must be pursued without delay. • stand improvement and rehabilitation Considerable time and patient training are • decision support systems. needed for equipment operators to develop In the context of mixedwood forestry, applied technical expertise and an ethic for the residual ecology and silviculture were higher research and forest. Respect for natural diversity and adaptation development priorities than physiology, genetics, to successional tendencies must be emphasized forest protection, or wood science. Fish and because these attitudes have not been prominent in wildlife habitat management was considered the traditional forestry operations. In northern most important discipline to link with mixedwood Ontario, most equipment operators have been silviculture. Site classification, economics, biodi- trained in the context of rapid timber production versity, social issues, and recreation were viewed as on large-scale clearcuts. It is a positive sign for somewhat important linkages, and issues related to mixedwood management that some Ontario climate change were considered to be least important. companies have invested in cut-to-length Considering the survey results, the Mixedwood harvesting and forwarding systems designed for Silviculture Program at the Ontario Forest partial cutting with reduced impacts on the site Research Institute will emphasize four key areas: and residual stand. entire-rotation silvicultural prescriptions and The complexity of mixedwood forests requires performance standards, ecosystem processes and considerable skill to manage stands and predict successional pathways, landscape effects of outcomes. Thus, training courses, applied workshops, mixedwood management, and growth and yield. and demonstration areas must be planned to allow Operational trials will be designed to permit researchers, resource managers, and equipment effective application and refinement of silvicultural operators to exchange information␣ regularly. prescriptions. Non-timber values will be addressed
110 Conclusions agencies, universities, and non-timber interest groups. The Boreal Mixedwood Forest is becoming Attitude barriers and knowledge gaps are among more prominent in the research and development the obstacles to the implementation of mixedwood activities of the Ontario Ministry of Natural silviculture in northern Ontario. Creative solutions Resources and Natural Resources Canada, Ontario to these challenges will require a focused effort by Region. Co-operation between these two silviculturists. The opportunity to realize multiple organizations is helping to ensure an efficient benefits from an underutilized resource justifies a approach to addressing mixedwood issues in the provincially co-ordinated mixedwood initiative. province. A Canada-wide network of mixedwood Support for such a program is anticipated because stakeholders should be established to facilitate the defining characteristics of mixedwood manage- communication and minimize duplication of ment, such as careful harvesting and conservation effort. Applied information must be shared of diversity, are acceptable to an increasingly between jurisdictions because public expectations critical general public. Furthermore, preliminary are out-pacing current improvements in research results support the biological and mixedwood management practices. economic feasibility of proactive mixedwood management in northern Ontario. References Inventory systems and standards for stocking and free-to-grow attainment must be refined to Alexander, M.E. and D.L. Euler. . Ecological accommodate species mixtures. Appropriate role of fire in the uncut boreal mixedwood incentives and training programs for wood-using forest. In Boreal mixedwood symposium. industries would improve the protection of Proc. symp. Sept. –, , Thunder Bay, conifer␣ understoreys, residual overstoreys, and Ont. R.D. Whitney and K.M. McClain (edi- soil␣ stability. tors). Can. For. Serv., Sault Ste. Marie, Ont. The status of mixedwood silviculture in Ontario COJFRC Symp. Proc. O-P-. pp. –. could be advanced most effectively by refining Armson, K.A. . The boreal mixedwood for- harvest systems to perpetuate healthy mixed- ests of Ontario: past, present, and future. In species crops. Successful mixedwood management Management and utilization of northern also requires more serious attention to mid- mixedwoods. Proc. symp. Apr. –, , rotation silviculture, such as controlling stand Edmonton, Alta. J.K. Samoil (editor). Can. density and quality. The costs of such operations For. Serv., Edmonton, Alta. Inf. Rep. NOR-X- should be balanced against the reduced . pp. –. regeneration costs associated with mixedwood Baker, J.B. . Ecosystem management research management. The inherent capacity of in the Ouachita Mountains: pretreatment mixedwoods to produce a wide range of resource conditions and preliminary findings. Proc. values demands a high level of interaction between symp. Oct. –, , Hot Springs, Ark. disciplines and maximum use of technology such U.S. Dep. Agric. For. Serv. South. For. Exp. as geographic information systems ₍GIS₎ and Sta., New Orleans, La. Gen. Tech. Rep. SO- decision support systems. The transition to more . p. proactive mixedwood management can be Bedell, G.H.B. . Variation in growth, regen- facilitated through carefully targeted eration, and quality of balsam fir, jack pine demonstration and education programs. and spruce within their natural ranges. In The opportunities for sustainable supplies of Proc. three symposia on wood quality. Ont. multiple resource values from Ontario’s Boreal Dep. Lands and Forests, Toronto, Ont. Res. Mixedwood Forest are only beginning to be Rep. No. . pp. –. realized. However, the magnitude and urgency of Brace Forest Services. . Protecting white the challenges will require a co-ordinated spruce understories when harvesting aspen. management and research effort through For. Can. and Alta. For. Serv. Edmonton, partnerships between resource managers, wood- Alta. Canada-Alberta Partnership Agreement using industries, federal and provincial research in For. Rep. No. . p.
111 Brooks, D.J. and G.E. Grant. . New perspec- Jones, R.K., G. Pierpoint, G.M. Wickware, J.K. tives in forest management: background, sci- Jeglum, R.W. Arnup, and J.M. Bowles. . ence issues, and research agenda. U.S. Dep. Field guide to forest ecosystem classification Agric. For. Serv., Pac. NW For. Exp. Sta., for the Clay Belt, Site Region e. Ont. Min. Portland, Oreg. Res. Pap. PNW-RP-. p. Nat. Resour., Toronto, Ont. p. Burkhart, H.E. and A. Tham. . Predictions Koven, A. and E. Martel. . Reasons for from growth and yield models of the per- decision and decision. Class environmental formance of mixed-species stands. In The assessment by the Ministry of Natural ecology of mixed-species stands of trees. Resources for timber management on Crown M.G.R. Cannell, D.C. Malcolm, and P.A. lands in Ontario. Ont. Environ. Assessment Robertson (editors). Blackwell Scientific Pub- Board, Toronto, Ont. No. EA--. p. lications, London, U.K. pp. –. Lieffers, V.J. and J.A. Beck. . A semi-natural Chan, S.S., S.R. Radosevich, and J.D. Walstad. approach to mixedwood management in the . Carbon allocation, morphology and prairie provinces. For. Chron. :–. physiology of Douglas-fir (Pseudotsuga McClain, K.M. . Definition and distribution menziesii Mirb. Franco) and red alder (Alnus of the boreal mixedwood forest in Ontario. rubra Bong.) saplings in response to light In Boreal mixedwood symposium. Proc. and soil moisture availability. In Vegetation symp. Sept. –, , Thunder Bay, Ont. competition and responses. Proc. rd annual R.D. Whitney and K.M. McClain (editors). vegetation management workshop, Feb. –, Can. For. Serv., Sault Ste. Marie, Ont. COJFRC , Vancouver, B.C. E. Hamilton and S. Symp. Proc. O-P-. Watts (compilers). Can. For. Serv. and B.C. MacDonald, B. and D. Weingartner. . Defini- Min. For. and Lands, Victoria, B.C., FRDA tion of the Boreal Mixedwood Forest. Ont. Rep. No. . pp. –. Min. Nat. Resour., Toronto, Ont., Boreal Debyle, N.V. . Integrated use in mixedwood Mixedwood Notes No. -. forests - a challenge. In Northern mixedwood Norokorpi, Y. . Natural structure and devel- ’. Proc. symp. Sept. –, , Fort St. opment of forests as the basis for alternative John, B.C. A. Shortreid (editor). For. Can., silvicultural methods in northern Finland. Victoria, B.C. FRDA Rep. No. . pp. –. Swed. Univ. Agric. Sci. Umea, Sweden. Rep. Forest Research Advisory Council of Canada. No. . p. . Forest research priorities in Canada, Opper, M.A. . Present utilization of species . For. Chron. :–. in the boreal mixedwood forests in Ontario: Gordon, A.G. . Nutrient cycling dynamics in industrial perspective. In Boreal mixedwood differing spruce and mixedwood ecosystems symposium. Proc. symp. Sept. –, , in Ontario and the effects of nutrient remov- Thunder Bay, Ont. R.D. Whitney and K.M. als through harvesting. In Resources and dy- McClain (editors). Can. For. Serv., Sault Ste. namics of the boreal zone. Assoc. Can. Univ. Marie, Ont. COJFRC Symp. Proc. O-P-. for Northern Studies. Ottawa, Ont. pp. –. pp.␣ –. Hearnden, K.W., S.V. Millson, and W.C. Wilson. Schuler, T.M. and F.W. Smith. . Effect of spe- . A report on the status of forest regen- cies mix on size/density and leaf area rela- eration. Ont. Min. Nat. Resour. Indep. For. tions in southwest pinyon/juniper woodlands. Audit Comm., Toronto, Ont. p. For. Ecol. Manage. :–. Irland, L.C. and D.I. Maass. . Regional per- Sims, R.A., W.D. Towill, K.A. Baldwin, and G.M. spective: forest conditions and silvicultural Wickware. . Field guide to the forest trends in the northeastern USA. For. Chron. ecosystem classification for northwestern :–. Ontario. Ont. Min. Nat. Resour., Toronto, Ont. p.
112 Smith, S.M. . What did you expect? In Man- Weingartner, D.H. . Spacing and thinning in agement and utilization of northern aspen and mixedwood: some thoughts, theo- mixedwoods. Proc. symp. Apr. –, , ries and observations. In Space to grow: Edmonton, Alta. J.K. Samoil (editor). Can. spacing and thinning in northern Ontario. For. Serv. Edmonton, Alta. Inf. Rep. NOR-X- Proc. symp. June –, , Sault Ste. . pp. –. Marie, Ont. V.F. Haavisto, C.R. Smith, and C. Spectranalysis, Inc. . Development of a spa- Mason (editors). For. Can. and Ont. Min. tial forest data base for the eastern boreal Nat. Resour., Sault Ste. Marie, Ont. Joint forest region of Ontario. For. Landscape Rep. . pp. –. Ecol. Prog., Ont. Min. Nat. Resour., Sault Weingartner, D.H. and G.B. MacDonald. . A Ste. Marie, Ont., For. Fragmentation and survey of mixedwood research and develop- Biodiversity Proj. Rep. No. . p. ment needs in Ontario. Ont. Min. Nat. Suffling, R., C. Lihou, and Y. Morand. . Con- Resour., Sault Ste. Marie, Ont. For. Res. Inf. trol of landscape diversity by catastrophic Pap. No. . p. disturbance: a theory and a case study of fire Yang, R.C. and R.D. Fry. . Natural succession in a Canadian boreal forest. Environ. Man- following harvesting in the Boreal Mixed- age. : –. wood Forest. In Boreal mixedwood sympo- Uhlig, P.W.C. and J. Baker. . Provincial Eco- sium. Proc. symp. Sept. –, , Thunder logical Land Classification Program prospec- Bay, Ont. R.D. Whitney and K.M. McClain tus. Ont. Min. Nat. Resour., Sault Ste. Marie, (editors). Can. For. Serv., Sault Ste. Marie, Ont., For. Res. Inf. Pap. No. . p. Ont. COJFRC Symp. Proc. O-P-. pp. –.
113 Investigations of Planting White Spruce under a Trembling Aspen Canopy
DAVID TANNER, S. CRAIG DELONG, AND ANDREA EASTHAM
Abstract tremuloides) and white spruce (Picea glauca) were very common throughout the boreal forest. In Boreal forests are facing extreme pressure to certain areas and especially near human settle- produce conifer and hardwood products as well as ments, repeated disturbances related to forestry, to provide other resource uses. Regulations and agriculture, and range management have removed silvicultural treatments have been largely aimed at the spruce seed source, resulting in large areas of promoting white spruce after clearcutting. This is pure aspen. Underplanting aspen stands with white accomplished through plantation establishment spruce may generate stands similar to natural and management, usually including mechanical site mixedwoods. If implemented, this technique could preparation and herbicides. Alternative provide a socially acceptable alternative to management strategies are required to satisfy clearcutting and single-species management. timber and other societal demands. Underplanting This study was initiated to explore the potential aspen stands with white spruce appears to be a for underplanting aspen stands with white spruce. viable alternative management strategy. Several In this silvicultural system, –-year-old aspen benefits may accrue using this strategy. These stands, which have already undergone significant include: reducing site preparation and vegetation natural thinning, would be underplanted with management costs; reducing root rot problems; white spruce without treatment to reduce aspen reducing growing season frost damage; retaining stocking. After approximately years, the trees to provide protection and thermal cover for overstorey aspen would be removed while wildlife and to reduce visual resource impacts; and protecting the understorey spruce. It is improving nutrient status of white spruce. hypothesised that aspen would still regenerate by root suckering in gaps between the young spruce, Introduction resulting in a mixedwood stand. After another – years, when spruce and aspen reach harvestable Boreal forest ecosystems in British Columbia are size, the site would be clearcut. Aspen would again facing extreme pressure to produce conifer and regenerate and the site would be ready for hardwood products as well as to provide areas for underplanting in another – years (Figure ). agriculture, range, recreation, and wildlife. A recent This project has been pursued in three phases. timber supply analysis for the Dawson Creek Phase investigated previously underplanted aspen Timber Supply Area in British Columbia indicated stands in Alberta. In phase , we determined the that the current Allowable Annual Cut ₍AAC₎ for most suitable stands for underplanting by hardwoods can not be supported (B.C. Ministry of investigating the light and moisture availability Forests ). Therefore, management to produce under trembling aspen stands that varied in basal both coniferous and deciduous volume on the area, density, and moisture regime. The criteria same area of land (i.e., mixedwood management) developed in phase enabled the selection of three may be advantageous, especially if it can be aspen stands that were underplanted in phase . To demonstrated that total volume yields will be derive meaningful comparisons, we also selected greater than for pure coniferous or pure three clearcuts that approximated the aspen deciduous␣ stands. underplant sites in soil texture, slope position, Historically, mixed stands of aspen (Populus aspect, and biogeoclimatic site unit.
114 points within a . ha area, on a × m Aspen Spruce sampling grid. At each grid position, sets of instantaneous measurements were taken, one above 1st harvest 2nd harvest all understorey vegetation and the other at a height 100 of cm above ground level, approximating the 80 height of a planted seedling. Each measurement set
Percent yield Percent 60 40 represented an average of instantaneous readings underplanting 20 taken in the four cardinal directions. All 0 000 020 040 060 080 100 120 140 160 measurements were taken during clear sky Time (years) conditions, between and PST, during the period of peak vegetative development (July – Schematic showing the proposed management August ). An overall estimate of the light level regime for spruce planted under pure aspen stands. beneath the canopy was calculated using the mean PPFD at each grid position, and this was related to basal area, density, and moisture regime infor- Characterizing Micro-environment of mation for each stand (Figure ). Basal area for Aspen Stands each grid position was calculated using a relascope and then averaged across all grid positions to Sample sites were selected within a km radius obtain stand estimates. Stand density was of Dawson Creek, B.C. (°' N, °' W). All determined by conducting stem counts and sites were located in the Peace variant of the moist calculating the mean of four × m quadrants warm Boreal White and Black Spruce subzone randomly located within the sample area. These (BWBSmw), which has a mean annual precipitation data demonstrate that, in general terms, light of mm and a mean annual temperature of availability decreases as basal area, density, or .°C. Aspen plot data collected for a classification moisture availability increases (Figure ). of the seral aspen ecosystems of the BWBSc Continuous PPFD measurements were made at (DeLong ) were used as a starting point to low, medium, and high basal area mesic sites in the select the sites. Sampling sites were chosen to summers of and . Hourly averages were represent a range of basal areas, densities, and recorded at each site from May to September moisture regimes in –-year-old aspen stands. A using quantum sensors (LIS, LI-COR Inc., Lincoln, total of five sites were selected for study (Table ). Neb.) mounted on stands and connected to During the summers of and , dataloggers (CR, Campbell Scientific Co., Logan, photosynthetic photon flux density ₍PPFD₎ Utah). Six sensors were located in each stand. Each measurements were obtained along designated sensor was placed at the centre of a m radius area transects at each of the five selected sites using a that was kept free of understorey vegetation. The hand-held integrating radiometer (Sunfleck basal area and density associated with each sensor Ceptometer, Decagon Devices Inc., Pullman, location was assessed by measuring all trees within Wash.). Measurements were made systematically at an m radius, tree-centred plot. These data, when multiplied by , yielded basal area or density Combination of basal area and moisture regime values per hectare. attributes for sampled sites Data collected from the quantum sensors indicated that low basal area stands received more light than moderate or high basal area stands on Moisture regime sunny days, but that light reception on cloudy days Relative basal area Submesic Mesic Subhygric was not markedly different across the range in basal area (Figure ). To determine the effects of Low x measured levels of light on seedling growth, the relationship between light levels and white spruce Medium x x x seedling photosynthesis must be determined. Work High x by G.R. Lister and W.D. Binder (B.C. Ministry of Forests Experimental Project , unpublished
115 800 800 800 Above 700 700 700 Below
) 600 600 600 -1 • s
-2 500 500 500
400 400 400 mol • m µ 300 300 300
PPFD ( 200 200 200
100 100 100
0 0 0 Low Moderate High Low Moderate High Submesic Mesic Subhygric Relative density Relative basal area Moisture regime
Amount of available light above and below understorey vegetation assessed near solar noon (between 1100 and 1400 PST) for different stand conditions (columns indicated by different letters are significantly different at P < 0.05).
1000 data) enabled construction of a generalized curve ) 900 a High -1 800 Moderate for relative apparent photosynthesis of white spruce • s
-2 700 as a function of PPFD. This curve is described by the Low 600 following equation: 500 (x – )
mol • m 400 y =
µ 300 [a + b(x – )] 200 where y = % of relative apparent photosynthesis,
PPFD ( 100 x = light intensity [PPFD], 0 700 900 1100 1300 1500 1700 1900 a = . , and b = .. Continuous light measurements collected under
1000 aspen canopies were converted to relative apparent
) 900 b photosynthesis using this equation. These estimates
-1 High 800 were then totalled and compared with the relative • s Moderate -2 700 apparent photosynthesis that would take place at 600 Low 500 maximum (i.e., unshaded) PPFD levels. From these
mol • m 400 data, we were able to develop a regression µ 300 relationship between the percentage of maximum 200 potential photosynthesis and density (Figure ).
PPFD ( 100 0 The relationship proved to be very strong for the 700 900 1100 1300 1500 1700 1900 range of stand conditions measured (r = .). Time of day (PST) In addition to light data collected from each stand, soil moisture potential was assessed using Contrasting light availability a) on a sunny day gypsum moisture blocks. At each site, five groups and b) a cloudy day under aspen canopies of of three soil moisture blocks were located at a different relative basal areas. depth of cm below the humus/mineral soil interface at the apices of an equilateral triangle with -cm sides. Measurements were obtained using a hand-held moisture meter at approximately
116 -week intervals from June to August . 0.00 Although soil moisture data were collected during -0.20 an extremely dry summer ₍₎, soil moisture -0.40 Subhygric potentials for the majority of the growing season -0.60 Submesic -0.80 did not indicate more than moderate moisture Mesic stress (Figure ). Therefore, available soil moisture -1.00 did not appear to be a significant constraint to -1.20 underplanting aspen stands. -1.40
Soil moisture potential (MPa) -1.60 150 160 170 180 190 200 210 60 Day of year Y = 55.35 – 0.015X N = 18 R2 = 0.87 Soil moisture potential throughout the growing 50 season on three circummesic sites. Lines on the graph at -0.20 and -1.0 MPa correspond to 40 moderate and high moisture stress, respectively.
+␣ operational, within and between aspen 30 understorey and clearcut environments. • To compare the survival and growth of + 20 stock, cultured under shade cloth versus cultured in full sun, and planted under an
10 aspen␣ canopy. • To compare the survival and growth of shade- % of maximum potential photosynthesis adapted stock planted on brushed microsites to 0 000 500 1000 1500 2000 stock planted on brushed and screefed Density (stems per hectare) microsites under an aspen canopy. • To compare the seedling micro-environment Regression demonstrating the percentage of under an aspen canopy to that in a clearcut. available light throughout an entire growing The overall design of the experiment is a season, as continuously sampled by a LI-COR randomized, incomplete-block, split-plot design. quantum sensor, as a function of density Each site constitutes a statistical block and there of␣ aspen. are three blocks per main plot treatment. Table summarizes the various treatments. Underplanting Aspen Stands Summary of treatments Collected light and moisture information indicated that the micro-environment under aspen stands was suitable for spruce growth, and Main plot Split plot also generated criteria useful in selecting sites for underplanting. The data suggested that we would Clearcut • 1 - 1+0 psb research (full sunlight) be most successful with mesic or subhygric sites • 2 - 1+0 psb operational of low to moderate basal area. Using these data, Aspen • 1 - 1+0 psb research (full sunlight) stock grown specifically for this project were underplant • 2 - 1+0 psb operational planted under selected aspen canopies and in • 3 - 1+0 psb research (66% full clearcuts. Three clearcuts and three aspen underplant sites were located in April and May of sunlight)
. The objectives of the study are as follows: • 4 - 1+0 psb research (66% full sunlight) + screefing • To compare the survival and growth of two white spruce stock types, + research and
117 Environmental Measurements Live Dead One aspen understorey site and one clearcut site each had positions selected to record micro- Underplant Clearcut environmental information. Locations were 100 selected to cover the range of stand conditions Percent 80 60 (aspen understorey) or expected vegetative 40 development (clearcut). Each position was 20 equipped with a LI-COR quantum sensor, gypsum 0 soil moisture blocks, and thermistors for B1 B2 PUG B3 B4 OH Site measuring air and soil temperatures. Quantum sensors provided continuous PPFD information as in the studies described earlier. Air temperature Percentage of seedling survival after 2 years by thermistors were mounted at cm above ground main plot treatment and individual site (B1, B2, level and soil temperature thermistors were buried PUG, B3, B4, and OH are individual sites, or cm below the humus/mineral soil interface. All replicates of the main plot treatment). thermistors recorded daily minima, maxima, and means. Three soil moisture blocks were located at each micro-environmental position. To provide Normal Frost-damaged continuous soil moisture potential information, one of the moisture blocks was connected directly to a datalogger. In addition to this equipment, the 1993 1994 100 understorey and clearcut sites were outfitted with a Percent 80 relative humidity sensor, which recorded average 60 hourly temperature and relative humidity. 40 20 0 Two-year Results Underplant Clearcut Underplant Clearcut
The project is still in its infancy and any results should not be construed as definitive. However, Percentage of frost-damaged seedlings in 1993 and 1994 by main plot treatment. several interesting trends have been observed. Second-year survival data indicate no significant differences between main plot treatments (Figure␣ ). Seedling mortality appears to be a in the fall, and with greater frequency throughout function of microsite differences (poor planting the growing season. All of these factors could location) or unexpected events (trampled by cattle) contribute to frost damage of spruce seedlings. rather than of treatment differences. The percentage of seedlings assessed as chlorotic One of the factors that was expected to be was also observed to be greater in the clearcut significantly different between main plot treat- environment (Figure ). One of the reasons for this ments was the occurrence of frost damage. White could be nutrient deficiency, but this hypothesis spruce in plantations in the boreal forest can will remain unanswered until years after experience as much as % frost damage outplanting, when foliar analysis will be done. (L.␣ Herring, B.C. Ministry of Forests, pers. comm., Seedlings in the aspen understorey, where chlorosis ). Although our data do not reflect such a high was nearly absent (< %), may be able to obtain percentage of damage, frost damage was much inputs from nutrient-rich aspen litter. In contrast, greater in the clearcut environment in both seedlings grown in clearcuts do not have the and (Figure ). Micro-environmental data same␣ availability of nutrients as they are often confirms these observations (Figure ). In both planted in microsites with negligible organic and , frost events were more common and horizon development. severe in clearcut environments. Also, frost events The two-year results of tree growth and vigour in the clearcut occurred later in the spring, earlier were somewhat surprising as seedlings planted in
118 aspen understorey sites out-performed seedlings reflect the nursery culture from the previous year, planted in clearcuts. We expected that clearcut and values reflect growing conditions. environments, experiencing greater light levels, Values for underplant conditions are higher would lead to seedlings with superior and than those for the clearcut. Analysis of variance accelerated growth. demonstrated that these differences were Figure depicts the number of interwhorl buds statistically significant between these main plot by site and main plot treatment. Values for treatments (P = .) at α = . (see Table ).
Underplant Clearcut 15 15 10 10
5 1993 5 0 0 -5 -5 -10 -10 139 152 165 178 191 204 217 230 243 256 141 154 167 180 193 206 219 232 245 258 C at 15 cm) °
15 15
Temperature ( Temperature 10 10
5 1994 5 0 0 -5 -5 -10 -10 131 146 161 176 191 206 221 236 251 266 131 144 157 170 183 196 209 222 235 248 261 Day of year Day of year
Mean minimum growing season air temperatures for main plot treatments in 1993 and 1994.
20 1993 1994 Clearcut Underplant Normal Chlorotic
100 10
Percent 80 60 40 20
Number of interwhorl buds 0 0 Underplant ClearcutClearcut B1 B2 PUG B3 B4 OH Site
Percentage of chlorotic seedlings in 1994 by Number of interwhorl buds assessed at the end main treatment. of each of the first two growing seasons by main plot treatment and individual site.
119 Similarly to the number of interwhorl buds, 12 annual height increment for was greater in the R-100 Underplant O-100 9 underplant environment, and across all sites R-66 (Figure ). Analysis of variance again R-66-S 6 demonstrated significant differences between main Clearcut plot treatments (P = .) at α = . (Table ). In contrast to main plot differences, analysis of 3 height increment by stock type (Figure ) did not Height increment (cm) demonstrate any appreciable differences. Two-year 0 results indicate that shade-adapted stock offer no B1 B2 PUG B3 B4 OH height growth advantages over operational stock. Site
Second-year annual height increment of stock 12 types by main plot treatment and individual site (R-100 = research stock grown in full sun; O-100 = operational stock grown in full sun; 9 Underplant R-66 = research stock grown in 66% full sun; and R-66-S = research stock grown in 66% full 6 Clearcut sun and planted in screefed microsites).
3 Height increment (cm) 0 B1 B2 PUG B3 B4 OH Site
Height increment in 1994 by main plot treatment and individual site.
ANOVA table comparing the number of interwhorl buds in 1994 in clearcut and underplant environments
Source df Mean square F-ratio P
Main plot treatment 1 189.78 13.215 0.022 Error mean square site 4 14.361 – –
ANOVA table comparing 1994 annual height increment in clearcut and underplant environments
Source df Mean square F-ratio P
Main plot treatment 1 1354.78 15.063 0.018 Error mean square site 4 89.941 – –
120 Summary Preliminary results indicate that underplanting aspen stands with white spruce offers an acceptable Other potential advantages to this approach exist alternative to growing white spruce in clearcuts. that are not explicitly tested in the research design. Two-year results of seedling growth and micro- Providing and maintaining tree cover, especially climate data indicate that in some instances aspen coniferous species (which provide thermal cover), understoreys may provide better conditions for should prove beneficial to wildlife. Maintaining seedling growth. Because the data collected are tree cover should also improve the visual aesthetics preliminary, caution should be exercised before of the site, especially after the first harvest when definitive conclusions are drawn. It is feasible that the coniferous understorey will be preserved. long-term data may contradict earlier findings. Competing vegetation development is expected to Also, we wish to expand the data set of seedlings be less in the understorey. Assessment of growing in clearcut environments. competing vegetation under aspen overstoreys Planned future data collections on the planted indicated that it was not a significant competitor at white spruce seedlings will help to determine if the any time in the growing season. This is in accord results collected to date are indicative of general with other studies conducted in the boreal forest trends. These collections include more growth (Lieffers and Stadt ). Competing vegetation analyses (including stem diameters), examination development in understorey environments is of physiological response (photosynthetic important to assess because species such as efficiency, carbon allocation), and foliar analysis to bluejoint (Calamagrostis canadensis) can be determine seedling nutrient status. significant and severe competitors to planted coniferous seedlings in clearcut environments. References Another advantage to this underplanting approach is the potential for reducing losses to B.C. Ministry of Forests. . Dawson Creek TSA tomentosus root rot (Inonotus tomentosus). Because timber supply analysis. Victoria, B.C. p. white spruce is absent from the site for DeLong, C. . A field guide for identification approximately – years every second rotation, and interpretation of seral aspen ecosystems the potential for inoculum build-up in susceptible of the BWBSc, Prince George Forest Region. hosts is reduced, and the inoculum may be B.C. Min. For. Land Manage. Handb. No. . temporarily eliminated. This is important in a Lieffers, V.J. and K.J. Stadt. . Growth of system where tomentosus root rot has been known understory Picea glauca, Calamagrostis to affect up to % of the spruce within a given canadensis, and Epilobium angustifolium in area (R. Reich, B.C. Ministry of Forests, pers. relation to overstorey light transmission. comm., ). Can. J. For. Res. :–.
121 Vertical Stratification in Some 50-year-old Mixed-species Stands in the Interior Cedar–Hemlock Zone
IAN R. CAMERON
Abstract differences, such as genetic variation and microsite conditions, can contribute to the development of Patterns of vertical stratification were analyzed in size hierarchies (Weiner␣ ). Divergent growth some -year-old, mixed-species stands of the patterns are accentuated by competition and Interior Cedar–Hemlock zone, Kamloops Forest chance events (Waller␣ ; Smith␣ ). Foresters Region. Fixed-area plots were located in stands describe the process as “differentiation into crown with different species composition. Western classes” (Smith␣ ). redcedar, western hemlock, and Douglas-fir were Mixed-species stands can have greater variation the most common species. More than % of the in vertical structure than monocultures. If the plots supported four or more species. Analysis of constituent species have intrinsic differences in vertical stratification showed that western white height-growth pattern and shade tolerance, the pine and Douglas-fir tend to be the tallest trees stand may become stratified in height by species when present. Hybrid spruce, western redcedar, (Larson␣ ). When the differences between and western hemlock dominated the upper stratum species are large, the stand may appear to have only when the other species were not present. layers of foliage, with a separate species dominating each layer or stratum (Smith␣ ). Stratification is Introduction a common phenomenon in mixed-species stands of the boreal, temperate, and tropical regions The forest stands of the Interior Cedar–Hemlock (Oliver␣ ). biogeoclimatic zone or ICH (Lloyd et␣ al.␣ ) have While the terms “differentiation” and regenerated following large, stand-replacement “stratification” both describe variation in height- fires (Johnson␣ et␣ al.␣ ; Parminter␣ ). The growth rate, it is useful to reserve “differentiation” stands are predominantly single-cohort, mixed- for variation within species and to reserve species stands (Oliver and Larson␣ ), although “stratification” for variation between species (Oliver remnants are common. With native tree species, and Larson␣ ). The vertical stratification of a the ICH supports a wide variety of stand structures. mixed-species stand might then be described by The numbers and proportions of the constituent comparing the heights of the tallest trees of each species vary widely, even for stands initiated by the species and ignoring the differentiation within same disturbance. General patterns of species species. This paper uses that approach to provide composition have been identified and related to an overview of observed patterns of stratification elevation, aspect, and moisture regime (Lloyd in natural, fire-origin stands at approximately et␣ al.␣ ). Studies of the structure and dynamics years of age. of these stands have begun only recently. This is a preliminary analysis of data collected The vertical organization of a stand is a product during the summer of␣ as part of an ongoing of the many factors that influence tree-to-tree study of stand dynamics in the ICH. The main variation in height growth. These include species project is a retrospective study of mixed-species characteristics and differences in establishment age. stand dynamics that uses both chronosequences Even single-species plantations, however, usually and stand reconstruction techniques. Some of the develop a vertical hierarchy. Studies in plant plots described in this paper were subsequently communities have shown that seemingly subtle selected for stand reconstruction. The stem analysis
122 measurements from reconstruction were not That is, Pb was defined as the predominant height available for this preliminary analysis. Those of plot b based on the tallest trees per hectare
measurements will provide a better picture of without regard to species. Sab was defined as the height-growth patterns and changes in mean height of the tallest trees per hectare of stratification over time. species a in plot b. The current stratification of each plot was described by expressing the Methods predominant height of a component species (S) as a proportion of the predominant height of the plot Sample stands were selected from within the (P). That is: ICHmw and ICHmw subzones in the Kamloops S Forest Region (Lloyd et␣ al.␣ ). I attempted to R = x ab ab P identify all stands (inventory polygons) that were b between and years old (Inventory Age Classes where: Rab = the relative predominant height of and ) by cross-referencing forest cover maps species a in plot b with biogeoclimatic subzone maps. The initial Sab = the predominant height of species a in sampling was conducted in stands easily accessible plot b
by road. The ecosystem association, age, species Pb = the predominant height of plot b composition, and stratification were assessed during a reconnaissance visit. Geographic locations and ages of plots Two temporary sample plots were established in each stand. The nearest road sign (kilometre mark) Location No. of plots Age of dominants (yr) was used as a reference point. At each location, two circular plots were established at distances of m and m from the road edge, on a bearing Sugar Lake 8 54 perpendicular to the direction of the road. Plot size Cayenne Creek 8 40 varied with stand density so that at least trees in Harbour Lake 18 52 the dominant, codominant, and intermediate crowns classes would be included. The plots varied Craigallechie 10 49 in size between . m radius (. ha) and . m radius (. ha) in even multiples of . ha. All trees greater than . m in height (living and Results dead) were numbered and measured. Tree measurements included diameter-at-breast-height Species Composition In total, tree species were (dbh) and height. In each plot we selected a observed in the plots: western white pine (Pinus minimum of three trees that spanned the range of monticola), lodgepole pine (Pinus contorta var. tree heights and measured their heights with latifolia), interior Douglas-fir (Pseudotsuga conventional angle-distance methods using a menziesii var. glauca), paper birch (Betula clinometer and measuring tape. The heights of all papyrifera), black cottonwood (Populus␣ trichocarpa), other trees in the plot were estimated from ocular trembling aspen (Populus tremuloides), hybrid comparisons with the measured trees. Increment white spruce (Picea engelmannii x glauca), cores were collected from the largest trees of each subalpine fir (Abies lasiocarpa), western hemlock species. In total, plots were sampled. The (Tsuga heterophylla), western redcedar (Thuja number of plots and mean age of dominant trees plicata), and willow (Salix sp.) The overall species in each geographic location are summarized in composition is shown in Table . Western redcedar, Table . western hemlock, and Douglas-fir are the species Predominant height—the mean height of the that occur most frequently in this data set. tallest trees per hectare—was calculated for All of the plots were located in mixed-species each plot and for each species within each plot. stands. Thirty-seven of the plots (%) had four or
Predominant height, based on the tallest trees per hectare, is slightly greater than conventional top height, which is based on the trees per hectare of largest dbh.
123 more species. Considering only the upper canopy the tallest white pines were almost always the trees (taller than % of predominant height), tallest trees in the plot. Douglas-fir generally were plots (%) had three or more species. Only five the tallest trees in the absence of white pine. plots had a single species in the upper canopy. Hybrid spruce, western redcedar, and western hemlock dominate the upper stratum only when Species composition (by basal area) over all the other species are not present. sample plots Discussion Species Percent basal area Larson ₍₎ identified four types of height-growth Western redcedar 26 pattern: asymptotic, sigmoid, linear-fast, and linear-slow. These patterns, together with the Western hemlock 22 information from shade tolerance tables Douglas-fir 18 (Baker␣ ), can be used to group species into guilds (Ashton␣ ) of similar developmental Western white pine 9 characteristics. Paper birch 9 Table shows such a grouping for the ICH Trembling aspen 5 species. In the absence of stem analysis information, the rate and pattern attributes were Hybrid white spruce 5 inferred from other sources. For instance, relative Lodgepole pine 4 predominant height was substituted for height- growth rate to distinguish between “fast” and Black cottonwood <1 “moderate.” This is reasonable only because the Willow <1 stands sampled in this study are all approximately Subalpine fir <1 the same age. The shape attributes (e.g., asymptotic or linear) are included to distinguish the characteristic, asymptotic shape of the birch and Vertical Stratification The ranking of species by aspen growth from that of the other species. relative predominant height (R) is shown in The guilds offer an initial prediction of Table␣ . White pine has the highest values of R. stratification for different species mixtures at age This means that if the plot contained some white . Mixtures composed of members of the same pine trees—other than advance regeneration—then guild are not expected to exhibit marked stratification.
Relative predominant height by species
Species Height (m) Number of plots Standard error
Western white pine 100 17 1.5 Douglas-fir 93 30 2.6 Trembling aspen 83 20 3.1 Paper birch 81 20 2.7 Lodgepole pine 81 14 4.7 Hybrid spruce 77 19 2.9 Western redcedar 76 35 2.3 Western hemlock 75 25 2.8
124 Species guilds based on height-growth pattern and shade tolearnce
Species Height-growth pattern Shade tolerance class Guild
Western white pine linear-fast intermediate I Douglas-fir linear-fast intermediate I Trembling aspen linear-moderate intolerant I
Paper birch asymptotic very intolerant II Lodgepole pine asymptotic intolerant II
Hybrid spruce linear moderate tolerant III Western redcedar linear moderate very tolerant III Western hemlock linear moderate very tolerant III
References Oliver, C.D.␣ . Similarities of stand structures and stand development processes throughout Ashton, P.M.S. . Establishment and early the world: some evidence and applications to growth of advance regeneration of canopy silviculture through adaptive management. In trees in moist mixed-species forest. In The The ecology and silviculture of mixed-species ecology and silviculture of mixed-species for- forests. M.J. Kelty, B.C. Larson, and C.D. ests. M.J. Kelty, B.C. Larson, and C.D. Oliver Oliver (editors). Klewer, Boston, Mass. (editors). Klewer, Boston, Mass. pp. –. pp.␣ –. Baker, F.S. . A revised tolerance table. J. For. Oliver, C.D. and B.C. Larson. . Forest stand :–. dynamics. McGraw-Hill, New York, N.Y. ␣p. Johnson, E. A., G.I. Fryer, and M.J. Parminter, J. []. Typical historic patterns of Heathcott.␣ . The influence of man and wildfire disturbance by biogeoclimatic zone. climate on the fire frequency in the Interior Table adapted from Old-growth forests: prob- Wet Belt forest, British Columbia. J. Ecol. lem analysis.␣ . B.C. Min. For. Res. :–. Branch, Victoria, B.C. Unpubl. rep. p. Larson, B.C.␣ . Pathways of development in Smith, D.M.␣ . The practice of silviculture. mixed-species stands. In The ecology and th ed. John Wiley and Sons, New York, N.Y. silviculture of mixed-species forests. M.J. p. Kelty, B.C. Larson, and C.D. Oliver (editors). Waller, D. M.␣ . The genesis of size hierarchies Klewer, Boston, Mass. pp. –. in seedling populations of Impatiens capensis Lloyd, D., K. Angove, G. Hope, and C. Meerb. New Phytol. :–. Thompson.␣ . A guide to site identifica- Weiner, J.␣ . Variation in the performance of tion and interpretation for the Kamloops individuals in plant populations. In Plant Forest Region. B.C. Min. For. Victoria, B.C. population ecology. A.J. Davey, M.J. Land Manage. Handb.␣ No. . Hutchings, and A.R. Watkinson (editors). Blackwell Scientific Publications, London, U.K. pp.␣ –.
125 Habitat Management for Game and Parasite Control by Prescribed Burning
KEN KYLLO
Abstract Use of Prescribed Burning in Habitat Management Spring and sometimes fall burning has been a way of life. It was used by Indians to maintain and All my life, I have lived and worked in the Peace enhance game ranges. Fur traders used it to River and Williston Lake areas of northeastern increase game stocks to provide meat, the staple of British Columbia, except for years of high school their diet and food for their sled dogs. The shrewd at Vancouver Tech. I have been an active guide and old Scots traders used chemicals for low-cost outfitter for years. My guiding area encompasses predator control., just as foresters now use poisons some square miles [ km] of the western to economically control brush and weeds. All early watershed of the Rocky Mountains, bordering ranchers used fire to create pasture and clear the Williston Lake and the Finlay River. At one time it land. The evolution of biologists brought a new also included all watersheds on the north side of breed of game managers with more sophisticated the Peace Arm of Williston Lake. and complicated methods. Tried and true Evolution has caused many changes to habitat procedures had to be studied and evaluated. during my lifetime. The extraction of hydro power, Science and technology had proven that the age- timber, minerals, oil, and gas have had a old, tried and true method of habitat management considerable impact on the environment, wildlife, was effective, although not altogether for reasons and the recreation resource required to sustain a previously acknowledged. Evolutionary trends of viable outfitting and resort industry. the last years lean farther and farther towards My experience with habitat enhancement and long studies, with endless reams of information broadleaf management started at age . On a hot filed in lengthy reports and computer banks. The spring day my father gave me a box of wooden real problem is that too much information matches and sent me off on my saddle horse to set confuses the issue. Who is qualified to take the fires. The purpose was to burn brush from the hills time to search through all the information, and undergrowth in the aspen to improve pasture separate the logical from the illogical, come up for our cattle and horses. It also provided a cost- with workable plans and objectives and carry it effective means of clearing bottom lands that through? The computer cannot. It takes knowledge, we␣ eventually cultivated. A box of matches cost combined with practical experience and a personal five␣ cents. commitment, to get on with the job. With the Dear old “Aunty Mac,” bless her soul, had a swing of the pendulum and present trends, I severe case of nerves every spring. She was always foresee further over-zealous protection of timber much relieved when burning was finished. Our stands and broadleaves to the elimination of fires never burned out of control. Dad was ever wildlife habitat and, eventually, all wildlife. I doubt cautious to not burn the coniferous forests. that this symposium will create very many new This conference is about growing aspen and stands of fibre. If the costs were directed to habitat birch. My concerns are twofold. The first is that enhancement in the field, many new continuing priority will be directed toward conserving and jobs could be created. The pendulum has to swing growing softwoods, to the extent that all other uses in balance. Knowledge and experience of people in are forgotten. Some ranger districts are so the field must be considered and incorporated in concerned with spruce and pine that there is planning and put to use. absolutely no consideration for enhancement of
126 wildlife pastures. Smoky the Bear has them para- on parasites and burning. The reply was “no,” but noid. They believe that even a spark will most it was known that fire killed snails, which are the certainly demolish vast stands of the Queen’s carrier for lung worm in sheep. timber. My second concern is that all available Next fall, at our annual convention, biologists funding will continue to be expended for studies came with more charts and graphs. These showed and re-studies, to the extent that little or no that sheep on unburned range were severely funding will ever be available for real practical work. infected with lung worm and had a % lamb crop. Spring and sometimes fall burning has been a A similar band on burned range had almost no way of life. It was used by Indians to maintain lung worm and an % lamb crop. and␣ enhance game ranges. Fur traders used it to Science and technology had proven that the age- increase game stocks to provide meat, the staple of old, tried and true method of habitat management their diet and food for their sled dogs. The shrewd was effective, although not altogether for reasons old Scots traders used chemicals for low-cost previously acknowledged. Here we use a pendulum predator control, just as foresters now use poisons to explain technology versus experience. When on to economically control brush and weeds. All early a level plane the pendulum is in balance. As it is ranchers used fire to create pasture and clear tilted, the balance becomes unequal. Tilted too far, the␣ land. it falls flat. Guide outfitters used fire to rejuvenate over- Trends over the last years lean toward long grown jungles and vegetation in remote mountain studies, with endless reams of information filed in valleys in order to improve game ranges and horse lengthy reports and computer banks. The real pastures. problem is that too much information confuses the Outfitters were doing such a good job of build- issue. I contend that when the pendulum tilts too ing game stocks that Fish and Wildlife personnel, far and falls over, all the information may just as wishing to gain credibility, also became involved. It well be filed on a roll of tissue paper. The end use was a pleasant break from office drudgery to fly the for that is obvious. areas in a Super Cub or helicopter. Foresters joined Who is qualified to take the time to search the flights and started to issue burning permits. through all the information, separate the logical Permits were always required. Until then, no one from the illogical, come up with workable plans had ever bothered to apply and Forestry never and objectives and carry it through? The computer intervened. can not. It takes knowledge, combined with The evolution of biologists brought a new breed practical experience and a personal commitment, of game managers with more sophisticated and to get on with the job. complicated methods. Tried and true procedures All too often in these days of conflicting had to be studied and evaluated. In the Northern bureaucracies, technology, power politics, and Rockies, Stone Sheep, the prime marketable ecological opportunists, the job is considered to be resource, were subjected to intensive research. the completion of a study. By then, the consultants One fall, at the annual convention of the have milked all funds from the budget and the Northern British Columbia Guides Association, politicians and bureaucrats feel that they have biologists made a presentation on burning. taken care of the situation. Anyway, no further Comparisons of test sites showed that burned areas funds remain to carry out procedures and greened up a few days earlier and stayed green a recommendations. Funding can usually be made few days later. They concluded that the advantage available for further study of the same topic— from burning was increased range. Fires also kept perhaps from a different perspective or for another the hillsides clear, thus allowing sun and wind to jurisdiction of bureaucracy. We all agree that some reduce snow cover, improving winter range. research is necessary, but there has to be Burning benefits almost all ungulates. I can tell the compromise and balance. difference when my horses have had the benefit of The water license for the Bennett Dam burned range. imposed an obligation on B.C. Hydro to improve I had observed severe tick infestations on horses habitat around the new lake to compensate for the wintering on unburned ranges with heavy moose flooding of vast winter ranges. The Habitat populations, so I asked if research had been done Compensation Fund has contributed about
127 $ a year, since , but less than % of With the swing of the pendulum and present this money has gone to habitat enhancement. The trends, I foresee further overzealous protection of remainder has been spent on studies, public timber stands and broadleaves to the elimination relations, and administration. of wildlife habitat and eventually, all wildlife. I There are many alternatives to re-invest resource doubt that this symposium will create very many income back into the hinterlands and short-term new stands of fibre. If the costs were directed to renewable resources. Labour-intensive industries habitat enhancement in the field, many new such as big game guiding and tourism would bene- continuing jobs could be created. fit and produce added and continuing employment. From my own experience, I would like to give Forest practices are eliminating much of our you an example of a comparison between game stocks, yet the underfunded Fish and Wildlife technology and practical experience. At age , Branch is expected to carry on. Now they must with a graduation diploma in automotive bear another burden, expending extra time, mechanics from Vancouver Technical High School, personnel, and money, just to police environmental I joined the workforce as an apprentice mechanic. damage caused by logging. It was rather a shock when the realization hit home Benefits from prescribed fires are well known that my hard-earned technical diploma had little and documented. Outfitters were doing a fine job meaning in the workplace. on their own. However, present restrictions are After years of intensive education I did not causing more difficulty and expense each year. even know how to do a simple lube job. We had The Ministry of Forests now imposes unrealistic always been told to ensure that all bolts were restrictions to burning plans, should they even tightened securely. Eager to succeed, I did just that, be␣ granted. to the frustration of the foreman who patiently Wilderness and big game, a renewable resource, drilled out all the broken bolts. Remember, Van. provides enjoyment and satisfaction to wildlife Tech. was a technical school. I knew all the theory, viewers. It also provides recreation and exercise for but when it came to practical application, I was hunters and employment for many who would totally lost. The only appropriate term to describe otherwise be unemployed. Non-resident hunters the dilemma was that I was an “educated idiot.” and tourists bring in new capital to our provincial The pendulum has to swing in balance. economy. Guide outfitters and other resource users Knowledge and experience of people in the field deserve to be recognized. They should be afforded must be considered and incorporated into planning co-operation and an opportunity to continue their and put to use. way of life. As a lifelong citizen of this great province, I am Integrated resource management was a much concerned. That is why I have journeyed miles used and abused conundrum. It garnered publicity [ km] for this opportunity to speak for and suggested co-operation. The sad reality was preservation of wildlife habitat, wildlife, and the that the term only referred to spruce, pine, and resulting employment opportunities. balsam. Now aspen and birch are also included.
128 Simulation of the Long-term Impacts of Alder–Douglas-fir Mixtures on the Sustainability of Site Productivity Using the Ecosystem Management Model FORECAST
XIAOHUA WEI AND J.P. (HAMISH) KIMMINS
Abstract additions or accelerated nutrient cycling (Comeau and Sachs ). The long-term consequences of alder–Douglas-fir Nitrogen-fixing red alder (Alnus rubra Bong.) is mixtures to the sustainability of site productivity a widely distributed species in the Pacific are examined in a series of simulations using Northwest that has received considerable attention FORECAST. Four output parameters (stem biomass both as a potential commercial timber species and harvested, forest floor mass, total site nitrogen as a source of biologically fixed nitrogen (Cole capital, and the mass of decaying wood) are used et␣ al. ; Binkley , ; Van Miegroet et␣ al. as indicators of sustainability. Results suggest that ; Binkley et␣ al. ; Comeau and Sachs ). on medium-quality, low-elevation coastal sites the Through the process of symbiotic nitrogen combination of stems per hectare alder with fixation, red alder can play an important role in Douglas-fir, with the alder cut down and maintaining long-term site productivity, allowed to resprout at year , may be the particularly on nitrogen-deficient sites. Van appropriate mixture to sustain these indicators. Miegroet et␣ al. ₍₎ reported that the nitrogen- Stem-only harvesting with an -year rotation fixation rates for red alder range from – sustained long-term site productivity better than ␣kg•ha- •yr-. Alder has been managed as a shorter or longer rotations, and better than whole- nutritional nurse crop on sites with adequate tree harvesting. The importance of including moisture and mineral nutrition (alder has a high competition from understorey species in simu- demand for P). It has limited value on fertile, N- lations of mixedwood management is discussed. rich sites (Binkley ). These are preliminary results. Further work is Managing alder–Douglas-fir mixtures has been needed to explore this issue in more detail, and investigated by Atkinson and Hamilton ₍₎, Cole final simulation-based recommendations for et␣ al. ₍₎, Miller and Murray ₍₎, Binkley mixedwood management must await the ₍₎, and Comeau and Sachs ₍₎. Binkley ₍₎ completion of the individual-tree, ecosystem reported that mixing red alder and Douglas-fir management model FORTOON. (Pseudotsuga menziesii [Mirb.] Franco) has potential for increasing Douglas-fir growth and Introduction ecosystem productivity on N-deficient sites. Cole et␣ al. ₍) reported that red alder-dominated Intensive forest management practices with high ecosystems accumulated kg • ha- •yr- more levels of biomass utilization (e.g., whole-tree nitrogen than the Douglas-fir-dominated harvesting) and short rotations cause significant ecosystem, and the rate of within-ecosystem ecosystem nutrient loss, and may eventually reduce element cycling is faster in alder stands than in long-term site productivity. One method of Douglas-fir stands. Van Miegroet et␣ al. ₍₎ counteracting this loss of sustainability is to change examined the effect of red alder and alder harvest from monoculture to mixed-species management on the distribution and flux pattern of N, P, K, Ca, employing nitrogen-fixing broadleaf species with and Mg, and on site fertility. They found that conifer crop species. Such mixtures can maintain symbiotic nitrogen fixation by alder led to a or augment site productivity through nutrient significant accumulation of nitrogen, both in
129 above-ground biomass components and in the soil, modelling approach used in FORECAST can be found on the order of – kg • ha- •yr-. The increase in in Kimmins et␣ al. () and Kimmins (). available soil N also stimulated nitrification, which The objectives of this study were: () to identify in turn triggered nitrate-mediated cation leaching alder–Douglas-fir mixture options that are optimal loss and resulted in soil and solution acidification for four indicators: stem biomass harvested, forest in the upper part of the soil profile. Soil acidifi- floor mass, site nitrogen capital, and the mass of cation by red alder was also shown to reduce P decaying wood; and () to evaluate the interactions availability, which can reduce future growth of red between alder–Douglas-fir mixtures and different alder (Van Miegroet et␣ al. ). management regimes (utilization level and Managing alder–Douglas-fir mixtures requires rotation␣ length). careful manipulation of stand structure because the The total stem biomass harvested includes two species have different ecological characteristics stemwood and stembark biomass that is harvested and growth patterns. The more rapid height growth at the end of each rotation. Total forest floor mass of red alder results in light competition between is the total mass of forest floor at the end of each Douglas-fir and alder if both are planted at the rotation. Total site nitrogen capital is the soil same time. Cole et␣ al. ₍₎ reported that alder can nitrogen capital at the end of each rotation, compete with conifers for both water and light, including total nitrogen in woody materials, litter, and can also cause physical damage to conifers. and the mineral soil. Total mass of decaying wood Although information is available concerning is the mass of sapwood and heartwood present on the short-term effects of alder–Douglas-fir mix- the ground at the end of each rotation. The tures, few field investigations have been undertaken simulation period is years. on the long-term effects. The long-term effects of alder–Douglas-fir mixtures on the sustainability of Model Calibration site productivity need to be investigated to design sustainable mixedwood management strategies. In FORECAST requires calibration data that describe the the absence of long-term field data, simulation following factors for each of the species in the models are useful for examining the long-term simulation. Data are also needed for each of at effects of different management regimes. Comeau least two (preferably three) sites that vary in and Sachs ₍₎ used the FORCYTE- model nutritional site quality: (Kimmins et␣ al. ; Kimmins ) to examine . Trends over time in well-stocked monoculture the effects of different stocking levels of alder on stands: the growth of Douglas-fir. They found that the • biomass accumulation in different biomass greatest yield of Douglas-fir and alder stemwood components biomass is achieved at alder densities of stems • tree height and stand density (in naturally per hectare with a -year rotation length for alder thinned stands) and an -year rotation length for Douglas-fir. • stem size frequencies However, no simulations have been undertaken • litter decomposition (loss of mass and change that examine the long-term effects of different in nutrient concentrations). mixtures (e.g., relatively higher initial alder densities [ stems per hectare], but cut down . Data that define: and allowed to resprout at the earlier stages) interacting with different management regimes. • shade as a function of foliage biomass • foliage light adaptations (photosynthetic light FORECAST is a stand-level ecosystem management simulation model. It explicitly simulates the effects saturation curves) of nutrient availability and competition for light • tissue nutrient concentrations and various and nutrients on the growth of trees and plants. aspects of nutrient cycling (e.g., nitrogen The model uses empirical data on tree and plant fixation rates, foliage leaching) biomass accumulation over time, together with • litterfall and tissue senescence (e.g., sapwood- data on key processes such as photosynthesis, to-heartwood conversion) detrital decomposition, and nutrient cycling as a • non-growing season photosynthesis by ever- basis for simulations. A detailed description of the greens and shade effects on height growth.
130 These data are used in “setup” programs to Simulation Results establish various indices of canopy function, resource allocation, nutrient uptake demand and Table shows the four alder–Douglas-fir internal cycling, shade-related tree and branch management scenarios that were identified. The mortality, and tree size frequencies. They also best scenario was then determined by comparing provide a guide to how tree growth and various the simulation outputs of the four scenarios over a ecological processes vary as soil nutrient -year period. availability changes. ₍ ₎ Data sets prepared by Kimmins et al. a, b Four alder–Douglas-fir mixture scenarios, with an were supplemented by data from Comeau and initial stocking level of 1200 stems per hectare of Sachs ₍₎, with additional data from Sachs and Douglas-fir. Stem-only harvesting with removal of Trofymow ₍), Kurz ₍₎, and others (see 85% of stemwood and stembark was applied details in Comeau and Sachs ). Detailed only to Douglas-fir at the time of harvest. Branch, descriptions of site qualities and indices for the leaves, and roots were left on the site. There was Douglas-fir and alder data sets were given by no commercial harvesting of red alder. Comeau and Sachs (). Once the setup models for FORECAST have been Scenarios Red alder density (stems per calibrated and the quality of the input data eval- hectare) and management uated by means of the setup output graphs, the various indices and growth descriptions derived 1 No red alder from the setup programs are used in the ecosystem management program ECOSYTM. Before making 2 400, with no alder control management runs, however, it is necessary to 3 400, alder cut and left on the define the “initial state” of the simulated ground in year 12; alder resprouts ecosystem. The predictions of FORECAST, especially from cut stumps in year 13 for the first two or three rotations, are sensitive to the assumptions made in preparing this initial 4 400, alder removed in the year 30; no alder resprouts state. For the runs reported, we created an ecosystem state that reflects the following history. Two -year rotations of Douglas-fir were The temporal patterns of four output simulated in the no-nutrient feedback mode, which parameters (total stem biomass harvested, total ensures that historical patterns and rates of forest floor mass, total site nitrogen capital, and biomass production, accumulation, and turnover total mass of decaying wood) were used to indicate are included. At the end of each rotation, the the sustainability of site productivity. The stem Douglas-fir were cut and left on the ground to biomass harvested is a direct indicator of achieved simulate natural disturbance by wind. One -year site productivity, while total floor biomass and site rotation was then simulated with nutrient feedback nitrogen are indirect or potential indicators of site switched on. At the end of this rotation, a stem- productivity. Decaying wood is a source of only harvest was applied, removing % of asymbiotic nitrogen fixation, and the N-fixed rates stemwood and stembark. This created an initial- in decaying wood depend on its mass (Jurgensen state file with a total mass of forest floor and et␣ al. ). However, too much decaying wood decomposing litter of . Mg/ha and a soil may cause soil acidification (Van Miegroet et␣ al. humus level of . Mg/ha. The former value is ). Thus, decaying wood as an indicator of site rather high and the latter value is low since the productivity remains questionable from a nutrient calibration data failed to transfer sufficient perspective. However, we included it as an decomposing material to the humus box. Because indicator in this study because natural and the model approximates the decomposition rate of managed forests differ in decaying wood, and the oldest decomposing material to the rate given because decaying wood plays an important role in for humus, it is not thought that this error of maintaining some aspects of biodiversity. representation has had a significant effect on the After selecting the appropriate alder–Douglas-fir simulation performance.
131 mixture scenario (as defined by the limited per hectare) and alder rotation lengths (, , and simulations), its effect on the sustainability of site years) on the biomass of Douglas-fir. Table productivity when interacting with different shows that the greatest amount of harvested management regimes (utilization levels and stemwood biomass is achieved by either higher rotation lengths) was evaluated by comparing the densities (e.g., trees per hectare) of alder with four indicators in a further series of simulations. For all simulations, growth was modelled assuming site index at years of m for (a) Rotation 3 100 Douglas-fir and m at years for red alder. Rotation 2 Rotation 1 80 Effect of Different Mixture Scenarios Four alder– Douglas-fir scenarios (Table ) were used to 60 investigate the effects of different mixtures on 40 long-term site productivity. Values for the four
indicators for those four management scenarios Site N capital (%) 20 over a -year period representing three -year , 0 rotations are illustrated in Figures a, b, and c. 1 2 3 4 Results showed that scenario is the best choice for Scenario the highest levels of harvested stem biomass, total forest floor mass, total site nitrogen capital, and (b) the second-highest level of decaying wood. Rotation 3 100 Scenarios and had the lowest values for three Rotation 2 Rotation 1 of␣ these indicators, suggesting that unless alder– 80 Douglas-fir mixtures are managed properly, they␣ may not perform as well as the pure Douglas- 60 fir option. 40 Compared with the monoculture Douglas-fir, scenario achieved a –% improvement in the 20
values of the four indicators summed over the Decaying wood mass (%) 0 three -year rotations. 1 2 3 4 Additional simulations investigated the effect of Scenario different alder densities (e.g., and stems (c) Rotation 3 100 100 Rotation 2 Rotation 1 Rotation 3 80 80 Rotation 2 Rotation 1 60 60 40 40 20 Forest floor mass (%) 20 0 1 2 3 4
Stem biomass harvested (%) 0 Scenario 1 2 3 4 Scenario Site N capital (a), decaying wood mass (b), and forest floor mass (c) for the three successive 80- Total stem biomass harvested for three successive year rotations and the four mixture scenarios. 80-year rotations and the four mixture scenarios. Data expressed as % of the highest value within Data expressed as % of the highest value within the indicator data set. The 100% scale values for the indicator data set. The absolute value of the three indicators are 1299 kg/ha, 70.3 Mg/ha, 100% scale value was 1701 Mg/ha. and 448.6 Mg/ha, respectively.
132 Five alder–Douglas-fir mixture scenarios, with an initial stocking level of 1200 stems per hectare of Douglas-fir. Stem-only harvesting with removal of 85% of stemwood and stembark was applied at the end of each 80-year rotation. Branches, leaves, and roots were left on the site. The utilization level of red alder was the same as Douglas-fir.
Rotation 1 Rotation 2 Rotation 3 Alder scenario (density; rotation length) Species (Mg/ha) (Mg/ha) (Mg/ha)
200 trees; 36 years; no resprouting fir 530.2 539.2 539.2 alder 49.8 51.9 51.9 total 580.0 591.1 591.1 400 trees; 12 years; resprouting fir 522.6 590.1 588.6 alder 6.9 6.7 6.7 total 529.5 596.8 595.3 400 trees; 12 years; no resprouting fir 507.1 565.6 561.6 alder 6.9 6.7 6.7 total 514.0 572.3 568.3 400 trees; 24 years; no resprouting fir 446.0 508.4 502.9 alder 44.8 44.2 45.0 total 490.8 552.6 547.9 400 trees; 36 years; no resprouting fir 312.3 183.4 148.5 alder 82.2 86.4 87.6 total 394.5 269.8 236.1
shorter rotation lengths (– years), or lower scenario (no alder). The results (Table ) indicate densities (e.g., trees per hectare) with longer that alder–Douglas-fir mixture scenario with rotation length (e.g., years). This was also stem-only harvesting increased productivity in identified by Comeau and Sachs ₍₎. The subsequent rotations, while the same mixtures with mixtures with densities of stems per hectare whole-tree harvesting decreased productivity in and -year or longer rotation lengths of alder subsequent rotations. The decline over the three significantly decrease the stemwood biomass of -year rotations is most rapid in scenario with Douglas-fir, and are not appropriate for alder– whole-tree harvesting. Table also shows that Douglas-fir mixedwood management. scenario with stem-only harvesting achieved Simulation results also showed that no alder more sustainable forest floor biomass and site N trees were left at the end of each -year rotation. compared with whole-tree harvesting. The mass of They died out because of competition. decaying wood in the second rotation dropped to about half of the value in the end of the first Effect of Different Utilization Levels The rotation, but there was little change between utilization levels investigated in this study were rotations and . The high loading of decaying whole-tree harvesting and stem-only harvesting. wood at the end of the first rotation is a reflection Whole-tree harvesting was defined as the removal of the “legacy” of decaying wood that was present of % of stemwood and stembark, and % of on the site as defined in the initial-state file. Our branches and leaves. Stem-only harvesting was conclusion is that the appropriate mixture of these defined as % removal of stemwood and four alder–Douglas-fir management scenarios stembark. Rotation length was years and alder– would be scenario with stem-only harvesting. Douglas-fir mixture scenario was compared with
133 Four indicators of sustainability over three 80-year rotations with two harvesting levels (whole-tree harvesting [WTH] and stem-only harvesting [SOH]) for alder–Douglas-fir scenarios 1 and 3. The results are expressed as a percentage of the highest value obtained within each of the four indicators.
Stem biomass Forest Site nitrogen Decaying (Douglas-fir) floor mass capital wood mass Rotations Management scenario WTH SOH WTH SOH WTH SOH WTH SOH
1 3 93.8 88.6 81.8 100.0 71.7 99.2 89.9 89.9 1 83.8 79.2 75.0 93.6 69.8 95.2 100.0 100.0 2 3 80.0 100.0 66.3 96.5 66.0 100.0 43.7 36.0 1 67.5 78.7 55.5 86.0 61.3 95.1 30.3 48.0 3 3 75.6 99.7 63.0 96.1 65.1 99.1 40.0 34.6 1 60.3 78.5 48.8 85.8 56.9 94.8 26.0 47.0
Note: Scenario has no alder.
Whole-tree harvesting did not prove to be a The Need to Represent the Entire Plant sustainable management option, in spite of the Community nitrogen inputs from alder. However, this conclusion is sensitive to the definition of the The simulations assumed a simple alder–Douglas- initial-state. Higher or lower initial values for fir system. This is not realistic. Herbs and shrubs forest floor mass, site N capital, and decaying wood can have a significant competitive effect on the in the initial state file would result in the different growth of both Douglas-fir and red alder. Tables values and patterns of change from those presented and demonstrate the importance of a more in Table . complete description of the simulated plant community. These scenarios simulated the Effect of Different Rotation Lengths The effects of effects of the herb fireweed and the shrub different rotation lengths (, , , and years) salmonberry on the growth of both Douglas-fir were examined for scenario with stem-only and red alder as a function of the timing of their harvesting. The simulations revealed a tradeoff invasion of the site (Table ). Values of stem between the shortest and longest rotation lengths biomass, foliage biomass, and top height at the end (Table ). The shortest rotation produced the of a -year simulation, or the maximum values highest amount of harvested stem biomass but the achieved by those species that are shaded out lowest floor mass, site nitrogen capital, and mass of before the end of the simulation, are presented for decaying wood over the -year simulation each scenario in Table . period. The longest rotation produced the reverse The results show that fireweed and salmonberry pattern: lowest stem biomass but largest values for can have a dramatic effect on the growth of the other three indicators. Douglas-fir and red alder if these herbs and shrubs Management for sustainable long-term site invade in year , the year before Douglas-fir productivity requires a balance between timber planting (scenarios , , ). However, their negative production and the maintenance of site fertility. effects are significantly reduced if they invade in The -year rotation length with scenario appears year (scenarios , , ). This suggests that to give the best overall performance, with close to vegetation management or control is required if the maximum stem biomass, good organic matter dense salmonberry or fireweed becomes established and nitrogen conservation, and intermediate levels on the site being simulated. of decaying wood.
134 Comparison of the four indicators under four different rotation lengths, for mixedwood scenario 3 and monoculture Douglas-fir (scenario 1) over the 240-year period
Stem biomass Forest Site nitrogen Decaying Rotation Management (Douglas-fir) floor mass capital wood mass length scenario (Mg/ha) (kg/ha) (Mg/ha) (Mg/ha)
40 years 3 1821 250 924 10.7 (six rotations) 1 1419 203 781 11.6 60 years 3 1763 352 1172 13.5 (four rotations) 1 1398 309 1074 21.2 80 years 3 1701 431 1287 24.5 (three rotations) 1 1395 385 1231 33.2 120 years 3 1257 513 1620 51 (two rotations) 1 1136 466 1539 49 Initial state – – 254 1341 32.6
Scenarios used to investigate the effects of fireweed, salmonberry, and a bryophyte community on the growth of both Douglas-fir and red alder (from Kimmins 1993)
Year of establishment Scenarioa Red alder Fireweed Salmonberry
1–b –– 216–– 3161– 416–1 51611 61655 7165– 816–5 9111 10 5 1 1 11 10 1 1 12 1 – – 13 5 – – 14 10 – – a All scenarios include a bryophyte community. Douglas-fir ( stems per hectare) was planted in year and spaced to stems per hectare in year . Whenever red alder is represented, stems per hectare were simulated. Red alder were not removed during the spacing in year . Scenarios vary in the year of invasion of red alder, fireweed, and salmonberry. Alder establishment in year , , and represents invasion before Douglas-fir canopy closure. Establishment in year represents invasion following Douglas-fir spacing. b A dash signifies that the species was not included in the simulation.
Maximum values of stem biomass, foliage biomass, and top height for each of the species over a 40-year simulation. Scenarios described in table 5.
135 Douglas-fir Red alder Fireweed Salmonberry Bryophyte Scenario SBa FBb THc SB FB TH SB FB TH SB FB TH SB FB
1 293 16.7 35.2 –d – – – – – – – – 26.4 10.5 2 295 16.9 35.2 0.7* 0.09 1.5 – – – – – – 26.3 10.5 3 100 9.8 22.0 0.1* 0.03 0.9 3.0 0.6 2.0 – – – 7.2 6.3 4 8.0 1.0 15.0 0.1* 0.03 0.9 – – – 10.0 1.5 2.8 14.2 11.2 5 0.4*e 0.1 1.6 0.1* 0.03 0.9 2.3 0.5 2.0 10.1 1.5 2.8 4.2 3.5 6 288 16.8 34.2 0.6* 0.08 1.4 1.8 0.4 2.0 3.3 0.6 2.8 12.1 7.5 7 286 16.5 34.3 0.8* 0.10 1.6 2.2 0.5 2.0 – – – 13.2 8.1 8 298 17.0 34.9 0.6* 0.08 1.4 – – – 6.0 0.9 2.8 15.1 8.8 9 2.0 0.2 11.2 49 5.80 4.8 1.9 0.4 2.0 9.0 1.5 2.8 5.9 4.8 10 0.4* 0.1 1.2 0.1* 0.30 0.9 2.3 0.5 2.0 10.1 1.5 2.8 4.2 3.5 11 0.4* 0.1 1.8 0.1* 0.30 0.9 2.3 0.5 2.0 10.1 1.5 2.8 4.2 3.5 12 39 4.1 21.9 96 9.10 12.2 – – – – – – 30.9 15.3 13 323 18.4 36.5 7.6* 0.90 9.8 – – – – – – 25.7 10.8 14 300 16.8 35.3 0.8* 0.10 1.7 – – – – – – 26.1 10.4 a SB = stem biomass (Mg/ha). “Stem” for bryophytes means total bryophyte biomass. b FB = foliage biomass (Mg/ha). c TH = top height (m). d Species not included in simulation. e Species eliminated by the end of the 40-year simulation are flagged with an asterisk.
Invasion of alder in year in the absence of invading in year had little effect on well- minor vegetation had almost no effect on Douglas- established Douglas-fir. fir (scenario ). Alder invasion in year at trees Establishment and management of alder– per hectare reduced Douglas-fir stem biomass by Douglas-fir mixtures clearly requires an under- % (scenario ), whereas alder invasion in years standing of tree-understorey relationships as well and resulted in increases in Douglas-fir growth as tree-tree relationships. This argues strongly for of % and %, respectively (scenarios and ). an ecosystem approach to mixedwood management The presence of fireweed and salmonberry from rather than a community approach. Computer year reduced the growth of alder by about % models that ignore the competitive role of herbs when alder invaded in year (scenario ). Alder and shrubs in mediating establishment of, and was eliminated by fireweed and salmonberry when competition between, coniferous and deciduous alder invasion was delayed to years , , and trees will therefore have a limited role in the design (scenarios to , , and ). Douglas-fir was elim- of mixedwood management strategies. inated by a dense community of fireweed and The simulations presented here used a stand-level salmonberry established in year (scenario ), but model and an “opaque blanket” representation of was able to achieve about % of the stemwood in tree canopies (see Kimmins ). While this repre- scenario in the presence of fireweed that sentation works well for even-aged, monoculture established in year (scenario ), and % in the forests, it has significant limitations for the simu- presence of salmonberry which established in year lation of mixed stands. The model can represent (scenario ). Fireweed, salmonberry, or a mixture herbs and shrubs and their interactions with trees.
136 Individual-tree models that are spatially explicit References offer a significant advance over most existing models and have potential application in mixed- Atkinson, W.A. and W.I. Hamilton. . The wood management planning and research. So far, value of red alder as a source of nitrogen in most of these models deal only with light Douglas-fir/alder mixed stands. In Utilization competition, although the model developers and management of alder. D.G. Briggs, D.S. express their intention to eventually incorporate DeBell, and W.A. Atkinson (compilers). U.S. soil processes. The models still do not include Dep. Agric. For. Serv. Gen. Tech. Rep. PNW- minor vegetation. Our team is developing FORTOON: . pp. –. an individual-tree, stand-level ecosystem model Binkley, D. . Nodule biomass and acetylene that will have all the simulation capabilities of reduction rates of red alder and Sitka alder FORECAST. In addition, it will also represent uneven- on Vancouver Island, British Columbia. Can. aged (as well as even-aged) stands, mixed stands, J. For. Res. :–. the full range of silvicultural systems, and a wide .␣ . Ecosystem production in Douglas- range of management practices. If this model is fir plantations: interactions of red alder and successful in meeting its design objectives, it site fertility. For. Ecol. Manage. :–. should be a valuable tool for mixedwood research Binkley, D., P. Sollins, R. Bell, D. Sachs, and D. and management. An initial version of FORTOON Myrold. . Biogeochemistry of adjacent should be available in late . conifer and alder-conifer stands. Ecology ():–. Conclusions Cole, D.W., S.P. Gessel, and J. Turner. . Com- parative mineral cycling in red alder and The simulations reported here show that an Douglas-fir. In Utilization and management appropriate alder–Douglas-fir mixture can achieve of alder. D.G. Briggs, D.S. DeBell, and W.A. a potential yield increase of –% compared with Atkinson (compilers). U.S. Dep. Agric. For. monoculture Douglas-fir management. We antici- Serv. Gen. Tech. Rep. PNW-. pp. –. pate that a more detailed investigation may reveal Comeau, P. and D. Sachs. . Simulation of the the potential for an even greater increase. consequences of red alder management on From a nutrient perspective, the simulations the growth of Douglas-fir using FORCYTE-. suggest that the optimum strategy for the B.C. Min. For. and For. Can., Victoria, B.C. sustainable management of Douglas-fir ecosystems FRDA Rep. No. . p. would be a mixture of Douglas-fir trees per Jurgensen, M.F., M.J. Larsen, R.T. Graham, and hectare and red alder, cutting the alder in year A.E. Harvey. . Nitrogen fixation in and letting them resprout, stem-only harvesting, woody residue of Northern Rocky Mountain and an -year rotation length conifer forests. Can. J. For. Res. :–. Understorey vegetation (e.g., fireweed and sal- Kimmins, J.P. . Scientific foundations for the monberry) may require management to achieve the simulation of ecosystem function and man- potential benefits from alder–Douglas-fir mixtures. agement in FORCYTE-. Can. For. Serv. North. Most existing computer models are unsuitable For. Cent., Edmonton, Alta. Inf. Rep. NOR- for the design of mixedwood management options. X-. p. For this, researchers and forest managers need an Kimmins, J.P., N. Price, and M.J. Apps. [a]. ecosystem-level model that incorporates: herbs, Red alder: a selected bibliography and com- shrubs, and probably bryophytes in addition to pilation of literature data for use in conjunc- trees; an explicit representation of individual tree tion with the ecosystem management model crowns; nutrient requirements, nutrient cycling, FORCYTE-. For. Can. North. For. Cent., and nutritional interactions of different species; Edmonton, Alta. Unpubl. contract rep. and, if natural regeneration options are to be simulated, a representation of seedbeds and seedling establishment.
137 Kimmins, J.P., N. Price, R. Oran, and M.J. Apps. Sachs, D. and J.A. Trofymow. . Testing the [b]. Douglas-fir: a selected bibliography performance of FORCYTE- against results and compilation of literature data for use in from the Shawnigan Lake thinning and conjunction with the ecosystem management fertilization trials on Douglas-fir. For. Can., model FORCYTE-. For. Can. North. For. Pac. For. Cent., Victoria, B.C. Inf. Rep. Cent., Edmonton, Alta. Unpubl. contact rep. BC-X-. p. Kimmins, J.P., K.A. Scoullar, and M.J. Apps. Van Miegroet, H., D.W. Cole, and P.S. Homann. . FORCYTE- user’s manual for the bench- . The effect of alder forest cover and al- mark version. For. Can. North. For. Cent., der forest conversion on site fertility and Edmonton, Alta. ENFOR Proj. P-. p. productivity. In Sustained productivity of Kurz, W.A. . The significance of shifts in car- forest soils. Proc. th N.Am. For. Soils Conf. bon allocation patterns for long-term site S.P. Gessel, D.S. Lacate, G.F. Weetman, and productivity research. In Research strategies R.F. Powers (editors). Fac. For. Univ. B.C., for long-term site productivity. Proc. IEA/BE Vancouver, B.C. pp. –. workshop. W.J. Dyck and C.A. Mees (edi- Urban, D.L. and H.H. Shugart. . Individual- tors). For. Res. Inst. Rotorua, New Zealand. based models of forest succession. In Plant IEA/BE Rep. No. . pp. –. succession theory and prediction. D.C. Miller, R.E. and M.D. Murray. . The effects Glenn-Lewin, R.K. Peet, and T.T. Veblen of red alder on growth of Douglas-fir. In Uti- (editors). Chapman and Hall, London. lization and management of alder. D.G. pp. –. Briggs, D.S. DeBell, and W.A. Atkinson Waring, R.H. and J.F. Franklin. . Evergreen (compilers). U.S. Dep. Agric. For. Serv. Gen. coniferous forests of the Pacific Northwest. Tech. Rep. PNW-. pp. –. Science :–.
138 SUMMARIES OF POSTERS
Presented at The Silviculture of Temperate and Boreal Broadleaf-conifer Mixtures Workshop held February 28 and March 1, 1995 in Richmond, British Columbia
139 Control of Spruce Weevil Damage By Introduction of Alternative Commercial Species (FRDA II Project 93-03)
KEN DAY
Abstract Treatment 1: Mixed-bag Planting
The poster describes an experiment for planting Planting The treatment will be block planting with spruce and lodgepole pine in a mixture as a foil for an initmate mixture of lodgepole pine and spruce, spruce weevils. at a density of stems per hectare. Optimum inter-tree spacing will be . m and minimum Introduction distance will be . m. The anticipated benefits of this treatment are: This trial was established to look at the practicality of establishing spruce plantations with a mixture • spruce will be overtopped by the lodgepole pine, of other commercial species as a foil for spruce resulting in smaller leader diameters and weevils. While research to date has concentrated on reduced heat sums on the leader; stands planted in , which already exhibit • the taller lodgepole pine will hide the shorter significant weevil damage, it seems prudent to spruce trees, rendering them less apparent to the examine alternative stand establishment regimes. weevils; Two small blocks will be planted as part of this • the rapid nutrient uptake and cycling by the study. The areas are described as: pine will benefit the spruce, since these sites are relatively nutrient degraded by burning in / • Unit D (. ha); Opening A-; logged and mechanical site preparation in . , burned , planted ; site preparation , planted . Stand Tending The block plantings will receive the • Unit E (. ha); Opening A-; burned , following stand-tending treatments: logged ; site preparation , planted . The units will each be divided in half and planted with two treatments. The treatments are Year Age Treatment described below. The two planting units were site prepared with a brush blade in summer and 1996 3 Manual brushing contain round piles. Extensive areas of exposed mineral soil and suckering from stumps and roots 2008 15 Juvenile spacing will result in aggressive colonization by willow and 2023 30 Commercial thin to 800 stems per cottonwood, and to a lesser extent by aspen, birch, hectare, retaining 30% lodgepole pine and alder. Planting stock for this trial are: 2043 50 Commercial thin to 350 stems per hectare, pure spruce • Lodgepole pine: PSBB + Seedlot ; 2073 80 Final harvest, 300 stems per hectare grown at PRT, Chilliwack. at 50 cm dbh • Spruce: PSBB + Seedlot ; grown at HiGro, Quesnel.
140 Treatment 2: Mixed-clump Planting Distance between clumps: m/ha d == . m Planting Trees will be planted in clumps of seven √ . m/ha trees, at clumps per hectare. Each clump will Clump diameter (assuming intertree distance of m): consist of four lodgepole pine and three spruce, planted in the arrangements shown at Figure . Circumference m cd === . m Spacing within the clumps is m between trees, ππ with minimum planting distance set at . m. The anticipated benefits of this treatment are: Planting densities are based on the following assumptions and calculations: • spruce will be overtopped by the lodgepole pine Basal area capacity = m/ha and adjacent deciduous, resulting in smaller leader diameters and reduced heat sums on Desired final crop = cm dbh spruce trees at the␣ leader; Desired final crop = years • the taller lodgepole pine and adjacent deciduous will hide the shorter spruce trees, rendering Intertree distance (within clumps) = m them less apparent to the weevils; Clumps established at final crop density. • the rapid nutrient uptake and cycling by the pine and deciduous will benefit the spruce, since cm Radius of final crop tree: r == . m these sites are relatively nutrient degraded by burning in / and mechanical site Basal area of one cm crop tree: preparation in ; BA = π r = π× . = . m • brush control need only take place within clumps, and the intervening space will be left in Number of stems per hectare at final crop deciduous cover; m • increased component of deciduous will provide spacing: sph ==. per hectare . m biodiversity in forest cover; and • deciduous trees such as birch, aspen, and cottonwood will be available for harvest in commercial thinnings.
5.72 m N Sx Pl Sx Pl Pl Sx Sx Pl Sx Sx Pl Pl Pl Pl
5.72 m
Sx Pl Sx Pl Pl Sx Sx Pl Sx Sx Pl Pl Pl Pl
Clump layout and planting design.
141 Boreal Mixedwood Understorey Protection Demonstration Trial in Fort Nelson, B.C.
KERRY C. DESCHAMPS AND ROBERT HALL
Abstract Objectives
The poster describes a demonstration area that A mixedwood understorey protection demonstra- illustrates mixedwood silviculture systems to tion area was set up jointly between CCMC and protect immature spruce understorey while the Canadian Forest Service ₍CFS₎ to meet the harvesting mature aspen in Fort Nelson, B.C. following objectives: • Demonstrate and test different harvesting Introduction techniques for protecting understorey spruce and encouraging aspen regeneration. In northeastern British Columbia, a large portion • Quantify the amount and type of understorey of the landscape is dominated by mixedwood spruce damage (broken tops, bark scrapes, etc.) forests. These are typically characterized by an resulting from the various harvesting techniques. aspen overstorey and a spruce understorey. It is the • Monitor the subsequent development of new presence of white spruce as an understorey aspen regeneration and residual spruce. component that poses significant operational • Document the costs and benefits of protecting problems, especially where spruce is greater than the understorey by taking into consideration % by volume or greater than stems per timber and non-timber values. hectare. Particularly challenging is the prescription • Evaluate the feasibility of a two-pass harvesting of harvesting methods to remove the merchantable system, while maintaining visual quality objectives. aspen, while protecting the vulnerable spruce • Promote this project through public education underneath. Research and operations conducted in and the development of interpretive trials, tour Alberta and Ontario clearly demonstrate that packages, and a video. harvesting operations in mixedwoods can be conducted effectively in a manner that preserves Demonstration Area the immature spruce understorey component and promotes healthy future stand development. The demonstration area is located km east of In the Fort Nelson Forest District, much interest Fort Nelson on the Sierra Yoyo Road within the has been expressed in mixedwood management Boreal White and Black Spruce ₍BWBSmw⁾ and the development of prescriptions to protect biogeoclimatic subzone. The -ha site was split understorey spruce and promote aspen regene- into four treatment units to accommodate the ration. Discussions with Canadian Chopsticks following four understorey protection treatments: Manufacturing Company Ltd. ₍CCMC₎ and Ministry of Forests representatives clearly show the need and • Selection logging and random skidding, with a their willingness to participate in setting up a cm dbh limit cut for both aspen and spruce; demonstration area to showcase exemplary • Conventional understorey logging and random management practices in mixedwood forests. skidding: this area will be the control; • Group retention and random skidding: patches will be identified and left unharvested; and • Conventional understorey protection logging and designated skidding, with the use of landings and preassigned skid trails.
142 Before harvesting the following activities took machine productivity data. Helicopter time was place: supplied by CCMC for CFS staff to videotape and photograph the demonstration area before and • CCMC staff developed the pre-harvest silviculture during harvesting operations. After harvesting, CFS prescriptions. staff relocated some of the plots to assess • CCMC staff flagged the boundaries, roads, harvesting damage on the residual spruce. landings, and retention patches. Monitoring the subsequent development of new • CFS staff established ten m circular plots in aspen regeneration and residual spruce will be each treatment unit and two m circular done annually for the next years. plots in the retention patches (totalling plots Future work will include remeasurement of in all). Plots were established along survey lines plots, and establishment of signs, interpretative spaced at m apart. A metal pin was driven trials, videos, and reporting of results. into the ground at each plot centre and all coni- fers were tagged, dbh and height measured, and Acknowledgements condition noted. All hardwoods were measured for dbh and some were sampled for height. Funding and in-kind support for developing the Harvesting started in late October and Mixedwood Understorey Protection Demonstration finished in early December . The type of Area was provided by the following agencies: equipment used included a Timberjack Feller • Canadian Chopstick Manufacturing Co. Ltd. Buncher, a Timberjack Grapple Skidder, a • Natural Resources Canada - Canadian Forest Cat ELC Carrier with DH Processing Head, a Service Cat Carrier with DH Processing Head, a • Green Plan - Forest Practices Cat Butt’n Top, and D and DK crawlers for • Ministry of Forests - Fort Nelson Forest District roads, deactivation, and piling. During the logging • Canada-B.C. Forest Resource Development operation, a CCMC supervisor was stationed on-site Agreement ₍FRDA₎. to monitor the operation and to gather cost and
143 Effects of Red Alder on Stand Dynamics and Nitrogen Availability (EP-1121)
P.G. COMEAU, G. HARPER, B. BIRING, AND K.D. THOMAS
Abstract Objectives
Replacement series and additive experiments were The objectives of this study are: intitiated in to examine both short- and long- • to improve understanding of both the term effects of red alder density and proportion on competitive and beneficial effects of red alder stand dynamics. The results from these when grown with conifers; experiments will broaden our understanding of • to document and demonstrate the effects of nutrient cycling and ecosystem dynamics within different amounts and spatial arrangements of these mixedwood stands. The long-term data will red alder on the growth and survival of conifers provide calibration and verification of canopy light and hardwoods, stand dynamics, crown penetration and ecosystem models. characteristics, nitrogen availability, nutrient cycling, and long-term productivity. Introduction Field Experiments Red alder is found throughout the Coastal Western Hemlock biogeoclimatic zone and is the most Replacement Series Experiment In this study, red abundant broadleaf species in coastal British alder and Douglas-fir were planted in a series of Columbia. Red alder litter influences nutrient five proportions (Table ) at a total density of cycling in the forest and can also contribute to site ␣ trees per hectare (. m spacing), following an nitrogen capital and long-term productivity experimental design protocol prepared by the through the process of symbiotic nitrogen fixation. Oregon State University Hardwood Silviculture Co- Juvenile growth of red alder is much more rapid operative. One installation was planted at East than that of most conifers. Three-year-old red Wilson Creek in and another at Holt Creek in alder can grow – m/yr in height, and can rapidly (Table ). Each installation consists of one overtop neighbouring conifers. This can lead to replicate of each of the five treatments described substantially reduced light availability, which may in␣ Table . cause physical damage to crop trees. The degree of light reduction and the amount of damage to Additive Experiment Additive experiments were conifers depends largely on the density and size established at five locations (Table ). Douglas-fir of␣ the red alder component of the stand. We and western redcedar were planted in all plots at initiated studies in to examine both short- and total densities of trees per hectare (with the long-term effects of red alder density and two species planted in equal proportions at proportion stand dynamics. This study had five alternating planting spots), and one of seven major components: “broadleaf” density treatments (Table ) was replacement series field experiments; applied. Because of space limitations, treatments additive field experiments; were not replicated on individual sites and not all “cluster” experiment; treatments could be applied at all sites. Problems modelling light penetration through alder with survival of planted trees due to site canopies; and conditions, vegetation competition, and browsing ecosystem modelling. were encountered at two locations (Shawnigan Lake and Surrey Nursery).
144 Replacement series experiment treatments
Treatment % alder % Douglas-fir Trees per hectare Spacing (m)
1 100 0 742 3.67 2 50 50 742 3.67 3 25 75 742 3.67 4 11 89 742 3.67 5 0 100 742 3.67
Field installations
Zone Moisture Nutrient Treatment Year Study Site District regime regime
Replacement East Wilson Creek Sunshine Coast CWHdm 3 c 1–5 1992 series Holt Creek Duncan CWHxm 5 d 1–5 1994
Additive Waterloo Creek Port Alberni CWHdm 3 c 1–8 1992 Gough Creek Sunshine Coast CWHdm 3 c 1–8 1992 Shawnigan Lake Duncan CWHxm mixed d 1–6 1993 Surrey Nursery Chilliwack CDFdm 6 d 1–8 1993 Holt Creek Duncan CWHdm 5 d 1–8 1994
Cluster Holt Creek Duncan CWHdm 5 d – 1994
Additive experiment treatments
Treatment Species Trees per hectare Spacing (m)
1– 0– 2 Red alder 50 14.2 × 14.1 3 Red alder 100 10.0 × 10.0 4 Red alder 200 7.1 × 7.1 5 Red alder 400 5.0 × 5.0 6 Bigleaf maple 50 14.2 × 14.1 7 Sitka alder 200 7.1 × 7.1
145 Cluster Experiment Eight patches of red alder were (Figure ) are the same as for the Gough Creek, planted at Holt Creek in May , in a ha area Waterloo Creek, and Holt Creek additive (Figures that had previously been planted with Douglas-fir and ) installations. in March . Patches of alder, × m, were located at least m apart within the opening and Modelling Light Penetration through Red Alder at least m from the edge of the adjoining stand. Canopies Two alder densities were each randomly assigned to four patches— trees per patch ( trees per The amount of light that penetrates red alder hectare or m spacing) or trees per patch canopies varies as a function of: ( trees per hectare or m spacing). • stand characteristics (number, size, spatial arrangement, and crown architecture of red alder); • season (the locations of the sun and leaf Measurements development of the canopy); • time of day; and Treatment plots are × m (. ha) at East • weather conditions. Wilson Creek, Waterloo Creek, and Gough Creek, and × m (. ha) at Holt Creek. In each Light is one of the major factors involved in treatment plot, stem diameter and height were interactions between red alder and conifers. A measured within a . ha permanent model is currently being developed for estimating measurement plot. Soil samples are being collected the amount of light penetrating red alder canopies. to document changes in soil nitrogen capital, pH, The model will estimate light penetration through and organic matter content. A climate station was alder stands of various sizes and configurations erected at each installation to provide data on solar and demonstrate how light penetration varies radiation, soil and air temperature, soil moisture, diurnally and seasonally. Model output will include and rainfall. Data collection regimes for East estimates of hourly averages and of hourly, daily, Wilson Creek and Holt Creek replacement series weekly, and seasonal totals.
500 Dr 1992 Dr 1993
400 Dr 1994 Fd 1992 Fd 1993 300 Fd 1994 Height (cm) 200
100
0 000 50 75 89 100 Proportion of Douglas-fir (%)
East Wilson Creek replacement series: tree heights for 1992, 1993, and 1994 (Fd = Douglas-fir, Dr = red alder).
146 500 Cw 1992 Cw 1993 Cw 1994 400 Fd 1992 Fd 1993 Fd 1994 Dr 1992 300 Dr 1993 Dr 1994
200 Height (cm)
100
0 000 050 100 200 400 Stems per hectare (alder)
Gough Creek additive installation: tree heights for 1992, 1993, and 1994 (Cw = western redcedar; Fd = Douglas-fir; Dr = red alder).
500 Cw 1992 Cw 1993 Cw 1994 400 Fd 1992 Fd 1993 Fd 1994 Dr 1992 300 Dr 1993 Dr 1994
Height (cm) 200
100
0 000 050 100 200 400 Stems per hectare (alder)
Waterloo Creek additive installation: tree heights for 1992, 1993, and 1994 (Cw = western redcedar, Fd = Douglas-fir, Dr = red alder).
147 Ecosystem Modelling 300 250 Ecosystem models are useful tools for exploring the 200 trade-offs between the beneficial and detrimental 150 Total effects of red alder on stand growth and on long- Red alder 100 term site productivity. The FORCYTE- ecosystem Douglas-fir 50 Harvested biomass model has been calibrated for red alder, Douglas- (thousands Mg/ha) 0 fir, western redcedar, and western hemlock. 0 20 40 60 80 100 Simulations using FORCYTE- indicate that between % Douglas-fir and well-spaced stems of red alder per hectare will increase Douglas-fir yields by up to Results from a simulated replacement series % (Figure ). However, this study and others experiment showing total stemwood biomass indicate that intermixing alder with Douglas-fir yield over 80 years. FORCYTE_11 simulation was will lead to substantial losses in the conifer done for a medium-quality Douglas-fir site, with component unless alder densities are less than 1000 total trees per hectare (planted); red alder ␣ trees per hectare. Simulations with mixtures of were harvested at age 40, and Douglas-fir at red alder and western redcedar, or with red alder age 80 (Comeau and Sachs 1992). and western hemlock, provide similar results. However, performance of western redcedar or Douglas-fir (where alder is precluded from the western hemlock declines when red alder densities stand). However, establishment and retention of exceed trees per hectare. red alder per hectare appear to maintain soil In simulated replacement series experiments, nitrogen and site productivity. maximum whole-stand yield was achieved with a Further refinements in the capabilities of this stand composed of % Douglas-fir and % red model are under way, including simulation of the alder (Figure ). Douglas-fir yield was highest in effects of different spatial arrangements of red the stand with only a % red alder component. alder. Data from field experiments will be used for Simulations suggest that site productivity may calibration and validation of model predictions. decline with successive -year rotations of pure Acknowledgements
300 This research has been supported by FRDA I, FRDA II, Red alder Forest Renewal BC, and the B.C. Ministry of 250 Douglas-fir Forests. Support from the Hardwood and Vegetation Management Technical Advisory 200 Committee is gratefully acknowledged. 150 Reference 100
biomass (Mg/ha) Comeau, P. and D. Sachs. . Simulation of the Harvested stemwood 50 consequences of red alder management on the growth of Douglas-fir using FORCYTE-. 0 B.C. Min. For. and For. Can., Victoria, B.C. 000 050 0100 0200 0300 0400 0500 1000 2000 Initial red alder density (stems per hectare) FRDA Rep. No. . p.
Effects of initial red alder density on cumulative stemwood biomass yield of Douglas-fir and red alder for a site index (SI) of 30 m at 50 years and red alder SI of 17 m at 50 years. Results are based on simulations using FORCYTE_11 (Comeau and Sachs 1992).
148 Response of Hybrid Poplar to Phosphorus Fertilization and Water Supply
R. van den DRIESSCHE
Abstract Experiment 1
The optimal nutrient status for different hybrids Objectives The purpose of this work was to examine being planted on the east side of Vancouver Island response of hybrid poplar (Populus trichocarpa x P. is largely unknown. Two experiments were deltoides) to phosphorus (P) level and water supply. established to examine the response of hybrid Methods Poplar cuttings were grown in m deep poplar to phosphorus and water supply; and to containers made from cm diameter water pipe. two␣ sources of both phosphorus and nitrogen. Each container was filled with peat and vermiculite Results following one growing period are presented potting mixture into which triple superphosphate and discussed. fertilizer had been incorporated to provide one of four levels of P (Table ). Each container was Introduction planted with one of two clones of hybrid poplar, and two moisture regimes were set up by varying Fast growing hybrid poplar plantations are being the frequency of watering. Nitrogen, potassium, established in the Fraser Valley and on the east sulphur, copper, and zinc were applied in solution coast of Vancouver Island. Intensive culture of the during the experiment. The four factors (Table ) plantations is necessary to obtain the full growth were fully randomized and the containers were potential of the poplar hybrids. Besides clean placed in the open at × cm spacing. The cultivation of the planting sites, fertilization is experiment started on May and finished on likely to be required. The optimal nutrient status August , after weeks of growth. for different hybrids being planted on the east side of Vancouver Island is largely unknown, and there Results has been some evidence of phosphorus deficiency. Total growth Average dry weight growth by the The summer drought of this region may also plants during the -week experiment, from lowest influence response to mineral nutrition. to highest P treatment levels, were: ., ., .,
Treatments in Experiment 1
Factor Levels Cumulative number
Phosphorus (g per container) 0.0 0.5 1.0 4.0 4 Clones 1 2 8 Water dry wet 16 Replications 1 2 3 4 5 80
149 and . g. Thus the . g P treatment increased sand, and dolomitic limestone and P treatments dry matter production by .%. were worked cm into the surface. Nitrogen, Shoot and root growth Phosphorus affected shoot potassium, copper, and zinc solutions were applied and root dry weights similarly. Response to P was in solution. much reduced in the dry treatment. Shoot:root dry The experiment was designed as a weight ratio increased at the highest level of P × × × × factorial. Factors were fully treatment. Root length per plant was initially randomized down to clones, which split each plot. decreased by P treatment before increasing at the Sand beds were roofed with polyethylene to highest P level. prevent excessive water accumulation in plastic- Leaf Area Specific leaf area was strongly increased lined beds. by increase in P treatment, resulting in greater leaf Results First-season results indicated that triple area per plant at the . and . g P treatments. superphosphate (TSP) increased shoot dry weight Photosynthesis Photosynthesis, transpiration, and at kg/ha, but kg/ha depressed shoot weight stomatal conductance were reduced by increase in to the control level. Rock phosphate (RP) had no P level to . g, but further increase had little effect. significant effect on shoot weight. The more rapid decrease in transpiration than in Shoot dry weight of clone was significantly photosynthesis with increase in P level indicated related to percentage of leaf P in RP treatments, that P increases instantaneous water use efficiency. where leaf P was generally below .%. Clone Phosphorus treatment resulted in a steady increase shoot dry weight was unrelated to percentage of in water potential. Photosynthetic rates per plant, leaf P in TSP treatments, where leaf P was calculated from rate per unit leaf area and total leaf generally above .%, and presumably adequate. area per plant, showed that the rate per plant did Clone showed a maximum shoot dry weight in not decline much until g of P were added, and the range .–.% P regardless of P source. that . and . g P increased the rate per plant under wet conditions. Discussion Clones Shoot dry weight and shoot:root ratio of clone was greater than that of clone , but Growth response of hybrid poplar to phosphorus otherwise clonal differences were not observed. can be large (%), but is reduced under dry soil conditions. There was no evidence that higher level Experiment 2 of P increased response under dry conditions in this experiment. However, treatment with rates Objectives To examine the response of hybrid between . and . g P might have shown such a poplar to two sources of P and two sources of response in dry soil. Phosphorus level had nitrogen (N). important effects on specific leaf area and specific Methods Raised wooden beds, . m deep and root length. The overall effect was to increase . m wide, were divided into forty-eight m long photosynthesis per plant under wet conditions and plastic-lined plots. These were filled with building also to increase instantaneous water use efficiency.
Factors and levels for Experiment 2
Factor Levels Cumulative number
P source triple superphosphate rock phosphate 2
N source (NH4)2SO4 Ca(NO3)2 2 P level (kg/ha) 0 125 250 500 16 Replication 1 2 3 48 Clone 1 2 96
150 Increases in growth at the . and . g P level are per plant at the g P treatment may have not explained fully by the increase in leaf area per explained the high water potential in this plant and could possibly be related to differences treatment. in respiration. Differences in dry matter distribu- The lack of relationship between shoot dry tion appear to be unlikely causes of the response. weight and and foliar P levels above about % in The effect of P treatment in increasing water clone suggested that phosphorus requirements of potential was interesting, since the effect of this clone were relatively low, and that it was watering regime on water potential was not insensitive to higher levels. Overall results significant. The decrease in stomatal conductance supported the conclusion that optimal with P level presumably accounted for much of concentrations of P in foliage sampled in early July this increase, but the large increase in root length are in the region of .–.%.
151 Simulation of the Growth of Mixed Stands of Douglas-fir and Paper Birch Using the FORECAST Model
D. SACHS
Abstract literature data were used. The model was then used to examine the potential effects of different mixed- The FORECAST ecosystem model was used to wood management strategies on the long-term simulate the growth of mixed stands of Douglas-fir productivity of these sites. (Pseudotsuga menziesii) and paper birch (Betula papyrifera) in the Interior Cedar–Hemlock Model Calibration biogeoclimatic zone ₍ICH₎. The model was calibrated using information from current and recent studies FORECAST is an ecosystem model that explicitly in the Kamloops Forest Region, as well as a variety simulates the effects of nutrient availability and of literature data. A number of -year mixed- light competition on the growth of trees, shrubs, wood management scenarios were simulated using herbs, and mosses. It is the latest in the FORCYTE a replacement series design with a constant density series of ecosystem models (Kimmins et al. ). of stems per hectare and a rotation length of The model requires data on height growth, years. Pure stands of birch provided slightly biomass accumulation, nutrient content, light higher yields than pure Douglas-fir stands and attenuation, and photosynthetic efficiency of each dominated the mixtures at birch densities greater species simulated. Additional data describing than stems per hectare. Mixtures containing decomposition rates for detritus are also required. less than stems per hectare of birch showed For this simulation, the calibration for tree productivity losses in later rotations. These are stemwood and height growth of both tree species dependent on the model assumptions and the was based on predictions from the B.C. Ministry of quality of the input data. Forests VDYP and TIPSY yield projection models. Data on juvenile height growth of birch and Introduction Douglas-fir in the ICH came from Simard and Vyse ₍₎. Data on the biomass and nutrient content of Mixed stands of Douglas-fir (Pseudotsuga a chronosequence of birch stands was taken from a menziesii) and paper birch (Betula papyrifera) are study by Wang et al. ₍₎. Light attenuation and common in the Interior Cedar-Hemlock ₍ICH₎ photosynthetic efficiency curves for birch were biogeoclimatic zone of the southern interior of estimated using data from Wang et al. ₍₎. In the British Columbia. Recent forest management policy absence of available data, birch logs were assumed in the southern interior has eradicated birch and to decompose in years. Additional model created pure conifer stands where possible. calibration data came from the literature (Table ). However, growing concerns about the role of birch Literature data for coastal Douglas-fir were used to in the long-term productivity of these sites, and estimate decomposition parameters when no the increasing demand for birch logs, has brought interior Douglas-fir data were available. this policy into question (Simard and Vyse ). This project calibrated the FORECAST ecosystem Initial State and Model Testing model to simulate the growth of mixed stands of Douglas-fir and paper birch. Information from a Before using the FORECAST model for management number of current or recent studies in the simulations, an initial-state file describing the Kamloops Forest Region along with available forest floor detrital materials and soil nutrient
152 Additional sources of data for the calibration of the FORECAST model for these simulations.
Information Source
Douglas-fir biomass equations Gower et al. 1987, Brown 1978 Douglas-fir N content Clayton and Kennedy 1985 Douglas-fir needle decomposition Edmonds 1987, Harmon et al. 1990 Douglas-fir branch and bark decomposition Fogel and Cromack 1977 Douglas-fir log decomposition Sollins et al. 1987, Harmon et al. 1986 Douglas-fir fine root decomposition Sollins et al. 1980 Douglas-fir root biomass and turnover Fogel 1983, Santantonio and Hermann 1985, Vogt et al. 1986 Birch photosynthetic efficiency Jurik et al. 1988 Birch leaf decomposition Prescott (in progress), Bockheim et al. 1991, Berg and Staaf 1987, Berg and Wessen 1984, Van Cleve and Noonan 1975 Birch root biomass and turnover Safford 1974, Vogt et al. 1986 capital must be created. This was done by running to the highest level in the model, which resulted in the model with nutrient limitation to growth Douglas-fir outcompeting birch in later rotations. disabled for a -year simulation consisting of To lower nitrogen input when birch was not two rotations. The initial rotation started with a present, the constant annual nitrogen input due to mixture of Douglas-fir and birch that was grown non-symbiotic N fixation was lowered to a more for years, and then destroyed by a fire. This was reasonable . kg/ha. In addition, symbiotic N followed by another mixed stand that was clearcut fixation of approximately kg · ha- ·yr- was harvested after years. The resulting forest floor simulated for birch, based on initial laboratory was used as the starting point for all subsequent data from Simard et al. (in progress). In the simulations. FORECAST model, symbiotic N fixation is dependent Initial test simulations indicated that birch was on the total leaf biomass of the N-fixing species. much more sensitive to site nitrogen availability Thus stands of Douglas-fir do not receive excessive than was Douglas-fir. This is not surprising nitrogen inputs when birch is present at low levels since the nitrogen content of birch tissue is or absent entirely. As the amount of birch in a significantly higher than that of Douglas-fir. stand increases, the annual nitrogen input Simulated pure stands of birch demanded as much increases. As a result, the simulated mixture grows as kg N␣ ·␣ ha-␣·␣yr-, whereas Douglas-fir demand more consistently in later rotations (Figure b). rarely exceeded kg N · ha- ·yr-. Birch needed an This nitrogen input scenario was used in the annual nitrogen input of at least kg/ha to subsequent management simulations. maintain productivity in the simulations. For an initial test, the model was used to simulate a mixed Management Simulations stand containing Douglas-fir and birch seedlings per hectare at planting with an annual A series of pure stands and mixtures of different nitrogen input of kg/ha from non-symbiotic N initial planting densities of Douglas-fir and birch fixation (Figure a). However, this level of nitrogen were simulated. Each simulation consisted of four input far exceeded the demands of a pure Douglas- consecutive -year rotations. Each rotation ended fir stand, and its site productivity quickly increased with a clearcut harvest and replanting of the
153 400 Birch 1600 Birch 1200 Douglas-fir 1600 Douglas-fir 250 250 (a) 200 200 150 150 100
50 100 Stem biomass (Mg/ha) 0 1 2 3 4 Stem biomass (Mg/ha) 50 Rotation 0 1 2 3 4 250 (b) Rotation 200 Predicted stemwood biomass produced growing 150 each species separately over four consecutive 100-year rotations. 100
50 Additional initial planting densities were
Stem biomass (Mg/ha) simulated to further examine predicted 0 1 2 3 4 competition between Douglas-fir and birch in a Rotation replacement series design (de Wit ; Radosevich and Holt ) (Figure ). In the first rotation, Replacement series showing predicted birch was a better competitor than Douglas-fir, stemwood biomass accumulation of birch and providing a higher proportion of total yield in the Douglas-fir when planted in pure stands and mixed stands than would be expected if the two mixtures at a constant density of 1600 stems species were equally competitive (as indicated by per hectare over four consecutive 100-year the thin dashed straight line connecting the rotations. endpoints of the birch yield curve in Figure a). Douglas-fir provided less than the expected yield. mixture. The first two scenarios simulated pure Total yield remained constant for most mixtures, stands of birch and Douglas-fir (Figure ). Birch but dropped off slightly at high Douglas-fir showed a very slight decline in predicted yield in densities. Results in the fourth rotation (Figure b) the last two rotations. Douglas-fir showed a were similar, but yields of both species declined greater␣ decline in yield over four rotations. Birch when fewer than birch trees per hectare were dominated the mixed stands in all but the present in the stand. ␣birch: Douglas-fir simulation (Figure ). Total cumulative yield for the four rotations was Discussion approximately equal for all combinations containing birch (Figure ). The pure Douglas-fir The current calibration of FORECAST has some stands had the lowest predicted yield (Figure ). problems, but does indicate that birch may be The birch: Douglas-fir mixture was important in maintaining the long-term extended for an additional years to examine productivity of these sites. FORECAST was designed extreme long-term trends (Figure ). Douglas-fir to evaluate long-term trends, and not for yields increased slightly each rotation and appeared quantitative yield prediction. Also, every model is to plateau in rotations and . Total yield imperfect, and all results are a function of the remained stable for years. assumptions implicit in the model’s design or the
154 250 200 150 100
50 800 Birch 0 800 Douglas-fir 1 2 3 4
250 200 150 100 50 400 Birch 0 1400 Douglas-fir
Stem biomass (Mg/ha) 1 2 3 4
250 200 150 100 50 200 Birch 0 1400 Douglas-fir 1 2 3 4 Rotation
Stemwood biomass produced growing each species in combination over four consecutive 100-year rotations.
400 Birch 1200 Douglas-fir 250
1000 200
800 150 600 100 400 produced (Mg/ha) Stem biomass (Mg/ha) 50 200 Cumulative stem biomass
0 0 0:1600 1600:0 200:1400 400:1200 800:800 1 2 3 4 5 6 7 8 Initial stocking (Birch:Douglas-fir per hectare) Rotation
Predicted cumulative yield for four consecutive Predicted stemwood biomass produced growing 100-year rotations of pure and mixed stands of the species in combination for eight 100-year birch and Douglas-fir. rotations.
155 First rotation 250
200
150
100
50 Stem biomass (Mg/ha) 0 000 0400 0800 1200 1600 Douglas-fir per hectare 1600 1200 0800 0400 000 Birch per hectare
Fourth rotation 250
200
150
100
50 Stem biomass (Mg/ha) 0 000 0400 0800 1200 1600 Douglas-fir per hectare 1600 1200 0800 0400 000 Birch per hectare
Replacement series showing predicted stemwood biomass accumulation of birch and Douglas-fir when planted in pure stands and mixtures at a constant density of 1600 stems per hectare ( ▲ total stemwood biomass; ● birch stemwood biomass; ■ Douglas-fir stemwood biomass). quality of the input data. However, modelling year-old pure birch stand. However, FORECAST does efforts are useful for integrating data from a wide not simulate heart rot or the general decline of variety of studies to make projections, and for individual trees. The recommended rotation age indicating future research priorities. for birch in the ICH is – years, with The model indicates that birch stands require an merchantable volume actually declining in older average annual nitrogen input of at least – kg/ha stands because of the high incidence of decay to maintain productivity. This is not unreasonable (Simard and Vyse ). However, the birch yield given preliminary laboratory evidence that the model used to generate the input data for these rhizospheres of birch seedlings support a high level simulations clearly overestimates the volume in of associative N-fixing bacteria. Given that birch older stands. The birch stemwood biomass was assumed to provide a great deal of the accumulation data used as model input reaches its nitrogen input in this model run, it is not asymptote at about years. Hence, FORECAST is surprising that simulations of management overestimating the yield of pure birch stands scenarios with minimal birch content predicted because of poor input data. The input data for lower yields in later rotations because of nitrogen Douglas-fir stands shows continued stemwood limitation (Figure b). biomass accumulation beyond years. Therefore, FORECAST appears to overpredict birch yield in for any simulated rotation length greater than both pure and mixed stands. In reality, a -year- years, a pure Douglas-fir stand will produce more old stand of pure Douglas-fir would probably stemwood biomass than a pure birch␣ stand. contain more sound stemwood biomass than a - FORECAST may also overestimate birch yield in
156 mixtures because of the lack of simulation of developed by Dr. Hamish Kimmins and Kim individual tree canopies. This could cause an Scoullar with funding from the ENFOR program of overestimate of the competitive effect of birch on Forestry Canada. Douglas-fir. In FORECAST, the foliage of a species is simulated as an opaque blanket covering a References hypothetical hectare so that the competitive effect of just a few trees is spread equally over the entire Berg, B. and H. Staaf . Release of nutrients area. The opaque blankets of each tree species from decomposing white birch leaves and interact to compete for light. Therefore, as few as Scots pine needle litter. Pedobiologia :– birch trees per hectare significantly interfere . with Douglas-fir growth. In reality, so few birch Berg, B. and B. Wessen. . Changes in or- would not suppress Douglas-fir growth as greatly ganic-chemical components and ingrowth of as predicted, partly because trees grow in patches. fungal mycelium in decomposing birch leaf This is especially true in the ICH where root disease litter as compared to pine needles. is common. To more accurately simulate the Pedobiologia :–. growth of these mixed stands, there must be some Bockheim, J.G., E.A. Jepsen, and D.M. Heisey. spatial representation of individual tree canopies, . Nutrient dynamics in decomposing leaf or at least patches of trees. litter of four tree species on a sandy soils in Aside from the simulation of light competition, northwestern Wisconsin. Can. J. For. Res. the other main problem with this version of :–. FORECAST is that nitrogen is considered to be the Brown, J.K. . Weight and density of crowns only other factor limiting tree growth. In reality, of Rocky Mountain conifers. U.S. Dep. Agric. soil moisture is important, with birch being a For. Serv. Intermtn. For. Range Exp. Sta., much stronger competitor on the moist ICH sites Ogden, Utah. Res. Pap. INT-. p. (Simard and Vyse ). Other nutrients may also Clayton, J.L. and D.A. Kennedy. A comparison of be limiting, as evidenced by the high cation levels the nutrient content of Rocky Mountain in birch foliage. The model is capable of simulating Douglas-fir and ponderosa pine trees, . three limiting nutrients concurrently, but sufficient U.S. Dep. Agric. For. Serv. Intermtn. For. data for any nutrient other than nitrogen do not Range Exp. Sta., Ogden, Utah. Res. Pap. INT- exist at this time. . p. Despite these shortcomings, the FORECAST model deWit, C.T. . On competiton. Versl. provides some evidence that growing mixed stands Lanbouwkd. Onderz. :–. of birch and Douglas-fir should be feasible and Edmonds, R.L. . Decomposition rates and may offer higher long-term yields of fibre than nutrient dynamics in small-diameter woody pure stands of Douglas-fir. Under the current litter in four forest ecosystems in Washing- model assumptions, maintaining birch trees ton, U.S.A. Can. J. For. Res. :–. per hectare could sustain long-term productivity Fogel, R. . Root turnover and productivity of on these sites. These simulations should be coniferous forest. Plant and Soil :–. repeated when revised yield curves for birch in the Fogel, R. and K. Cromack, Jr. . Effect of ICH and field estimates of nitrogen-fixation rates in habitat and substrate quality on Douglas-fir birch rhizospheres are available. litter decomposition in western Oregon. Can. J. Bot. :–. Acknowledgements Gower, S.T., C.C. Grier, D.J. Vogt, and K.A. Vogt. . Allometric relations of deciduous (Larix Funding for this project was provided by the occidentalis) and evergreen conifers (Pinus Kamloops Forest Region and Research Branch, contorta and Pseudotsuga menziesii) of the British Columbia Ministry of Forests. A copy of the Cascade Mountains in central Washington. FORECAST model was graciously provided by Hamish Can. J. For. Res. :–. Kimmins and Kim Scoullar, the model developers. Harmon, M.E., G.A. Baker, G. Spycher, and S.E. FORECAST was based on the FORCYTE series of models Greene. . Leaf-litter decomposition in the Picea / Tsuga forests of Olympic National Park, Washington, U.S.A. For. Ecol. Manage. :–.
157 Harmon, M.E., J.F. Franklin, F.J. Swanson, P. Simard, S. and A. Vyse. . Ecology and Sollins, S.V. Gregory, J.D. Lattin, N.H. management of paper birch and black Anderson, S.P. Cline, N.G. Aumen, J.R. cottonwood in southern British Columbia. Sedell, S.W. Lienkaemper, K. Cromack Jr., B.C. Min. For. Res. Branch, Victoria, B.C. and K.W. Cummins. . Ecology of coarse Land Manage. Rep. No. . woody debris in temperate ecosystems. Adv. Sollins, P., S.P. Cline, T. Verhoeven, D. Sachs, and Ecol. Res. :–. G. Spycher. . Patterns of log decay in Jurik, T.W., J.A. Weber, and D.M. Gates. . old-growth Douglas-fir forests. Can. J. For. Effects of temperature and light on photo- Res. :–. synthesis of dominant species of a northern Sollins, P., C. Grier, F.M. McCorison, K. hardwood forest. Bot. Gaz. ():–. Cromack, Jr., R. Fogel, and R.L. Frederiksen. Kimmins, J.P., K.A. Scoullar, and M.J. Apps . . The internal element cycles of an old- FORCYTE- user’s manual for the benchmark growth Douglas-fir ecosystem in western Or- version. For. Can. North. For. Cent., Edmon- egon. Ecol. Monogr. :–. ton, Alta. ENFOR Proj. P-. p. Van Cleve, K. and L.L. Noonan. . Litter fall Prescott, C.E., R. Kabzems, S. Simard, C. Staley, and nutrient cycling in the forest floor of L. Zabek, and P.G. Comeau. . Mixedwood birch and aspen stands in Interior Alaska. forests: implications for litter decomposition. Can. J. For. Res. :–. These proceedings. Vogt, K.A., C.C. Grier, and D.J. Vogt. . Pro- Radosevich, S.R. and J.S. Holt. . Weed ecology. duction, turnover, and nutrient dynamics of John Wiley and Sons, New York, N.Y. p. above- and belowground detritus of world Safford, L.O. . Effect of fertilization on forests. Adv. Ecol. Res. :–. biomass and nutrient content of fine roots in Wang, J.R., S. Simard, and J.P. Kimmins. . a beech-birch-maple stand. Plant and Soil Physiological responses to thinning in paper :–. birch stands in the ICH zone of British Co- Santantonio, D. and R.K. Hermann. . Stand- lumbia. For. Ecol. Manage. :–. ing crop, production, and turnover of fine .␣ []. Above-ground biomass produc- roots on dry, moderate, and wet sites of ma- tion and nutrient accumulation in an age ture Douglas-fir in western Oregon. Ann. Sci. sequence of paper birch in the Interior For. ():–. Cedar–Hemlock Zone in British Columbia. Simard, S., C.Y. Li, and D.A. Perry. [n.d.]. Asso- For. Ecol. Manage. In press. ciative nitrogen fixation by paper birch and Douglas-fir seedlings grown in mixture in soils collected from the southern interior of British Columbia. In preparation.
158 Mixedwood Forests: Implications for Litter Decomposition
C.E. PRESCOTT, R. KABZEMS, S. SIMARD, C. STALEY, L. ZABEK, AND P.G. COMEAU
Abstract faunal interactions in mixed plantations, which increased the rates of decomposition and nitrogen A network of ongoing experiments in British release from litter. Development of hardwood Columbia is described, experiments that explore clumps after clearcutting white spruce in the boreal the implications of mixedwood management for forest contributes to acceleration of nutrient rates of litter decomposition in forests. The cycling because of higher concentrations of influence of tree species composition is being nitrogen and phosporus in foliar litter of aspen assessed by comparing rates of decay of foliar litter and poplar and subsequent faster decomposition of different species and relating this to the (Paré and Van Cleve ). Conifer litter often chemical “quality” of each litter. The influence of decomposes more rapidly when mixed with the forest floor is being compared by measuring hardwood leaf litter (Taylor et al. ), suggesting rates of decomposition of a single litter type on that decomposition of conifer litter may be faster different types of forest floors. Finally, the in mixed plots. The influence of mixed litters of influence of mixed forests is being assessed by either hardwood or conifer have been less comparing rates of decomposition of pure and consistent (Blair et al. ). The forest floor that mixed-species litter in pure and mixed stands. The develops under mixed stands may also influence results will be used to predict rates of the rate of decomposition and nutrient release decomposition under different management from litter. McClaugherty et al. ₍₎ noted a slight scenarios, such as mixedwoods. tendency for litter to decompose more rapidly in forests of its own species than does transplanted Introduction litter, but it has also been suggested that litter will Recent interest in management of mixedwood decompose more rapidly on N-rich sites, regardless forests in British Columbia, including boreal of species. Most of these observations are from spruce and aspen, interior Douglas-fir and birch, studies of different forest types, in which not only and coastal Douglas-fir and alder, has highlighted the species, but site factors such as fertility and our limited understanding of the interactions climate, also vary. Testing the pure effect of species among species in mixed forests, particularly as they on decomposition requires plantations of different influence below-ground processes. Studies in other species on a single site. forests have demonstrated that seral stages or The following questions are being addressed in a interplanting with other species, particularly network of ongoing studies in British Columbia: hardwoods, can significantly improve growth of • Are there consistent differences in decay rates of regenerating conifers, partly through improved litter of different tree species in British nutrition of trees in mixed forests. In the United Columbia, and are these predictable from Kingdom, Sitka spruce in mixtures with larch or measures of the “quality” of the litter? pine grow significantly faster than those in single • Does litter decompose fastest on its own forest species plantations, and this has been linked to floor, or on N-rich forest floors? greater availability of nitrogen in the forest floor of • Does litter decompose more rapidly in mixed or mixed plantations (Carlyle and Malcolm ). pure forests? ₍₎ Williams and Alexander suggested that the • Does mixing litter of different species influence improved nitrogen availability in mixed plantations decomposition rates? was at least partly due to more complex fungal-
159 Methods mann spruce (Se), subalpine fir (Fs), amabilis fir (Fa), Douglas-fir (fd), trembling aspen (At), Freshly fallen foliar litter of each species was cottonwood poplar (Act), vine maple (Mv), and collected in autumn at each site. Fibreglass mesh red alder (Dr). bags, – cm on a side with pore size . or Duration ‒ . mm, containing . g of foliar litter, were used in all experiments. At yearly intervals for years, Results to Date During the first year, less of the bags of each type are removed from each plot, original g of litter remained at the coastal site, and the dry weight of material remaining in each but the relative ranking of species at the two sites bag is measured. Mass remaining at each time and was similar (Table ). decomposition constant will be determined for each litter type in each plot and compared among Experiment 2: Decomposition Rates of Litter of plots (to determine the influence of forest floor in Seven Tree Species in ICH Forests pure and mixed forests), among litter types (to determine the influence of litter quality), and in Questions Influence of litter quality, forest type, mixed and pure bags (to determine the influence and opening. of mixing litters). Sites Forests and adjacent cutovers in the ICH zone near Hidden Lake, Malakwa, and Adams Lake. Experiment 1: Decomposition Rates of Litter of 14 Tree Species in British Columbia Litter Types Trembling aspen, western larch, Douglas-fir, paper birch, white pine, lodgepole Question: Influence of litter quality. pine, and western redcedar
Sites Boreal spruce forest in the BWBS zone near Duration – Dawson Creek; coastal mixed conifer forest in the Results to Date After years, there was some CWH zone near Maple Ridge. variation in relative rates of decomposition of litter Litter Types Lodgepole pine (Pl), white pine (Pw), types (Table ). Decomposition was usually more ponderosa pine(Py), western hemlock (Hw), rapid in cutovers than in adjacent forests. western redcedar (Cw), western larch (Lw), Engel-
Dry weight of foliar litter of 14 tree species remaining after incubating for 1 year in coastal and boreal forest sites in British Columbia
Coastal Boreal Broadleaved (coastal)
Species Weight (g) Rank Weight (g) Rank Species Weight (g) Rank
Py 1.26 1.0 1.54 1.0 Mv 0.49 1.0 Pw 1.27 2.0 1.70 7.0 Dr 1.12 2.5 Lw 1.28 3.0 1.62 2.0 At 1.12 2.5 Fd 1.31 4.0 1.64 4.0 Act 1.35 4.0 Pl 1.34 5.5 1.67 5.5 Se 1.34 5.5 1.67 5.5 Fs 1.37 7.0 1.63 3.0 Hw 1.51 8.5 1.77 8.0 Fa 1.51 8.5 1.85 9.0 Cw 1.62 10.0 1.90 10.0
160 Dry weight of foliar litter of seven tree species remaining after incubating for 2 years in three ICH forests in interior British Columbia. Weight remaining of original 2 g and rank are shown.
Hidden Lake Malakwa Adams Lake
Species Forest Cutover Forest Cutover Forest Cutover
At 1.07 (1) 0.92 (1) 1.01 (1) 1.03 (1) 1.19 (5) 1.04 (1) Lw 1.09 (2) 1.18 (3) 1.05 (3) 1.29 (3) 0.97 (1) 1.29(6) Fd 1.11 (3) 1.14 (2) 1.15 (4) – 1.00 (2) 1.10 (3) Ep 1.34 (6) 1.20 (4) 1.02 (2) 1.37 (5) 1.20 (6) 1.08 (2) Pw 1.25 (5) 1.28 (5) 1.25 (6) 1.25 (2) 1.02 (3) 1.22 (5) Pl 1.18 (4) 1.35 (6) 1.16 (5) 1.33 (4) 1.04 (4) 1.16 (4) Cw 1.36 (7) 1.56 (7) 1.36 (7) 1.61 (6) 1.37 (7) 1.39 (7)
Experiment 3: Decomposition of Birch Leaf Litter Types Cedar, hemlock, and Douglas-fir are Litter in Forest Floors of Three Species collected from -year-old plantations near Maple␣ Ridge. Question Influence of forest floor. Duration – Sites Microcosm experiment, all at °C, % Results to Date Established in April . moisture.
Litter Types Paper birch leaf litter in forest floors 1.0 of lodgepole pine, Douglas-fir, and red alder. Alder Duration weeks Fir .8 Pine Results to Date Birch leaves lost weight fastest in alder forest floors and slowest in pine forest floors .6 (Figure ). This was not related to N availability, which was highest in Douglas-fir forest floors by the end of the experiment (Figure ). Weight loss .4
was related to the amount of frass in the bags, remaining (g) Weight indicative of soil faunal activity, which was greatest .2 in alder and least in pine.
Experiment 4: Decomposition of Cedar, Hemlock, 0.0 and Douglas-fir in Forests of Each Species 0 10 20 30 40 52 70 Weeks Questions Influence of litter quality and forest floor. Dry weight of paper birch leaf litter remaining Sites Four -year-old plantations of western during incubation in forest floors of lodgepole redcedar, western hemlock, and Douglas-fir in the pine, Douglas-fir, and red alder in laboratory CWH zone near Port Renfrew and Lake Cowichan. microcosms.
161 3.5 Results to Date During the first year, mixed litter Alder of both species decomposed more slowly than pure 3.0 Fir litter. There was not a consistent effect of forest Pine 2.5 type (Table ).
2.0 Experiment 6: Decomposition of Pure and Mixed Litter of Pine, Douglas-fir, and Birch in 1.5 Forests of Each Species
1.0 Extractable N (mg/g) Questions Influence of forest floor and mixing litter.
.5 Sites -year-old plantations of lodgepole pine, Douglas-fir, and paper birch in the Skimikin stump 0.0 trial in the ICH zone near Salmon Arm. 0 10 20 30 40 52 70 Weeks Litter Types Pine, fir, birch, mixed pine–fir, pine– birch, fir–birch. Concentrations of KCl-extractable N in forest Duration – floors of lodgepole pine, Douglas-fir, and red alder in laboratory microcosms. Results to Date Established in May .
Experiment 5: Decomposition of Pure and Experiment 7: Decomposition of Pure and Mixed Litter of Spruce and Aspen in Pure and Mixed Litter of Douglas-fir and Red Alder Mixed Forests Questions Influence of forest floor, mixing litter, Questions Influence of forest type, mixing litter, and N-fixing species. and mixed forest. Sites Three pure -year-old stands of Douglas-fir Sites One trembling aspen, one interior spruce, and red alder in the CWH zone near Maple Ridge. one mixed spruce-aspen forest in the BWBS zone Litter Types Douglas-fir, alder, mixed. near Dawson Creek. Duration – Litter Types Aspen, spruce, mixed. Results to Date Established in April . Duration –
Dry weight of foliar litter of aspen and spruce remaining after incubating for 1 year in pure and mixed stands of aspen and spruce in northern British Columbia. Weight remaining of original 2 g and rank are shown.
Aspen Spruce Mean Site Alone Mixed Alone Mixed Alone Mixed
Aspen 0.70 (4) 0.68 (1) 1.39 (3) 1.3 (1) 1.04 (3) 1.02 (1) Spruce 0.68 (3) 0.74 (3) 1.33 (1) 1.38 (2) 1.01 (2) 1.06 (2) Mixed 0.60 (1) 0.76 (4) 1.37 (2) 1.46 (3) 0.99 (1) 1.10 (3) Cutover 0.65 (2) 0.70 (2) 1.51 (4) 1.56 (4) 1.08 (4) 1.13 (4)
162 Summary McClaugherty, C.A., J. Pastor, J.D. Aber, and J.M. Melillo. . Forest litter decomposition in A network of experiments has been established to relation to soil nitrogen dynamics and litter determine the influence of litter quality, forest quality. Ecology :–. floor, and mixing litter on rates of decomposition. Paré, D. and K. Van Cleve. . Soil nutrient Results of these experiments and others addressing availability and relationships with the effects of climate, litter quality, opening size, aboveground biomass production on and fertilization will improve our ability to predict postharvested upland white spruce sites in rates of litter decomposition and nutrient recycling interior Alaska. Can. J. For. Res. :–. in British Columbia forests under different Taylor, B.R., W.F.J. Parsons, and D. Parkinson. management scenarios. . Decomposition of Populus tremuloides leaf litter accelerated by addition of Alnus References crispa litter. Can. J. For. Res. :–. Williams, B.L. and C.E. Alexander. . Interac- Blair, J.M., W. Parmalee, and M.H. Baere. . tions on mixing litters from beneath Sitka Decay rates, nitrogen fluxes, and decomposer spruce and Scots pine and the effects on mi- communities of single- and mixed-species crobial activity and N-mineralization. Soil foliar litter. Ecology :–. Bio. Biochem. ():–. Carlyle, J.L. and D.C. Malcolm. . Nitrogen availability beneath pure and mixed larch and spruce stands growing on a deep peat. Plant Soil :–.
163