3.6 Succession, Competition, and Responses to Disturbance

• Dense even-aged stands of paper birch can quickly develop after a disturbance, but once established, little ingress occurs, and mature birch stands are often characterized by a lack of young stems in younger age classes.

As elsewhere within the • Among British Columbia’s broadleaf species, only aspen is less shade- species’ natural range in tolerant than paper birch. Therefore, available light is one of the most North America, young critical factors that a manager can influence for successful management stands of pure paper birch of birch. As with other shade-intolerant species, birch’s shade intolerance are relatively uncommon becomes more pronounced with increasing stand age. in British Columbia. This is partly due to fire sup- The spatial distribution of birch across the landscape is constantly changing, partly pression and conifer- as a result of succession, but also due to changes in the type and scale of disturbance. centred silviculture At the landscape level, the dominant disturbance responsible for the current Interior practices during the patch pattern of birch was wildfire occurrence and settlement patterns at the turn of past several decades. the century. Wildfire has largely been eliminated by fire suppression programs and replaced by finer, community-scale disturbances such as root disease, windthrow, clearcutting, selective weeding, and cattle grazing. As a result, birch in maturing fire- Information Sources: seral forests is nearing its natural ecological rotation age (60–70 years) and is gradually being replaced by late-succession conifers. However, in many older stands birch does Buse and Bell 1992; persist in pockets created by root disease or other local disturbances. In recent clearcuts, Bergeron and regenerating birch is fragmented in small patches and often reduced to very small Dansereau 1993; Paré et components of any given site series. al. 1993; Vyse and DeLong 1994; Youngblood 1995; Simard 1996a.

3.6.1 Early-stage succession in pure paper birch

• Fire and other disturbances that expose mineral soil favour birch seedling establishment and sprouting from surviving roots.

The need for elimination • Birch can be an aggressive pioneer when it is not overtopped by shrubs. of early competition to In dense young birch stands, birch can be a strong competitor with under- paper birch seedlings is storey conifers, particularly those that are moderately or highly intolerant well known, but there is to shade. not sufficient information on the length of time over It is important for forest managers to distinguish between birch establishment which shrub competition and succession following broad-scale disturbances (such as fire or clear-cut harvesting), threatens survival or and birch’s successional responses to small-scale disturbances. growth of birch. Large-scale disturbances are typified by areas where paper birch and early seral conifers, such as Douglas-fir, western white pine, and western larch, invade sites where stand-replacement fires expose mineral soil and create open conditions. Stands with birch tend to burn only under special conditions such as in early spring before bud burst, following severe drought, or late in stand history when the proportions of conifers are greater.

Sprouting from adventitious buds around the root collar is a major source of birch regeneration after fire. Once established, rapid juvenile height growth allows birch to

35 dominate post-fire successional communities. Developing, pure, even-aged stands of birch permit little ingress and they self-thin rapidly. In some areas of British Columbia’s interior wet belt, mixed stands develop where veteran and/or adjacent conifers and birch survive the fire, and both disperse seed onto heterogeneous seedbeds created by complex fire behaviour. In this successional sequence, the presence of birch is characterized by a lack of birch in small size classes.

In the ICH zone, following large-scale disturbances such as fire or clearcutting, birch, spruce, and western hemlock seed-in within about 5–10 years. Western redcedar establishes over a longer period, often via layering. If birch is dense, conifers can be suppressed (especially moderate or shade-tolerant species such as Douglas-fir and western redcedar) or suffer mortality (especially shade-intolerant species such as white pine and western larch). Over time, size hierarchies develop and the stands are stratified Information Sources: vertically according to species composition and subsequent stand disturbances. Arlidge 1967; Van Wag- The other main kind of succession involving paper birch is in relation to small- ner 1969; Hibbs 1982; scale disturbances. This is generally a later stage of succession for birch, as outlined in Foster and King 1986; Section 3.6.2. For the manager, the main point is that birch does not reproduce well in Perala and Alm 1990b; established forests, where its occurence in later stages of forest succession is mainly in Safford et al. 1990; gaps created in the canopy following disturbances such as blowdown, fire, root disease, Simard 1990; Sims or insect-kill of conifers. Birch does regenerate in small gaps but does not grow well et al. 1990; Simard and because western redcedar, western hemlock, and other conifers may be released from Vyse 1992; McClure their suppressed position in the understorey. In larger gaps, birch may regenerate and Lee 1993; Wang where mineral soil is exposed and open light requirements exist. et al. 1995; Sachs 1996; Simard 1996a; Wang Table 9 lists the main trees, shrubs, herbs, mosses, and lichens that typically oc- et al. 1996. cur in association with paper birch in British Columbia.

36 TABLE 9. Dominant trees, shrubs, herbs, mosses, and lichens that occur in association with paper birch in British Columbia The species listed in this table are limited to those sufficiently prominent to be used in the names of site series where birch is common (see Appendices 2, A to E, derived from the Silviculture Interpre- tations Working Group 1994). Species are listed alphabetically by common name rather than by degree of association with paper birch. Common names follow Meidinger (1987).

Trees Herbs aspen Populus tremuloides bluebells Mertensia spp. cottonwood Populus trichocarpa bunchberry Cornus canadensis dogwood Cornus nuttallii coltsfoot Petasites spp. Douglas-fir Pseudotsuga menziesii currant Ribes spp. Douglas maple Acer glabrum fairybells Disporum spp. lodgepole pine Pinus contorta fireweed Epilobium angustifolium ponderosa pine Pinus ponderosa foamflower Tiarella spp. subalpine fir Abies lasiocarpa grouseberry Vaccinium scoparium water birch Betula occidentalis horsetail Equisetum arvense western hemlock Tsuga heterophylla Kentucky bluegrass Poa pratensis western larch Larix occidentalis lady fern Athyrium filix-femina western redcedar Thuja plicata oak fern Gymnocarpium dryopteris western white pine Pinus monticola ostrich fern Matteuccia struthiopteris western yew Taxus brevifolia parsley fern Cryptogramma crispa white spruce Picea glauca pinegrass Calamagrostis rubescens white spruce hybrid Picea glauca x engelmannii poison-ivy Toxicodendron rydbergii ______prickly rose Rosa acicularis Shrubs prince’s pine Chimaphila umbellata devil’s club Oplopanax horridus purple peavine Lathyrus nevadensis falsebox Paxistima myrsinites raspberry Rubus idaeus five-leaved bramble Rubus pedatus ricegrass Orzyopsis spp. gooseberry Ribes spp. scouring rush Equisetum spp. hazel nut Corylus cornuta sedge Carex spp. highbush cranberry Viburnum edule skunk cabbage Lysichiton americanum huckleberry Vaccinium spp. soft-leaved sedge Carex disperma juniper Juniperis communis Solomon’s seal Smilacina racemosa mock-orange Philadelphus lewisii soopolallie Shepherdia canadensis Nootka rose Rosa nutkana spiny wood fern Dryopteris expansa Oregon grape Mahonia spp. thimbleberry Rubus parviflorus oval-leaved blueberry Vaccinium ovalifolium toad-flax Geocaulon lividum pink spirea Spirea douglasii ssp. menziesii twinberry Lonicera involucrata Sitka alder Alnus viridis ssp. sinuata twinflower Linnaea borealis snowberry Symphoricarpus occidentalis wildrye Elymus spp. thimbleberry Rubus parviflorus wild sarsaparilla Aralia nudicaulis velvet-leaved blueberry Vaccinium myrtilloides wood horsetail Equisetum sylvaticum wintergreen Pyrola spp. Mosses and Lichens Cladina Cladina spp. knight’s plume Ptilium crista-castrensis Cladonia Cladonia spp. pipecleaner moss Rhytidiopsis robusta electrified cat’s-tail moss Rhytidiadelphus squarrosus sphagnum Sphagnum spp. feathermoss, red-stemmed Pleurozium schreberi step moss Hylocomium splendens

37 3.6.2 Later-stage succession in pure paper birch

• For late-stage succession, more information is available for Betula pendula stands in Europe than for paper birch in British Columbia. The international literature suggests that mature birch stands are characterized by a general lack of birch regeneration because of low light intensity and root competition, conditions comparable to those found in this province’s mature birch stands.

• Although paper birch can reach ages of about 150 years, the species often begins to decline about 70 years after the disturbance that provided circumstances favourable to birch vegetative or seedling reproduction.

Paper birch, wherever it Because paper birch is shade-intolerant, pure stands usually last only one becomes established, will generation without further major disturbance, and then are succeeded by understorey persist by sprouting. This conifers. Typical stages in birch stand development, from 0 to 80 years, are shown in ensures birch’s presence Figure 17. In the southern interior of British Columbia, there are common examples of over a broad range of birch stands succeeding to variable and complex mixtures that include one or more of successional stages. western redcedar, western hemlock, Douglas-fir, western white pine, or western larch. Birch is a common component of climax forests where gaps have been created by disturbances such as blowdown, fire, and root rot. The scattered distribution of birch among conifers and often aspen or balsam poplar (Figure 18) is typical of its occurrence in much of British Columbia.

Late-stage succession is obviously dependent on long intervals between fires. Reduced fire frequency can influence the successional pathway of stands that include Information Sources: birch. When the fire return interval is short, stands that contain birch (or aspen) usually Kinnaird 1974; Ohmann burn while still in the even-aged stage of development, and regeneration after a burn et al. 1978; Krajina et al. tends to be of the same species as before the burn. In contrast, where intervals between 1982; Perkins et al. 1988; fires are longer, successional pathways are increasingly influenced by canopy openings Comeau et al. 1989; caused by wind, insects, or diseases. Where managers of birch–conifer mixed stands Safford et al. 1990; wish to blend their silvicultural interventions with the current successional status of a Zasada et al. 1992; stand, they could be involved with a birch component that is either a result of early- Frelich and Reich 1995; stage recovery from a fire or logging disturbance or a result of later-stage succession Simard 1996b. related to smaller-scale canopy openings.

38 a c

b d

FIGURE 17. Typical stages in paper birch stand development: (a) seedling stage, and (b) sapling stage, (c) age 40 years, and (d) age 80 years. (Photographs by S. Simard and J. Mather)

39 FIGURE 18. Scattered birch in this stand, in mixture with balsam poplar, aspen, and interior spruce, is typical of many boreal mixedwood stands in northeastern British Columbia. (Photograph by K. Enns)

40 3.6.3 Development of mixed birch-conifer stands

• Mixed stands of birch and conifers can be grown more densely than pure birch, largely because they occupy separate root and crown growing spaces and have different growth rates.

There is evidence that • In the wet biogeoclimatic subzones and variants of interior British birch in high densities Columbia, light-demanding species such as western larch, western white can compete effectively pine, and lodgepole pine experience greater growth losses under the shade with conifers for light. In of paper birch than do moderately shade-tolerant species such as Douglas- boreal and sub-boreal fir and spruce, and even more so than do shade-tolerant species such as regions, birch is often the western redcedar and western hemlock. last early-seral species to die in stands that have Paper birch readily seeds into areas with soil disturbances caused by logging, been taken over by spruce slash burning, and mechanical site preparation. On sites where birch seeds-in densely or other conifers. or has sprouted from stumps, it can out-compete conifers. It may inhibit conifers where areas disturbed by harvesting and silvicultural practices are located near stands of mature birch trees or where residual birch are left on-site to allow seeding-in. Forest litter has long been recognized as a possible deterrent to tree seedling establishment. Litter from paper birch may play an important role in this regard; there is evidence from field tests in Minnesota that seed germination of various herbs, shrubs, and tree species (including white spruce) was lower in paper birch litter than in litter from aspen, red pine, white pine, or balsam fir.

Development of mixed stands following a large-scale disturbance in the southern Interior usually involves: rapid establishment of all species within 5–10 years; early dominance of shade-intolerant species such as paper birch; early suppression of understorey, shade-tolerant species such as western redcedar; later (after 40 or more years) creation of small openings due to mortality of birch and small-scale disturbances; release of suppressed-shade tolerant conifers; and subsequent re-sorting of species size hierarchies over time as birch drops out and longer-lived conifers occupy the upper stratum of the canopy. With 14 native tree species, the ICH zone supports a wide variety of stand structures. The numbers and proportions of the constituent species vary widely, even for stands initiated by the same disturbance (Cameron I.R. 1996). Measurements of even-aged, 10-year-old stands in the ICH zone of southern interior British Columbia showed that increasing birch density had strong effects on the size of neighbouring shade-intolerant western larch, moderate effects on the size of moderately shade-tolerant Douglas-fir, and little effect on the size of shade-tolerant western redcedar. Mixed-species stands, which are typical of birch’s occurrence in most of British Columbia, generally have greater variation in vertical structure than do single-species stands (Figures 18 and 19). Although details are not well known, there are indications that use of birch by hares, deer, and moose can have an important effect on stand density and structure of young birch stands.

In its boreal and sub-boreal setting, paper birch is often the last early-seral species to die in stands that have been taken over by spruce and other conifers. This is one reason why birches in boreal mixedwood stands are often larger than their associated broadleaf seral species (note balsam poplar in the example shown in Figure 11), and why birch is often a substantial component of boreal mixedwood stands where conifers and aspen have been harvested (Figure 20).

In the Kamloops and Nelson forest regions, birch is often considered a serious long-term competitor for conifers, although it is debatable whether the competition is as serious as some managers perceive. Vigorous and abundant birch can cause localized

41 FIGURE 19. Diverse stand structure in a 30-year-old mixed birch–conifer stand in the Spey Creek study area, Prince George Forest District, British Columbia. (Photograph by P. Comeau)

FIGURE 20. Harvest of boreal forest stands to use their aspen and conifer content for oriented strandboard and other products may leave behind residual birch. There is not yet detailed documentation of the future stands that will result from retention of birch as the dominant residual in such harvest areas.

42 threats to the survival and growth of intolerant species, such as western larch and lodgepole pine, and moderately shade-tolerant species, such as Douglas-fir and spruce. In the ICH zone of the Kamloops forest region, birch is not a problem for regeneration establishment, but may affect growth of conifers within the first 15 years after conifers are established. Recent work in British Columbia indicates that low densities of birch, when growing in intimate mixtures with Douglas-fir, may have positive effects by reducing stand mortality due to Armillaria root disease, maintaining long-term productivity, and enhancing stand diversity.

In a 135-year-old mixed species stand of the ICHmc2 variant, about 40 km northeast of Hazelton, birch that had originated within the first 10 years after a fire in 1855 occurred together with western hemlock, western redcedar, lodgepole pine, hybrid spruce, and subalpine fir. Within a single 10-year age class, paper birch had a relatively low variation in size (161% variation in height and 314% variation in dbh) compared to hemlock, which Information Sources: had the greatest variation (418% variation in height and 513% variation in dbh). Birch, like spruce, hemlock, and subalpine fir, had trees in the dominant and codominant Ahlgren and Ahlgren canopy layer that were younger than trees in the lower (intermediate) canopy layer. 1981; Perala 1989; Stem analyses revealed that the birch had averaged only 3 years to reach breast height. Sims et al. 1990; This rapid growth continued to average 30 cm/year for about 30 years, at which point Environmental Training increasing side shade began to slow birch height growth to less than 8 cm/year during Centre 1995; LePage 1995; the last 40 years in this 135-year-old stand. By 135 years, the birch had a mean crown Cameron, I.R. 1996; depth of about 8 m, their tops were 5–10 m below the level of the dominant conifer Simard 1996b. canopy, and individual trees were dropping out of the stand.

3.7 Paper Birch Growth and Yield

• Paper birch juvenile growth rates are moderate compared to those in black cottonwood, balsam poplar, or aspen. In the ICH zone, first-year birch seedlings have been recorded to reach 40 cm on good sites, but they average about 10 cm.

The characteristic • Height growth of birch on good sites is asymptotic, with rapid early height asymptotic height growth growth followed by marked slowing after 50 years. In contrast, birch on pattern of paper birch is poor sites has an overall slower rate of height growth that follows an easily distinguishable almost linear pattern. from that of associated conifer species, which Unlike alder, cottonwood, and aspen, paper birch does not exhibit exceptionally are characterized by rapid early growth rates but, compared to conifers, birch’s early height growth is still near-linear height impressive. Birch stems of sprout origin grow at faster rates than those of seed origin. growth patterns. For example, in the ICHmw3 variant, 10-year-old birch of sprout origin have been measured at an average of 7 m tall, while birch seedlings of the same age were only 4 m tall.

Greenhouse results usually cannot be applied to field conditions, but they are useful for determining the biological growth potential of a species. Paper birch has been grown to a height of 4.9 m and 2.5 cm diameter in 9 months, under favourable

43 Information Sources: nutrient and moisture conditions in a greenhouse. Such a growth rate is an unrealistic management goal but, as with other tree species, it does indicate that barriers to high Solomon and Leak 1969; growth rates in the field result from environmental factors rather than inherent biological Grant and Thompson limitations within birch. 1975; Safford 1983; Alberta Forest Service Site index curves have been developed for paper birch in British Columbia, Alaska, 1985; Leak et al. 1987; and the eastern United States. The British Columbia curves are reproduced in Figure Safford et al. 1990; 21 and the corresponding site index table is shown in Table 10. The data shown in Simard and Vyse 1992; Table 10 and Figure 21 are derived from Alberta Forest Service (1985) aspen site index Silviculture Interpretations estimates. Until new site index curves are developed from British Columbia birch data, Working Group 1994; these aspen-based curves are being used to estimate birch site indices. Cameron, I.R. 1996.

Interior paper birch

50

45 30

40

25 35

20 30

25 15 Height (m) 20 10 15

10 5

5

0 0 25 50 75 100 125 150 Breast-height age (yr)

FIGURE 21. Site index curves for paper birch in British Columbia, for a base age of 50 years at breast height, based on curves for aspen from the Alberta phase 3 inventory, validated by aspen data gathered in interior British Columbia. To date (June 1996) no validation of paper birch site index data was com- piled for British Columbia sample plots. See also the accompanying site index table and explanatory footnote in Table 10 (Alberta Forest Service [1985] as compiled by Thrower et al. 1994; T. Honer, pers. comm., July 1994).

44 TABLE 10. Site index table for paper birch in British Columbia, corresponding to site index curves of Figure 21 (Alberta Forest Service 1985, as compiled by Thrower et al. 1994)

bh age Top height (m) bh age (years) 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 (years)

Site index (m) 10 18 22 25 29 32 35 38 ------10 15 13 16 18 20 23 25 27 29 31 34 36 38 40 ------15 20 10 12 14 16 18 20 22 23 25 27 29 30 32 33 35 37 3840------20 258101213151718202123242627283031 33 34 35 36 37 38 40 ------25 30 7 9 10 12 13 14 16 17 19 20 21 22 24 25 26 28 29 30 31 33 34 35 36 38 40 ------30 356 8 910121314151718192021232425 26 27 28 30 31 32 33 34 35 37 38 39 40 - - - 35 40 6 7 8 9 11 12 13 14 15 16 17 18 20 21 22 23 24 25 26 27 28 30 31 32 33 34 35 36 37 38 39 40 40 45 5 6 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 27 28 29 30 31 32 33 34 35 36 37 38 45 50 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 50 555 6 7 8 8 910111213141516171819 20 21 22 23 24 25 26 27 28 29 30 31 31 32 33 34 55 60 - 5 6 7 8 910111212131415161718 19 20 21 22 23 24 25 26 26 27 28 29 30 31 32 33 60 65 - 5 6 7 8 8 9 10 11 12 12 13 14 15 16 17 18 19 20 21 22 23 24 25 25 26 27 28 29 30 31 32 65 70 - 5 6 6 7 8 9 10 11 11 12 13 14 15 16 17 17 18 19 20 21 22 23 24 25 26 26 27 28 29 30 31 70 75 - 5 5 6 7 8 9 9 10 11 12 13 13 14 15 16 17 18 19 19 20 21 22 23 24 25 26 27 27 28 29 30 75 80 - - 5 6 7 7 8 9 10 11 11 12 13 14 15 16 16 17 18 19 20 21 21 22 23 24 25 26 27 27 28 29 80 85 - - 5 6 6 7 8 9 910111213131415 16 17 18 18 19 20 21 22 23 23 24 25 26 27 28 29 85 90 - - 5 5 6 7 8 8 910111112131415 15 16 17 18 19 20 20 21 22 23 24 25 26 26 27 28 90 95 - - 5 5 6 7 7 8 910101112131414 15 16 17 18 18 19 20 21 22 23 23 24 25 26 27 28 95 100 - - 5 5 6 7 7 8 9 9 10 11 12 12 13 14 15 16 16 17 18 19 20 20 21 22 23 24 25 25 26 27 100 105 - - - 5 6 6 7 8 8 9 10 11 11 12 13 14 14 15 16 17 18 18 19 20 21 22 23 23 24 25 26 27 105 110 - - - 5 6 6 7 8 8 9 10 10 11 12 13 13 14 15 16 17 17 18 19 20 21 21 22 23 24 25 25 26 110 115 - - - 5 5 6 7 7 8 9 9 10 11 12 12 13 14 15 15 16 17 18 19 19 20 21 22 23 23 24 25 26 115 120 - - - 5 5 6 7 7 8 9 9 10 11 11 12 13 14 14 15 16 17 18 18 19 20 21 22 22 23 24 25 26 120 125 - - - 5 5 6 6 7 8 8 9 10 11 11 12 13 14 14 15 16 17 17 18 19 20 20 21 22 23 24 25 25 125 130 - - - - 5 6 6 7 8 8 9 10 10 11 12 13 13 14 15 16 16 17 18 19 19 20 21 22 23 23 24 25 130 135 - - - - 5 6 6 7 8 8 9 10 10 11 12 12 13 14 15 15 16 17 18 18 19 20 21 22 22 23 24 25 135 140- - - -556778991011121213 14 14 15 16 17 17 18 19 20 21 21 22 23 24 25 140 145- - - -556778991011111213 14 14 15 16 17 17 18 19 20 20 21 22 23 24 24 145 150- - - -556778891011111213 13 14 15 16 16 17 18 19 19 20 21 22 23 23 24 150 155- - - -556678891010111213 13 14 15 15 16 17 18 18 19 20 21 22 22 23 24 155 160- - - -556678891010111212 13 14 15 15 16 17 18 18 19 20 21 21 22 23 24 160 165 - - - - 5 5 6 6 7 8 8 9 9 10 11 12 12 13 14 14 15 16 17 17 18 19 20 21 21 22 23 24 165 170 - - - - - 5 6 6 7 7 8 9 9 10 11 11 12 13 14 14 15 16 17 17 18 19 20 20 21 22 23 23 170 175 - - - - - 5 6 6 7 7 8 9 9 10 11 11 12 13 13 14 15 16 16 17 18 19 19 20 21 22 23 23 175 180 - - - - - 5 5 6 7 7 8 9 9 10 11 11 12 13 13 14 15 16 16 17 18 19 19 20 21 22 22 23 180 185 - - - - - 5 5 6 7 7 8 8 9 10 10 11 12 13 13 14 15 16 16 17 18 18 19 20 21 22 22 23 185 190 - - - - - 5 5 6 7 7 8 8 9 10 10 11 12 12 13 14 15 15 16 17 18 18 19 20 21 21 22 23 190 195 - - - - - 5 5 6 6 7 8 8 9 10 10 11 12 12 13 14 15 15 16 17 17 18 19 20 21 21 22 23 195 200 - - - - - 5 5 6 6 7 8 8 9 10 10 11 12 12 13 14 14 15 16 17 17 18 19 20 20 21 22 23 200

Site index 5 6 7 8–9 10–12 13–17 ≥18 years to bh 9876543

The original data source for the table above is the Alberta Forest Service (1985). The table and the corresponding site index curves in Figure 21 were based on the following formulation:

H = 1.3 + (S - 1.3)b1/b2 b1 = 1 + exp {6.879 - 1.32 In 50 - 0618 In (S - 1.3)} b2 = 1 + exp {6.879 - 1.32 In A - 0.618 In (S - 1.3)} ytb = 1.33 + 38.56/S where: H = top height (m); S = site index (height at 50 years breast-height age); A = breast-height age (years); ytb = number of years to reach breast height (1.3 m); In = natural logarithm; and exp = exponential function to the base e. The height–age (site index) curves were developed from stem analysis of 207 dominant and codominant aspen trees located throughout Alberta. Site index ranged from about 9 to 24 m at 50 years breast-height age and included trees up to 90 years old. The Alberta data were validated with aspen data gathered in interior British Columbia (T. Honer, pers. comm., July 1994). The site index table and curves published in Thrower et al. (1994) are therefore considered valid for use in British Columbia. Lacking specific data for British Columbia birch stands, Thrower et al. (1994) recommended that the above site index data for aspen be used for paper birch in British Columbia.

45 3.7.1 Maturity ages and rotation lengths

• The average rotation age for paper birch is 70–80 years old. By 50–60 years, annual height growth is greatly reduced and many stands start to deteriorate by as early as 70 years, depending on site moisture availability.

• In British Columbia, where many broadleaf stands are mature and overmature, wood quality is a concern because, apart from firewood and oriented strandboard, there are currently few uses for poor-quality stemwood from broadleaf species.

Sprout-origin stems can Paper birch is short-lived and rarely lives beyond 140–150 years. Some old-growth reach maturity about birch in a research natural area in New England have been documented at 267 years, 10–15 years earlier than but this is atypical. By the age of 60–70 years (some researchers say 60–90 years), can seedling-origin stems. paper birch is fully mature, and usually vigour and quality can be expected to decline after about age 70. Sprout-origin stems can reach maturity about 10–15 years earlier than can seedling-origin stems, both because of the greater incidence of heartrot (which frequently originates from the mother stump) and faster juvenile growth rates in the former. Height differentiation occurs early in pure birch stands because suppressed trees die early and released trees rapidly grow into dominant crown positions. Intermediate trees may struggle for several years, but their struggle is reflected in low rates of growth.

Birch is among the early-succession broadleaf species that have high biomass Information Sources: accumulation rates in the first few years after disturbance. Recent studies spanning the Ohmann et al. 1978; east–west range of birch in Canada have confirmed the nutrient-cycling importance of Safford 1983; Leak 1985; this broadleaf species in early-stage succession. In the ICHmw3 variant in southern Hornbeck and Leak 1992; British Columbia, it was shown that, as stand age increased, an increasing proportion Simard and Vyse 1992; of annual biomass increment was allocated to stems, but nutrients were preferentially Massie et al. 1994; accumulated in the leaves. Average rates of nutrient accumulation in biomass were Wang et al. 1995, 1996. greatest in the early stages of stand development and less marked as stands aged.

3.7.2 Yield in pure paper birch stands

• Across the transcontinental range of paper birch, yields of mature birch usually vary between 100 and 300 m3/ha, but substantially higher yields occur in the ICH zone of the southern Interior.

• In British Columbia, site index at breast height age 50 years ranges from 25 to 30 m on good sites, 16 to 24 m on medium sites, and from 9 to 15 m on poor sites.

The most productive sites Through much of its geographic range, birch in mature stands average 25–30 cm for paper birch occur on dbh and over 20 m in height. On exceptional sites, individual old-growth birch have well-drained lowlands. been recorded as large as 76 cm dbh and 30 m high. As with many tree species, birch Steep colluvial slopes, is most productive on subhygric alluvial sites and least productive on xeric colluvial ridges, and rock outcrop sites (Figure 22). On subhygric sites in the ICH zone of southern interior British areas have the lowest Columbia, managers should expect mature birch to reach heights of 30–35 m and yields for paper birch. diameters of 50 cm or more. Yield of pure, mature paper birch stands is highly variable

46 a

b

FIGURE 22. Paper birch is most productive on subhygric alluvial sites (a) and least productive on xeric, colluvial slopes (b). Both sites are located near Mica Lake on the North Adams River, British Columbia. (Pho- tographs by S. Simard)

47 in the ICH zone, ranging from 150 m3/ha on submesic sites to 300–400 m3/ha on mesic sites and as high as 800 m3/ha on subhygric sites. Data from Alaska, Ontario, and New England indicate relatively similar birch yields over a broad geographic range—230– 270 m3/ha on good sites and 100–185 m3/ha on poor sites.

In eastern North America, birch site index ranges from 12 to 24 m at age 50, but in Alaska it ranges from 11 to 20 m at age 50. As expected, British Columbia site indices are within this range but average closer to the eastern Canadian–Lake states range than to the Alaskan range for birch site index. For instance, 33 examples of site series– site index correlations shown for paper birch in Table 11 indicate a site index range of 12.6–23.6 m at age 50 in British Columbia’s forest regions. The average for these 33 observations for paper birch index is 17.4 m at age 50. Gross merchantable volumes for individual birch trees of given heights and diameters in British Columbia are summarized in Table 12.

Although too detailed to reproduce in this handbook, tree volume tables recently published for paper birch in Alberta (Huang 1994) are probably applicable to paper birch in northeastern British Columbia. These Alberta tables provide estimates of individual birch tree volumes for merchantable lengths from a 0.3 m stump height to top diameters (inside bark) of 5 to 15 cm for birch trees that range from 5.1–81.0 cm dbh (outside bark) and from 3.1–39.0 m total tree height.

Of possible application to northeastern British Columbia, ecologically based site index curves for major tree species in Alberta suggest the following site index ranges for boreal ecosystems that are currently dominated by mixed broadleaf stands (made up of various proportions of aspen, balsam poplar, and paper birch):

Average site index at Site class breast-height age 50 (m) Good 20.0 Information Sources: Medium 16.0 Cooley 1962; Curtis Poor to low 12.0 and Post 1962; Gregory Unproductive <10.0 and Haack 1965; Hoyle and Bjorkbom 1969; Yield tables for fully stocked birch stands in British Columbia are not reproduced Solomon and Leak 1969; in this handbook. However, managers requiring such information may obtain age-related Alberta Forest Service estimates of height, diameter, volume, and mean annual increment for various specified 1985; B.C. Ministry of site indices, crown closure classes, and utilization classes by using the province’s Forests 1986; Safford Variable Density Yield Prediction (VDYP) database. It can be obtained from the Ministry et al. 1990; Simard and of Forests, Research Branch, Forest Productivity and Decision Support Section, Victoria, Vyse 1992; Huang 1994; B.C. V8W 3E7. Additional research is needed to provide answers for managers who Huang et al. 1994; would like British Columbia birch yield estimates in relation to initial densities of birch Thrower et al. 1994. or in relation to varying densities after thinning.

48 TABLE 11. Biogeoclimatic zones, subzones, variants, and site series currently represented by site index data for paper birch in British Columbia, listed in decreasing order of site index (Source: SIBEC [Site Index– Biogeoclimatic Ecosystem Classification] Project, D. Meidinger, pers, comm., Dec. 1995.)

Zone, Site Seriesa Average subzone and (number in brackets is the site series site index Region variant number in its respective subzone or variant) (m at 50 yrs)

Nelson IDFdm1 (05) FdLw - Spruce - Pinegrass 23.6 Vancouver CWHdm (05) Cw - Sword fern 23.5 Nelson ICHmw3 (04) CwFd - Soopolallie - Twinflower 22.1 Cariboo SBSmh (07) SxwEp - Devil’s club 21.7 Nelson ICHmw2 (04) CwFd - Falsebox 21.1 Nelson PPdh1 (06) AtAct - Snowberry - Horsetail 20.9 Nelson IDFdm2 (04) FdLw - Spruce -Pinegrass 19.1 Nelson IDFdm2 (05) SxwAt - Sarsaparilla 19.0 Cariboo SBSmh (01) SxwFd - Hazelnut 19.0 Cariboo SBSmh (06) SxwFd - Coltsfoot 18.3 Nelson ICHmw2 (07) Cwhw - Horsetail 18.2 Prince George SBSmk2 (01) Sxw - Huckleberry - Highbush-cranberry 18.0 Nelson ICHmw3 (07) CwHw - Devil’s club - Lady fern 17.9 Kamloops ICHmk1 (05) SxwFd - Gooseberry - Sarsaparilla 17.9 Kamloops ICHmk1 (01) CwSxw - Falsebox 17.8 Nelson PPdh1 (05) PyAct - Snowberry - Kentucky bluegrass 17.5 Prince George SBSmk1 (01) Sxw - Huckleberry - Highbush-cranberry 17.3 Prince Rupert ICHmc2 (04) CwHw - Devil’s club - Oak fern 17.0 Nelson ICHdw (03) CwHw - White pine - Devil’s club 16.4 Nelson ICHmw2 (01) HwCw - Falsebox - Feathermoss 16.0 Kamloops IDFxh1 (08) SxwFd - Douglas maple - Dogwood 15.7 Prince George BWBSmw2 (01) SwAt - Step moss 15.7 Cariboo SBSmw (08) Sxw - Devil’s club 15.7 Nelson ICHmw2 (03) FdCw - Falsebox - Prince’s pine 15.5 Nelson ICHmw3 (05) CwFd - Falsebox 15.4 Kamloops IDFmw2 (02) Fd - Snowberry - Bluebunch wheatgrass 15.3 Kamloops ICHmk2 (01) CwSxw - Falsebox - Knight’s plume 15.2 Nelson ICHdw (01) CwFd - Falsebox 14.8 Kamloops ICHmk1 (04) FdPl - Sitka alder - Pinegrass 14.8 Nelson MSdk (01) Sxw - Soopolallie - Grouseberry 14.6 Prince George BWBSmw2 (05) Sw - Currant - Horsetail 14.2 Nelson ICHdw (04) CwHw - Devil’s club - Lady fern 13.1 Nelson MSdk (05) Sxw - Soopolallie - Snowberry 12.6 a Numbers and names of site series are from Silviculture Interpretations Working Group (1994).

49 TABLE 12. Paper birch individual tree volume table, showing gross merchantable volume in cubic metres, calculated from equations by Demaerschalk and Kozak (1977) and as recorded in Reid Collins and Associates Limited (1992)

Tree volume for 30 cm stump height and 10 cm top diameter

Tree height (m)

DBH (cm) 10 15 20 25 30 35 40

15 0.05 0.08 0.10 0.13 0.15 — — 20 0.11 0.17 0.22 0.27 0.33 — — 25 0.19 0.27 0.36 0.44 0.53 0.62 0.71 30 0.27 0.39 0.52 0.64 0.77 0.90 1.0 35 0.37 0.53 0.69 0.86 1.0 1.2 1.4 40 0.49 0.68 0.89 1.1 1.3 1.5 1.8 Tree volume for 30 cm stump height and 15 cm top diameter

20 0.08 0.11 0.15 0.19 0.23 — — 25 0.16 0.24 0.31 0.39 0.47 0.54 0.62 30 0.26 0.36 0.48 0.60 0.72 0.84 0.96 35 0.36 0.51 0.67 0.83 0.99 1.2 1.3 40 0.47 0.66 0.87 1.1 1.3 1.5 1.7

3.7.3 Yields in mixed stands involving birch

• Unlike young stands, birch presence in stands 50 years or older results in a loss of overall volume growth in the stand, particularly where the companion conifer is a shade-intolerant species.

• The rapid early height growth of birch sprouts (1–2 m/year) is so much greater than that of its companion conifers (5–25 cm/year) that it is dif- ficult for the manager to grow mixed stands without intermediate cuttings.

On poor sites, conifers Height growth of paper birch in relation to that in associated Douglas-fir shows generally have better different growth patterns in coastal versus interior biogeoclimatic zones (Figure 23). stem quality and greater This figure is based on a study in the CWH zone of the Malcolm Knapp Research yields than do birch. Forest (coastal site) and in the ICHmw subzone near the northern end of Adams Lake (interior site). At both coastal and interior sites, birch had more rapid early height growth than did Douglas-fir. Eventually, Douglas-fir overtakes birch, but there are significant time differences in this process when coastal and interior locations are compared. On the coast, birch is overtopped by Douglas-fir at 35–40 years after stand establishment. In contrast, it takes at least 60 years for Douglas-fir to overtop birch in the ICHmw subzone. In the ICH zone, where conifer growth is usually not limited by moisture or nutrient availability, competition between birch and Douglas-fir appears to be controlled primarily by light. This competition for light is very important in the ICH zone because both species are relatively light-demanding, shade-intolerant species. Therefore, height growth is the key factor determining which species takes over the other in the process of stand development.

50 Information Sources: There is evidence that Douglas-fir in coastal biogeoclimatic zones has a more dense crown structure and higher foliage biomass than Douglas-fir in the interior of Simard 1990; Simard British Columbia. The greater shade created by coastal Douglas-fir crowns is the likely and Vyse 1992; Huang reason why birch height growth drops off earlier in coastal zones than in the Interior et al. 1994; LePage 1995; locations (Figure 23). In the Interior birch can exceed Douglas-fir in height growth for Wang 1997. at least 60 years.

Height (m) 40 Coastal site

30

20

10

0 0 20 40 60 80 100

40 Interior site

30

Paper birch 20

Douglas-fir 10

0 0 20 40 60 80 100 Age (yr)

FIGURE 23. Comparison of birch and Douglas-fir height growth in the CWH biogeoclimatic zone (coastal site, Malcolm Knapp Research Forest) and in the ICHmw subzone (interior site, near the northern end of Adams Lake) in British Columbia (Wang 1997).

51 3.7.4 Diseases, insects, and other damaging agents of paper birch

Die-back, a progressive • Many diseases affect paper birch, but the most important in older stands dying of branches and are three trunk rots: Phellinus igniarius, Fomes fomentarius, and twigs from the top down, Piptoporus betulinus. Most other diseases are associated with mechanical is a chronic problem of wounds that become entries for bacteria and fungi. paper birch. The cause is not known, but a • Paper birch is host to several insects and pathogens that attack birch at variety of stresses all stages of growth and involve all parts of the tree. may be involved. In British Columbia, pathogens of paper birch have not received as much attention as those of conifers. More information on birch pathogens is available from eastern parts of Canada where birch is a commercially more important species, relative to conifers, than it is in British Columbia. Table 13 summarizes the main diseases, insects, and other damaging agents of paper birch with information focusing on those most prevalent in British Columbia.

There is evidence that discoloration develops more rapidly in birch than in other broadleaf species. Stain generally accounts for a greater amount of cull than does decay. However, cull values vary depending on the desired use of birch wood. For example, Ontario cull studies of the heartwood of living trees revealed that losses due to stain and decay in mature birch trees can range from 15 to 45% of merchantable volume. Fungi responsible for significant levels of stain and decay in eastern birch include Phellinus igniarius, Cytostereum murrayi, Pholiota adiposa, Inonotus obliquus, Chondrostereum purpureum, and Peniophora cinerea. Fungi responsible for significant levels of stain and decay in paper birch in British Columbia (Allen et al. 1996) include Armillaria root disease (especially Armillaria ostoyae and A. sinapina), Fomes fomentarius, Phellinus igniarius, Piptoporus betulinus, and Inonotus obliquus. Inonotus obliquus is a significant problem because, once it is present, this fungus has a substantial vertical distribution through a birch stem. Fomes fomentarius, Piptoporus betulinus, and another decay fungus, Ganoderma applanatum, can very rapidly establish on dead standing birch trees. In circumstances where a manager has the option to recover valuable wood from birch trees that have recently died, this recovery must be done quickly before there is loss of wood quality and biomass from the spread of Fomes, Piptoporus, or Ganoderma. Often, birch in the early stages of stem decay has wood with attractive black lines (spalting) that have been used to advantage in the manufacture of furniture and wood panelling.

In addition to stain and decay, canker-causing fungi such as Nectria spp. can weaken birch, predisposing it to attack by other agents. Armillaria sinapina and A. ostoyae, two of the fungi causing Armillaria root rot, are weak parasites on broadleaf species and occur in birch trees of all ages. A. sinapina is found on both living and dead broadleaf trees. The vigour of birch trees influences their resistance to Armillaria; healthy trees on better sites show more resistance and often reach about age 60, while low-vigour trees can succumb as early as age 20.

Throughout the range of birch a condition known as redheart is locally severe. It is a common defect in paper birch that is associated with wetwood and later stages of discoloration in the central column of living birch trees. The term redheart has been applied to discoloured wood that has undergone changes as a result of increased moisture content and invasion by bacteria and non-decay fungi following an initial physical injury and entrance of fungi through branch stubs or bark wounds. The darkened wood resulting from these fungi is often sound, but the discoloration limits

52 birch use where clear white wood is required for a manufactured product. The proportion of redheart is greatest in overmature birch and in slow-growing trees. The incidence of discoloration from redheart can be reduced by avoiding injury to trees during logging, and by harvesting trees with injuries before stain and decay can develop. Silvicultural management that promotes early self-pruning of birch’s lower branches can help to reduce the entry of fungi that cause discoloration.

Die-back, a progressive death of twigs and branches from the crown downwards, is a serious and chronic problem of birch. Several researchers have examined possible causes, including age, drought, shallow rooting, defoliation, foliage diseases, and air and soil pollution. However, the circumstances causing birch die-back are not yet well understood. Information Sources: Common defoliators of birch are the birch skeletonizer (Bucculatrix cana- Basham and Morawski densisella), the forest tent caterpillar (Malacosoma disstria), and the gypsy moth 1964; Conklin 1969; (Lymantri dispar). Defoliation not only reduces annual growth but also weakens trees. Shigo 1969, 1986; This makes them susceptible to secondary attack by bark beetles, ambrosia beetles, Shigo and Larsen 1969; and several borer species, which can ultimately result in death. Of the borers, the bronze Ohman 1970; Allen 1986; birch borer (Agrilus anxius) does the most damage by attacking the crown and advancing Morrison et al. 1991; down the bole. Larvae feed on the bark and can effectively girdle stems. Also, three Wood and Van Sickle species of birch leaf miners (Lyonetia speculella, Fenusa pusilla, and Profenusa thomsoni) 1994; Braathe 1995; Van discolour and defoliate birch leaves. Lyonetia has been responsible for discoloured Sickle 1995; Allen 1996; stands of birch in parts of the Nelson, Prince Rupert, and Kamloops forest regions Allen et al. 1996; several years in a row. Birch stands in the Vancouver Forest Region have been affected E. Allen, pers. more by Profenusa thomsoni and Fenusa pusilla, which have caused moderate to severe comm., Feb. 1997. discoloration in recent years.

53 996) can cause serious losses and predispose trees to windthrow. can cause serious losses and predispose P. subacida In British Columbia, this trunk rot occurs on birch, alder, balsam poplar, and cottonwood. It causes decay in poplar, and cottonwood. It causes this trunk rot occurs on birch, alder, balsam In British Columbia, a light- heartwood. Early decay first appears as producing white rot in both sapwood and live and dead wood, is little Visible fruiting bodies indicate there with wood remaining quite firm. brown discoloration, in the tree. merchantable heartwood often and particularly damaging in old growth; occurring decays in British Columbia, One of most commonly a yellow- wounds. Early decay appears as can also gain entry to the heartwood through occurs as a sap rot but are soft and crumbly. wood later changing into small cubes that brown stain, with the decay of dead trees but can enter live trees through wounds and Plate-like, leathery fruiting bodies; important cause extensive damage. North America although present in British Columbia on several Found mainly on birch in other parts of broadleaved and coniferous hosts. Root and butt decay caused by Early decay appears as a yellow-white zone in the heartwood. Commonly found on many broadleaf species. a considerable volume of decay. Presence of a single fruiting body indicates in other parts of North America. Its potential occurrence in British This heartrot has been found on birch Columbia is not yet confirmed. and yellow birch elsewhere; one of few brown rotting fungi that Restricted to paper birch in British Columbia wood is yellowish-brown, and in advanced stages results in only attacks one broadleaved genus. Decayed in dead branches of dying trees; after death it spreads rapidly to powdery disintegration. It is often present bark and sapwood. Phellinus pini* Piptoporus betulinus Fomitopsis pinicola Ganoderma applanatum Laetiporus sulphureus* Perenniporia subacida Phellinus igniarius Fomes fomentarius identified as very common to paper birch. Information sources are identified in each subsection of this table. subsection of this identified in each sources are to paper birch. Information as very common identified Diseases, insects, and other damaging agents of paper birch. For diseases, names in boldface are organisms that Allen et al. (1 Allen that are organisms boldface names in For diseases, paper birch. of agents other damaging and insects, Diseases, brown crumbly rot white mottled rot brown cubical rot stringy butt rot hardwood trunk rot red ring rot brown cubical rot of birch white spongy trunk rot Type of damage Common name name Latin Incidence and severity DISEASES heart- rots TABLE 13. TABLE

54 then wood is highly ree’s response to injury with Armillaria. A. ostoyae are weakly pathogenic on living broadleaf trees, and occur as on living broadleaf trees, and occur are weakly pathogenic A. nabsnona , and is reported on birch elsewhere in North America, but its incidence on birch in British elsewhere in North America, but is reported on birch A. cepistipes , saprophytes on stumps of broadleaved trees after felling. saprophytes on stumps P. schweinitzii Columbia is not yet clear. Armillaria root rot is caused by many closely related but distinct species of caused by many closely related but distinct Armillaria root rot is years in ages in the Interior and those over 15 hosts and can infect trees of all pathogenic to coniferous growth trees and shrubs which show reduced also include a wide range of broadleaved coastal areas. Hosts death. mycelial fans at base of stem after before death, and red-brown foliage and and sparse foliage A. sinapina on birch and rarely on cottonwood. Prominent perennial black In British Columbia, this canker occurs from bark cankers; these cause severe damage to infected trees. A masses are a key characteristic, erupting to 100%) heartwood decay. single conk is indicative of extensive (50 of coral-coloured spots and sunken cankers are characteristic. Mostly broadleaved hosts involved. Masses but can be weakly pathogenic where wounds or stress weaken Acts mostly as a saprophyte on dead tissue tree. Initial decay is a reddish-brown stain; in advanced stages wood is dry and lightweight. Economic impact is Initial decay is a reddish-brown stain; in This fungus is a candidate for biocontrol of hardwood stumps. greatest on ornamental and orchard trees. on dead conifer wood, it also occurs on birch and other broadleaf Although this fungus is found mainly brown cubical rot. species. In advanced stages, it is a typical have not yet been recorded on birch in British Columbia, but their potential occurrence in the province was recognized by British Columbia, but their potential occurrence in the province was recognized have not yet been recorded on birch in degradation results. “Redheart” is such wood which is wet and dark red as a result of microbial activity. degradation results. “Redheart” is such wood phenolic compounds surrounding the injury — hence reddish discoloration in the central column. Bacteria and non-decay fungi can — hence reddish discoloration in the central column. Bacteria and non-decay phenolic compounds surrounding the injury and wood may be affected by microbial activity. Ultimately, decay fungi may be involved invade wounds, and moisture content of Nectria cinnabarina Inonotus obliquus Armillaria ostoyae A. sinapina A. cepistipes A. nabsnona Phaeolus schweinitzii* Chondrostereum purpureum Gleopyllum sepiarium P. schweinitzii , and Continued P. pini

brown cubical sap rot of birch Nectria canker Schweinitzii butt rot root disease Allen et al. (1996) by reference to their occurrence on birch elsewhere. Allen et al. (1996) by reference to their occurrence L. sulphureus, Type of damage Common name Latin name Incidence and severity Wood discolorationWood of injury and the t to discoloration than other hardwoods. It starts from some form Birch is more susceptible * sap rots silver leaf disease Cankers sterile trunk rot root rots Armillaria TABLE 13. TABLE DISEASES

55 causes discoloration of stands L. speculella , ants feeding on aphids decreased popula- B. pendula typically cause widespread, moderate to severe discoloration of birch foliage, typically cause widespread, moderate to F. pusilla and tions in early summer (not later in the season) and ants decreased numbers of other sap-feeding and leaf- tions in early summer (not later in the season) and ants decreased numbers of other chewing insects, although the effect on tree growth was slight. Leafhoppers are avid feeders on birch foliage; only heavy populations interfere with normal foliage Leafhoppers are avid feeders on birch are vectors of plant diseases. function and then weaken trees. Some leafhoppers minor. Severe infestations can cause premature leaf drop Damage by these sucking insects is generally trees. On and diminish aesthetic value of amenity These are the cause of injuries that promote defects in wood, but seldom result in tree mortality. of injuries that promote defects in wood, These are the cause As above. in the Nelson, Kamloops, and Prince Rupert forest regions. Tree health is not seriously affected unless in the Nelson, Kamloops, and Prince Rupert and branch and top die-back. leafmining is severe, which can induce stress The most common defoliators of birch; defoliation reduces annual growth and weakens trees, making defoliators of birch; defoliation reduces The most common infection, which can ultimately kill. them susceptible to secondary attack or foliage. Larvae of saddled prominent consume much Larvae chew leaf tissue and can cause complete defoliation when numbers are high. Damage usually Larvae chew leaf tissue and can cause consecutive years may kill twigs in upper crown and reduce stem occurs in late summer. Severe attacks in radial growth. Asian race is also a concern; it has a broad host preference In addition to the native race, the introduced for broadleaves and some conifers. P. thomsonii lower Fraser Canyon. often seen in the upper Fraser Valley and Most serious insect pest; attacks overmature and weakened trees by attacking crown and then trees by attacking crown and pest; attacks overmature and weakened Most serious insect girdle a tree. Larvae feed on inner bark and can ultimately progressing down bole. . sp . sp Xyloterinus politus Aphididae Trypodendron betulae and Phytobia pruinosa Oncopsis Erythroneura Malacosoma disstria Heterocampa guttivitta Bucculatrix canadensella Lymantri dispar Fenusa pusilla, Lyonetia speculella, Profenusa thomsonii Agrilus anxuis Continued birch aphids ambrosia beetle cambium miners leafhoppers forest tent caterpillar saddled prominent birch skeletonizer gypsy moth leafminers and leaf- mining sawflies Type of damage name Common Latin name severity Incidence and Defoli- ators Borers bronze birch borer TABLE 13. TABLE INSECTS

56 94; al. 1991; d et al. 1990; Hiratsuka et al. Young larvae overwinter in cases attached to the bark, and mine the leaves in spring and early summer. in cases attached to the bark, and Young larvae overwinter but results in little permanent injury. Damage can be unsightly, Browsing and girdling of stems damages birch seedlings and saplings. Voles and other rodents can consume birch Browsing and girdling of stems damages seeds. Browsing, especially over-browsing at the seedling stage, can reduce the amount of dominant birch in regenerating Browsing, especially over-browsing at the of surviving trees. Birch can be browsed quite heavily by deer for 5-6 years, stands or can impair the growth or quality within reach of deer. and must be continuously browsed to remain seedling stage, can reduce the amount of dominant birch in regenerating Browsing, especially over-browsing at the trees. stands or can impair the quality of surviving on the inner bark and by girdling large branches in the crown. Porcupine damage larger trees by feeding resulting in mortality and reduced stocking. Hares clip or gnaw bark of small seedlings, bark, or wound the stem by biting to obtain sap. Squirrels girdle stems by stripping off the in a regular pattern through the bark to feed on sap; they provide These sapsuckers peck rows of holes arranged entry for discoloration and decay fungi and may cause ring shake. Braathe 1995; Hiratsuka et al. 1995; Van Sickle 1995; Allen 1996; Allen et al. 1996. et al. 1995; Van Sickle 1995; Allen 1996; Allen Braathe 1995; Hiratsuka Mahdi and Whittaker 1993; Merler et al. 1993; Allen 1994; Humble and Stewart 1994; Simard and Vyse 1994; Wood and Van Sickle 19 Stewart 1994; Simard and Vyse 1994; Wood Merler et al. 1993; Allen 1994; Humble and Mahdi and Whittaker 1993; Fowells 1965; Conklin 1969; Shigo 1969; Shigo and Larson 1969; Ohmann et al. 1978; Allen 1986; Safford et al. 1990; Morrison et et al. 1978; Allen 1986; Safford et al. 1990; 1969; Shigo 1969; Shigo and Larson 1969; Ohmann Fowells 1965; Conklin Coleophora serratella . spp O. hemionus Erethizon dorsatum Lepus americanus Tamiasciurus hudsonicus Microtus Sphyrapicus varius Sphyrapicus nuchalis Odocoileus virginianus and Alces alces Many animals hinder regeneration of birch by eating seeds, browsing seedlings, or gnawing stems. regeneration of birch by eating seeds, Many animals hinder Aldous 1952; Stoeckler 1955; Hatcher 1960; Shigo 1963; Jordan and Rushmore 1969; Wolff 1976; Timmerman and McNicol 1988; Saffor Aldous 1952; Stoeckler 1955; Hatcher 1960; Shigo 1963; Jordan and Rushmore 1969; Wolff 1995; Steeger et al. 1996. Continued birch casebearer Type of damage Common name Latin name Incidence and severity Sapsuckers: Yellow-bellied Red-naped BIRDS Porcupine Snowshoe hare Red squirrel Voles and other rodents Common nameDeer (both white- name Latin tailed and mule) Moose Impact on paper birch ANIMALS TABLE 13. INSECTS Information Sources: Information Sources for Diseases and Insects: Information Sources

57 nds can Septoria result in ying back of twigs and C in tests. Birch grows best in soils free of frost, but in C in tests. Birch grows best in soils free ° canker occurs in seedlings, sun scald may increase seedling susceptibility to canker occurs in seedlings, sun scald Septoria uprooting of birch by wind. canker. branches in the crown, loss of tree vigour, even death over 5-6 years. Recent studies suggest it is caused by prolonged early years. Recent studies suggest it is caused loss of tree vigour, even death over 5-6 branches in the crown, frost injuries. cause of die-back that occurs after frost; fine root death is a secondary spring thaws and subsequent fires while mature trees are susceptible to moderately intense is thin and highly flammable; young trees Bark of paper birch ability decreases as root-collar sprouts; this post-fire sprouting young paper birch produce vigorous may survive. Fire-killed fire occurrences and thrives in environments with repeated fires can eliminate sprouting. Birch birch stands age. Severe boreal birch would be Without fire or human intervention, common on burned than unburned areas. regeneration is more as birch litter fires in spring can have high flammability and longer-lived species. Surface replaced by more shade-tolerant layers dry quickly in the sun. from intolerance to highly tolerant. Paper birch likely does not tolerate oxygen Reported reactions to flooding have ranged depletion or soil modification from flooding. to resist freezing to –80 Paper birch is one of four northern species annual thaw depth was only 64–76 cm. Alaska saplings have been observed where have been recorded in northeastern Alberta and are common in British Columbia. Stem bending and breakage from snow within an aspen–birch stand and within a spruce stand in Alaska were the same Bending and breakage of paper birch proportion in a heavy snow winter (1990–91). result in discoloration and permit entry of decay. Logging wounds to boles and root systems during partial harvest result in discoloration and permit entry treatments can be significant on birch. in in their first summer; germination is best in shade, but growth is better Seedlings require shade for 2 to 3 months to winter sun scald on south and southwest sides, especially following sunlight. Birch’s thin bark makes it susceptible collar is a problem for planted birch, particularly where there is no shading along release treatments. Sun scald at the root the entire stem. Where Continued Zasada 1971, 1986; Perala 1974; Van Wagner 1983; Haeussler et al. 1990; and Haeussler 1991; Perala Alm 1990a and Kozlowsky 1982; Norby and Kozlowski 1983; Ericsson and Schimpf 1986 Sakai and Weiser 1973; Safford et al. 1990 Sampson and Wurtz 1994 Shigo 1969 caused wou a common injury to the lower parts of birch stems. Mechanically On steep colluvial slopes, rolling rock is Marquis 1966; Watson et al. 1980 Information sourcesInformation Impact on paper birch Wind1988 Webb root rot fungi can In birch stands, high wind usually results in broken boles rather than uprooted trees. However, PHYSICAL FACTORS PHYSICAL Die-back1995Braathe 1986; Allen are d trees to other damaging agents. Symptoms problem of birch, predisposing This is a chronic DroughtFire Paré et al. 1993 Patrick 1969; Behrend and Birch seedlings are highly sensitive to moisture stress. TABLE 13. TABLE Flooding Krajina et al. 1982; Tang Freezing; Frost Snow load Blum 1966; Gill et al. 1973; Mechanical wounds Sun or heatFilip 1963; Blum and

58 vely has been growing on soils in this study is determined by the status Betula pendula Betula pollution have impacted forests. Acidic “Fundy fog” in New Brunswick pollution have impacted forests. Acidic 2 ) impact reduces stomatal conductance, induces necrotic lesions on leaves, and reduces growth of both ) impact reduces stomatal conductance, 2 unimportant. (mean pH 3.6 in growing season) is linked to deterioration of paper birch as foliar browning and increased susceptibility to to deterioration of paper birch as foliar browning and increased susceptibility (mean pH 3.6 in growing season) is linked causes discoloration, interveinal necrosis, and defoliation in broadleaf species. leaf spot fungus. Sulfur dioxide damage leaves; ozone and nitrogen oxides damage cause discoloration of leaves. PAN Fluoride induces marginal necrosis of silvering of leaf undersides. In Germany, (peroxyacetyl nitrate) causes bronzing or containing as much as 29 000 mg/kg total lead. The type of lead tolerance of containing as much as 29 000 mg/kg total a better tolerance to lead than most other species. of plant phosphate nutrition, but birch exhibits a of nitrogen and magnesium, inleaves such as paper birch and aspen, may have Pioneer species with high concentrations against high external acidity than shade-tolerant species. Site fertility is relati higher capacity to buffer their internal pH roots and stems. Nickel, cobalt, or copper (10–15 ppm) in mineral soil can reduce radicle elongation of paper birch roots and stems. Nickel, cobalt, or copper of smelting area since 1972 have not resulted in widespread regeneration germinants. Pollution controls in the Sudbury and SO birch where logging, fire, and heavy metal In Ontario, heavy-metal air pollution from mining and smelting repeatedly killed seedlings and sprouts in coppice woodland repeatedly killed seedlings and sprouts air pollution from mining and smelting In Ontario, heavy-metal transition between the were the only two tree species in a in older birch stands. Birch and red maple and reduced growth pollutant sources. at greater distances from smelter to smelter sites and other communities barren community adjacent Sulfur dioxide (SO Birch is highly susceptible to glyphosate application (foliar spray, hack-and-squirt cut-surface treatment); it is also spray, hack-and-squirt cut-surface to glyphosate application (foliar Birch is highly susceptible spot, hack-and-squirt, and foliar spray. and triclopyr applications in broadcast, susceptible to hexazinone Concluded Information sourcesInformation Impact on paper birch Malhotra and Blauel 1980; Malhotra and Blauel Norby and Kozlowski and 1982, 1983; Patterson Olson 1983; Addison et al. 1984; James and Courtin 1985; Cox et al. 1989; Beckett and Negusanti 1990; Safford et al. 1990; Watson et al. 1990; Eltrop et al. 1991; Myren 1994 Liu and Cote 1993 Boateng and Herring 1990; Boateng and Herring Haeussler et al. 1990; 1992; Prasad and Cadogan Biring et al. 1996 Buffering capacity Pollutants Herbicides Prasad 1985; TABLE 13. FACTORS CHEMICAL

59 4 MANAGING PAPER BIRCH IN MIXED STANDS

Mixed forests that include paper birch are common features of the province’s forested landscape, especially in the ICH, SBS, and BWBS biogeoclimatic zones. Paper birch–conifer mixtures are highly variable in both birch density and conifer species composition (Figure 24). Complex mixtures have developed through natural selection following fires, insect or disease outbreaks, or forest harvesting, except where intensive management has favoured rapid growth of a single conifer species. Until now, little intentional management of mixed stands has occurred in British Columbia.

There are practical reasons why mixed forests are requiring more attention from forest managers. First, current estimates are that mixed forests represent about 35% of British Columbia’s productive forest area. Second, quite apart from the fact that such mixtures occur naturally over a significant part of the province’s land area, the Forest Practices Code requires increased attention to their management. In support of these practical reasons for mixed forest management are biological reasons for carrying it out, based on factors defined by visual, biodiversity, forest health, nutritional, nurse crop, and yield criteria.

Most birch–conifer mixtures in the southern Interior are even-aged, and most management of these forests will continue to be done on an even-aged basis to mimic patterns of natural disturbance and succession. The vertical stratification of these stands comes not from age-class variation, but from species differences in inherent growth rates according to different shade tolerances. Site variation in productivity, competition, genetic differences, and chance events adds to variation in vertical stratification. Where there are local occurrences of uneven-aged class structures, they are often a result of continued, small-scale disturbances such as root diseases or selective logging, or are remnant patches of trees that survived fires to create islands of older stands within an overall younger, even-aged matrix.

Even-aged management with multiple species is appealing for several reasons: the diversity offered by multiple species and their vertical stratification has appeal for several forest uses beyond timber production; even-aged management mimics natural disturbances and succession; and widespread single-species management is perceived by both the public and scientists as being ecologically less desirable than multiple- species management.

Mixtures can be managed on a spatial scale (by alternating individuals, rows, or patches) or on a temporal scale (by alternating rotations of each species. In mixed forests, including those that contain birch, even-aged management of multiple species, whether in large areas or small patches, is sound ecosystem management because it creates stand conditions similar to those created by natural disturbances from fire, insects, or diseases. Most management of birch today involves stand-level rather than landscape-level decisions. The main circumstances under which foresters have to focus on larger landscape units rather than stands is in designing projects that aim to define the overall extent of mixed forests, and in building linkages with fish and wildlife habitat management.

Managers involved with stands that now contain birch are dealing with forest types that are mainly the result of natural succession processes following fire. However, silvicultural encouragement of new stands of birch typically involves regeneration of birch in recently cutover areas. This requires a major shift from conifer-centred management, where aggressive brushing and weeding practices to promote rapid

60 a

b

FIGURE 24. Mixtures of birch with conifers can range from birch as the dominant species (a) to birch as a few scattered individuals in mixed-species stands (b). (Photographs by J. Mather)

61 growth of commercially valuable conifers have minimized the prevalence of young birch- dominated stands. In earlier decades, low market values of paper birch motivated its removal from plantations to promote high-volume production of more commercially valuable conifers. Recognition of the high economic, social, and ecological cost of this practice has recently stimulated interest and research in managing paper birch in mixture with conifers.

For spatial mixtures it is necessary to ensure that fast-growing birch does not shade out desired conifers. However, birch–conifer mixtures can generally be maintained at a higher total density than can pure stands. This is partly because of the ability of each species to occupy separate root and crown growing spaces. In interior British Columbia, because of differences in shade tolerance, mixtures of birch with western redcedar and western hemlock may be planted at higher densities than mixtures of birch with Douglas-fir, western larch, or lodgepole pine. Shade-tolerant conifers initially require less growing space and lower light levels than shade-intolerant conifers because of lower resource requirements and slower juvenile growth rates. This is an important guideline for forest managers involved with paper birch management in mixed stands.

A current guideline in British Columbia is that areas now supporting mixed stands should be the focus of future mixed-stand silviculture. This guideline requires careful assessment of species suitability within the site series structure of the province’s Biogeoclimatic Ecosystem Classification system. However, more information is needed on the relative growth rates of individual broadleaf species and various broadleaf–conifer mixtures on specific site series.

A recent review of alternative silvicultural systems for Ontario’s boreal mixedwood Information Sources: forests suggests that shelterwood, strip cuts, and single-tree seed-tree systems are Perala and Alm 1989; most appropriate for birch. In the eastern part of paper birch’s range, the shelterwood Bell 1992; Simard and system is recommended for regenerating birch in areas where summer precipitation Vyse 1992, 1994; Mac- is limited in either amount or frequency, and where aspen reproduction might dominate Donald and Weingartner large clearcuts. Paper birch seedlings can grow well in 50% sun and can endure as 1994; Silviculture Inter- much as 90% shade for a few years, although they will not become established under pretation Working dense forest canopies. In Minnesota and Wisconsin, where paper birch has difficulty Group 1994; regenerating because of aggressive root suckering by aspens and frequent summer Cameron, I.R. 1996; droughts, the shelterwood system is gaining popularity. These various silvicultural Comeau 1996; alternatives, and the most appropriate site preparation approach for each alternative, Simard 1996b. require more testing in British Columbia ecosystems where birch is prevalent.

62 4.1 Controlling or Reducing Paper Birch Where It Is Not Wanted

• Birch will normally be maintained or encouraged as a part of mixed- species management, but where its reduction is desired the emphasis should be on selective brushing instead of broadcast approaches.

• Managers interested in removing birch can get good results by girdling, but there are difficulties because stems are often not round and significant sprouting can still occur at the stem base when birch is girdled at breast height. Manual brushing of birch has had mixed results.

Birch can be controlled Some responses of paper birch to various management treatments are summarized by several manual or in Table 14. Where the goal is to reduce birch’s abundance, there are several effective herbicidal treatments. approaches for spot treatments. These include manual cutting; cut stump application Birch stems of all ages of 2,4-D, glyphosate, or triclopyr; stem application of 2,4-D or glyphosate; and basal are highly responsive to bark application of 2,4-D or triclopyr. cut-surface treatments. In cases where it is desirable to remove birch to increase conifer growing space, one approach is to clear around individual conifers, creating successively larger free- growing radii as the conifers age. Birch can be either cut manually or cut with stumps treated with herbicide, treated by hack-and-squirt application of herbicide to stems, or application of herbicidal basal spray to stems. When birch is just cut, stump sprouts continue to recycle nutrients through foliage and fine root turnover, and to support diverse communities of soil micro-organisms. There is some evidence that the incidence of Armillaria root disease is higher among conifers where stumps of neighbouring plants are left untreated. Where stumps of birch are left untreated, the frequency of successive cleanings depends on the height of conifers and vigour of birch sprouts. In general, birch of sprout origin have a greater presence of heartrot than birch of seedling origin.

Chemical treatments to control birch have included some of the following agents and applications with good success. 2,4-D ester applied as an early foliar spray at a rate of 2.1 kg ae/ha and glyphosate applied as a late (September) foliar spray at 2.1 or 1.0 kg ai/ha resulted in 60–75% control for the first 3 years following treatment. With hexazinone applied in May as a broadcast spray at 2.2–4.3 kg ai/ha, injury after 2 years was about 50% lower at the lesser application rate than at the higher rate. Spot applications of hexazinone in May or June have also effectively controlled birch. Triclopyr has also successfully controlled birch when applied either as a foliar spray or as a basal bark treatment. Since registration in 1984, glyphosate has proven an effective broad-spectrum herbicide for reduction of herbaceous and woody competition in Canadian forestry applications. It is the most effective of three herbicides (glyphosate, triclopyr, and hexazinone) used in forest vegetation manage- ment requirements.

Trials of manual brushing of birch in the British Columbia interior showed that cutting between mid-June and mid-August is the most effective. Girdling larger diameter stems (5 cm or more) is growing in popularity for such species as birch, aspen, and cottonwood. Girdling of paper birch can be difficult to perform because the stems are not always circular, and birch girdled at breast height can have significant subsequent sprouting. Results from girdling birch in British Columbia have been mixed. Also, there is now a preference to leave some broadleaf species because of their contribution to long-term site productivity and role in providing wildlife habitat and wildlife trees. Reductions in cover and height of birch following broadcast glyphosate application significantly improve Douglas-fir growth; however, because birch is resistant to

63 TABLE 14. Response of paper birch to control measures designed to reduce or eliminate paper birch

Treatment Response Effectiveness as a control measure

MANUAL : cutting Sprouts from either root collar or stump, often Cutting birch stems is more effective for eliminating producing 6 or 7 sprouts from each cut stem; birch from sites if cut stems are mature rather than sprouting may be more vigorous if stems are cut in young; resprouting is relatively infrequent when cut dormant season. Winter cutting followed by low- stems are 70 or more years; most effective June- intensity spring burn gives fewer sprouts than August. cutting without a follow-up burn.

girdling Girdled trees sprout, but the numbers and vigour of Effective in all seasons. sprouts are variable, generally fewer than the num- bers of sprouts from cut stems. Double-frilled girdles or notched girdles ensure high rates of top mortality. HERBICIDE: foliar spray • 2,4-D ester applied as early or late foliar* spray Effectively kills or damages as per the following (rate 1.0–2.0 kg ae/ha for moderate control or herbicide injury scale: 2.0–3.0 kg ae/ha for severe control); • glyphosate applied as late summer foliar (rate • light (<25%); 1.5–2.1 kg ai/ha); • moderate (25–60%); • hexazinone can be at 3 rates: early foliar (1.0-–2.0 • severe (60–90%); and ai/ha for light control), early or late foliar (2.9–4.0 • very severe (90–100%). ai/ha for moderate control), or early foliar (4.0– 8.0 ai/ha for severe control); • triclopyr ester can be at 3 rates of foliar sprays: early foliar-late summer (2.0–3.0 kg ae/ha sum- mer for moderate to severe control), or early foliar-late summer (4.0–5.0 kg ae/ha for very severe control).

soil hexazinone applied at 3 rates: 1–2 kg ai/ha for light Effective according to rate in all seasons. application control; 2–4 kg ai/ha for moderate-severe control, and 408 kg ai/ha for severe control;

cut stump 2,4-D amine at 30–100% rate for severe control and Effective all seasons. glyphosate at 30–100% rate for severe control. stem glyphosate applied at 1 Cap/5 cm dbh for severe All-season efficacy for severe control. injection control.

hack-and- glyphosate at rate of 1 mL/5 cm dbh as early foliar- Good control, rated very severe; seasons as stated at squirt late summer application; splitting of birch bark may left. make treatment difficult.

basal bark triclopyr applied at 10–30% rate. All-season control at severe level.

SITE Treatments that incorporate mineral soil and organic For control, no disturbance or minimal disturbance PREPARATION: layers and expose mineral soil are excellent for precludes seeding-in by birch. germination and establishment of birch seed. Rapid seedling development of birch occurs on exposed mineral soil if seed trees are present.

PRESCRIBED Burns of moderate severity kill standing birch; Sprouts are more frequent, but seedlings are less BURNING burned stumps sprout less than unburned stumps; frequent on unburned than burned sites; moderately cut-and-burn treatments can eliminate as much as severe burns can dramatically reduce both sprouting 75% of root collar sprouts. and seeding.

GRASS SOWING Limits birch establishment by seed. Moderately effective. Information Sources: Coates et al. 1990; Haeussler et al. 1990; Wall 1990; Buse and Bell 1992; Simard and Vyse 1992; Prasad 1994; Newsome et al. 1995; Biring et al. 1996. * Definitions of foliar plant-growth stages are provided in Biring et al. (1996).

64 Armillaria root disease, helps improve long-term productivity, and enhances biodiversity, care should be taken not to affect birch more than necessary to ensure Information Sources: acceptable Douglas-fir performance. Further research is needed to identify the com- Revel 1983; Prasad 1989; petition threshold for birch so that it is not herbicidally controlled more than necessary. Coates et al. 1990; Haeussler et al 1990; A New Brunswick study of growth and resprouting that included paper birch Safford et al. 1990; revealed that resprouts from cut birch stems were not competitive under any of three Wall 1990; Hart and experimental spacings (1.5 x 1.5 m, 2.1 x 2.1 m, and 2.7 x 2.7 m). Sprouts are seldom Comeau 1992; Pitt et al. abundant enough to reproduce mature well-stocked stands. A study in Maine indicated 1993; Prasad 1994; that although over 75% of stumps sprouted, only 27% of the stumps had live sprouts Lees 1995; Biring after 2 years. Sprouting is known to occur at the base of standing live trees subjected to et al. 1996; Simard increased light exposure by removal of nearby trees (Figure 25), but few data exist on and Heinemann 1996. amounts of shade required to suppress resprouting in birch.

FIGURE 25. Birch that are exposed to increased light by removal of nearby trees often develop sprouts near their base.

65 4.2 Birch’s Role in Mixed-species Stands

Paper birch can poten- • Birch, like aspen, can serve as a natural nurse crop for conifers and tially benefit a number thereby improves soil conditions by enhancing nutrient cycling, protecting of companion species conifers from frost, insects, and pathogens, and by increasing stand wind- in British Columbia, firmness and stabilizing soils. including Douglas-fir, western redcedar, western • In birch–conifer mixed stands, as in other mixed stands, the main hemlock, lodgepole pine, management challenge is to understand when and for how long one mixed- western white pine, forest component is dominant over other species in the stand. ponderosa pine, western larch, interior spruces, Some economic analysts have suggested that better financial returns may be subalpine fir, aspen, possible by managing broadleaf species in mixtures rather than converting to conifers. balsam poplar, and Despite these suggestions, there are still circumstances when conversion is favoured black cottonwood. by managers. At present, the prime incentive for considering conversion of birch- dominated stands to conifers is to promote rapid growth of (currently) more commercially important conifers. The desired proportion of birch to conifers is very dependent on management objectives.

There is a high level of public support for measures to enhance the diversity of forests for wildlife and habitats. Habitat needs for many species of birds and mammals are best met by the diversity of successional stages typical of mixed forests that include broadleaf species. It is not surprising that demand is increasing for forest management that emphasizes species mixtures.

Temporal mixtures of birch and conifers are obtained by alternating rotations of each species. For example, 40-year rotations of paper birch may be alternated with 80-year rotations of Douglas-fir on patches as small as 20 ha. Use of such small patches results in a dynamic mosaic of relatively pure paper birch and relatively pure conifer patches across the landscape. On Phellinus weirii-infested sites where inoculum reduction is impractical, researchers recommend planting non-host species such as paper birch as an alternate rotation to susceptible conifers. Since P. weirii has limited ability to spread in moribund conifer root systems, its ability to infect can be substantially reduced when infection centres are planted to paper birch. Such an approach is impractical on Armillaria ostoyae-infected sites because the latter can remain infective in dead roots of conifer stumps throughout the succeeding rotation.

Paper birch is an important component of British Columbia’s present interest in how to manage and grow mixed stands (Figure 26). The province’s broadleaf species, including birch, present a challenging dilemma for forest managers. On one hand, they are fast-growing, shade-intolerant, early seral species that initially outgrow associated conifers. On the other hand, growth of broadleaf species such as birch will decline later in succession when they are overtopped by conifers. The silvicultural challenge is to understand when the broadleaf and conifer components have their respective natural advantages and to encourage these advantages at appropriate stages of mixed-forest rotations. Some analysts have recently suggested two important objectives for this challenge: (1) mixed stands must be managed according to the species involved, site conditions, and forest management objectives; and (2) knowledge gaps must be addressed and observations and experience involving mixed-forest manage- ment must be documented and shared.

66 In mixed birch–conifer stands, as in other broadleaf-conifer mixtures, the main management challenge is to recognize when and for how long one mixed-forest species has dominant height over others in the stand. The next step is to decide how and when to intervene silviculturally if either the broadleaf or conifer component is to be favoured for a given management objective (maximizing the total yield of woody biomass, forest health, nutrient relations, or enhancement of biodiversity and wildlife habitat values).

Birch usually does not present problems for conifer establishment and regeneration, but it can affect growth and survival of conifers 5–10 years after birch is established. Mature birch can be serious competitors to conifers in situations where harvesting and silvicultural practices result in uncontrolled seeding-in of birch. Several studies have demonstrated that shade-intolerant birch in high densities can compete effectively with conifers for resources, particularly light. In a retrospective analysis of competitive interactions in 10-year-old birch–Douglas-fir mixtures in the ICH zone in British Columbia, height and diameter growth of Douglas-fir decreased as a result of dense seeding-in of birch. Although high densities of birch can suppress growth of neighbouring conifers, there is mounting evidence that low densities of birch in mixtures with conifers can improve biodiversity, nutrient cycling, and survival in the presence of root diseases. In particular, birch appears to play an important role in redistributing nutrients and carbon among tree species through litter turnover and mycorrhizal linkages. Mixtures that include approximately 20% birch appear best suited for maintaining both high conifer production and long-term productivity following 100- year rotations. For example, recent investigations of the impacts of weeding removal of

FIGURE 26. An 80-year-old even-aged mixture of paper birch and Douglas-fir occurs with some western redcedar in the understorey on this mesic site in the ICHmw subzone on the west side of Adams Lake, British Columbia. Paper birch can coexist with up to 11 tree species in the ICH zone in the southern interior of the province: Douglas-fir, western redcedar, western hemlock, lodgepole pine, western white pine, ponderosa pine, western larch, hybrid spruce, subalpine fir, aspen, and black cottonwood. (Photo- graph by J. Mather)

67 paper birch upon the long-term productivity of birch/Douglas-fir stands in the ICH zone indicated that higher total stand productivity and higher Douglas-fir productivity is attainable if Douglas-fir is mixed with at least 200 birch per hectare than if Douglas- Information Sources: fir is grown alone. This beneficial influence of a birch component is believed to be due Gregory 1966; Hagglund to the nutrient-cycling role of birch. There is also evidence from northern Europe that and Peterson 1985; a birch component improves the growth of associated conifers, with the optimum birch Tham 1988; Perala and component ranging from 20 to 60%. For example, in Sweden, researchers found that Alm 1990b; Sims et al. 600–800 birch trees per hectare overtopping 2000–3000 Norway spruce per hectare 1990; Morrison et al. gave higher total yield than a corresponding stand of spruce without a birch overstorey. 1991; Pearce 1991; Can- nell et al. 1992; Bergeron Although birch–conifer mixtures are a normal component of much of interior and Dansereau 1993; British Columbia, especially in ecosystems of the ICH zone (Figure 26), birch Wayne and Bazzaz 1993; management has been hampered by conifer-oriented regulations. This situation is slowly Simard and Vyse 1994; changing as more forest managers accept that there are high costs in establishing and Bourque et al. 1995; maintaining pure conifer plantations, there are ecological and forest health risks in Frelich and Reich 1995; discouraging mixed birch–conifer forests, and there is increasing public expectation Johansson 1995; that something is wrong with attempts to eradicate paper birch (Figure 27). To meet MacDonald 1995; these changing circumstances and perceptions, the challenge for forest managers is to Comeau 1995, 1996; recognize that the transition from pure conifer to mixed-forest management is a positive Mielikäinen 1996; step towards “ecosystem management.” This transition requires definition and Simard 1996a, 1996b; acceptance of “mixture-friendly” standards to modify the province’s current conifer- Vyse 1996; Wang oriented, free-growing standards. et al. 1996.

FIGURE 27. Until recently, paper birch was typically piled in landings and burned or left to decompose following clearcutting. Markets are growing for good-quality birch for use in furniture and decorative veneer, and for lower-grade birch for oriented strandboard, dimensional lumber, and pulp. A major impedi- ment to commercial use of birch is a lack of awareness among landowners, contractors, and licensees about its value. If birch is not being commercially harvested, it is better ecosystem management to leave birch patches for biodiversity or birch regeneration, or to leave cut stems scattered on the forest floor for maintenance of long-term productivity. (Photograph by S. Simard)

68 4.3 Paper Birch–Armillaria Relations

• In the ICH zone, Armillaria studies in mixed birch–Douglas-fir stands have revealed circumstances where birch remained uninfected even in cases where birch roots were intertwined with infected Douglas-fir roots. These observations provide encouragement for the managed use of birch in sites with Armillaria occurrence.

• Research is now under way in the ICH zone to assess the hypothesis that, compared with mono-specific conifer plantations, mixed plantations of birch and conifers may reduce the spread of Armillaria and Phellinus root rot, increase ecosystem productivity, and increase biodiversity.

Birch’s greatest role in Tree species that are relatively resistant to Armillaria (paper birch, aspen, and Armillaria control is in balsam poplar) have a possible role for Armillaria control in sites where steepness of areas where steepness slope, soil characteristics, or other factors preclude stump removal of infected roots. of slope or other factors One approach is to plant these least-susceptible species close to infected stumps and preclude stump removal plant susceptible species farther away. When the number of infected stumps is high, of Armillaria-infected another approach is to plant alternate rows of susceptible and least-susceptible species. roots. Tests with alternate rows of Douglas-fir and birch indicated that this approach reduced both the number of Armillaria disease centres and the number of trees per disease centre. If birch or aspen is encouraged for this purpose, they are most effective before age 30. Evidence shows that their early-age resistance to Armillaria changes to moderate susceptibility as they approach age 50.

An important feature of most root pathogens is that they are adapted to long- term survival on the sites where they occur, but also have rather limited invasion strategies. Because clonal fungal genera, such as Armillaria, can be very long-lived, they can eventually occupy relatively large proportions of a forested land area even if establishment of new clones is a relatively rare event. The significance of this aspect of root pathogen biology for birch managers is that sites where root disease now occurs are going to be the sites of root disease presence long into the future. Armillaria-induced mortality rates of 1–2% per year and disease incidences as high as 10% now commonly occur in 5- to 10-year-old conifer stands in interior British Columbia. However, there is evidence that disease incidence is lower in birch–conifer mixtures than in pure stands of susceptible conifers. The degree to which birch is tolerant of Armillaria is uncertain, but some researchers have suggested that it can be susceptible. One concern is that, in Information Sources: locations where Armillaria is present, cutting birch can make matters worse by stimu- Perala and Alm 1990a; lating Armillaria development. However, the ecological importance of birch in the overall Simard 1990; Morrison maintenance of the ecosystem health of forest stands has been stressed, particularly et al. 1991; Deacon and on sites where root diseases are a problem for susceptible conifers. Fleming 1992; Morrison 1992; Simard and Vyse Armillaria fungi are intricately linked to several ecosystem processes such as 1992; Enns et al. 1993; nutrient cycling, successional changes, and biodiversity, some of which involve birch Merler et al. 1993; Stur- as a pioneer broadleaf species. Armillaria root disease provides dead, dying, and decayed rock and Garbutt 1994; trees for use by wildlife species. Canopy gaps created by Armillaria-induced conifer Norris 1995; Simard mortality are often invaded by birch and aspen, and such areas have high habitat value 1995, 1996; van der for several wildlife species. Such gaps increase spatial heterogeneity and the availability Kamp 1995; Morrison of snags, fallen logs, coarse woody debris, and patches of broadleaf species, all of which and Mallett 1996. are important for biodiversity within forest stands.

69 5 MANAGING TO ENCOURAGE BIRCH

This handbook assumes that, in most cases in British Columbia, birch is going to be managed in mixed stands (Section 4). However, in circumstances where a manager decides to promote birch in pure stands, the highest-value returns will be from production of high-quality boltwood, sawlogs, and veneer logs, although birch bark and birch sap production may also have implications in local circumstances (Section 6.3). A key reason for encouraging birch is that birch can grow well on sites that otherwise may not yield woody biomass usable for products such as pulp, oriented strandboard, or reconstituted boards. In some cases these sites for birch encouragement may also assist silvicultural objectives for conifer root-disease control, as well as providing connectivity of wildlife habitats and riparian zones. Non-timber values of birch such as the latter are discussed further in Section 6.

5.1 Site Evaluation for Regeneration and Management of Paper Birch

• Seed supply, characteristics of the seedbed, amount of shade, and weather conditions are the key factors in the success or failure of paper birch regeneration. A manager has most control over seed supply and amount of shade, some control over seedbed conditions (through site preparation), but no control over weather conditions and therefore over soil moisture or temperature during the germination period.

• Given the good knowledge that is available about birch germination, seedling survival, and early growth requirements, an important challenge for the manager is to determine the circumstances in which birch regeneration would be adequate without expensive measures. The focus can then be on sites where special measures are needed to enhance birch regeneration.

On poor sites, conifers It is fortunate that paper birch becomes established and grows well on a wide often have better stem range of soil and site conditions (Figures 7, 8, and 9). However, if intensive culture of quality and yield than birch is planned, the best sites for birch growth should be identified. This involves do birch. Therefore, one avoiding very wet sites with poorly drained soils and very dry sites with shallow soil management guideline is over bedrock or with very coarse sand or gravel on glacial outwash parent material. to maintain conifers on The best sites for intensive birch production are those with deep, rich, well-drained poor sites and pure birch loamy or loamy-sand soils overlying alluvium or glacial till, especially on sites that or mixtures of birch and experience only short periods of moisture deficit during the growing season. conifers on good sites. Within the broad range of sites that are acceptable for birch regeneration and management, a key concern for managers is competition from grass, herbs, shrubs, and other tree species. In young plantings of paper birch in the southern Interior of the province, dense layers of grass or herbs have severely reduced survival and growth of seedlings by limiting moisture availability and by altering the composition and activity of beneficial, below-ground, microbial communities. Even if they are only modestly dense, domestic grass-forb mixes can have serious impacts on birch regeneration. This is a common situation on sites that have been directly seeded as part of grazing lease management. Vegetation management options in these situations are limited both because birch is susceptible to herbicides and because many good birch sites are close to streams and rivers where herbicides are not acceptable for use.

70 Competition from shrubs should be minimized in all young birch stands, and is of greatest concern in the wetter subzones of the ICH and SBS zones, where dense shrub layers frequently develop. Birch should be kept free-growing by selective manual brushing around individual birch stems. Free-growing standards for birch are currently not available. Late successional species, such as western redcedar and western hemlock, are generally not a competition problem for birch, but there may be competition from fast-growing, shade-intolerant species such as lodgepole pine, western larch, or other broadleaved species. To maintain relatively pure birch stands, these shade-intolerant species can be weeded using the same free-growing criteria as that used for shrubs. Methods for removing unwanted trees include manual cutting, girdling, cut-stump glyphosate, and thin-line or wick application of triclopyr. It is currently unknown whether herbicide application will “flashback” to the birch crop tree. However, flashback may not be a concern, as there is little evidence of root grafting among paper birch, and root Information Sources: grafting is rare between different species. Marquis et al. 1969; Safford 1983; Tubbs and Studies in the northeastern United States have suggested that winter logging Reid 1984; Carmean results in better stocking of birch than summer logging. Winter-cut areas, if not well 1985; Fries 1985; Perala stocked with birch, tended to make up the unoccupied space with other northeastern and Alm 1990b; Safford broadleaf species. In contrast, summer-cut areas, if not well stocked with birch, tended et al. 1990; Hornbeck to have low stocking of all of the broadleaf species. The better stocking after winter and Leak 1992; Simard clearcutting was considered to be partly the result of less damage occurring to paper and Vyse 1992; Simard birch advance generation during winter logging than during summer harvests. 1996b.

5.1.1 Site evaluation for paper birch regeneration hazards

• Soil moisture and soil temperature are the factors that most directly affect birch seed germination and early seedling survival. The main determinants of moisture and temperature are weather conditions and seedbed type.

• Shade is better than full sunlight for birch germination, and mineral soil is better than humus, which in turn is better than leaf litter. Moist and shaded mineral soil is overall a good microsite when both germination and subsequent growth are considered. Leaf litter in full sunlight is a very poor seedbed for birch, and undisturbed litter seedbeds physically hinder germination and establishment of birch. Paper birch regenerates For managers with sites or circumstances permitting shade adjustment in the best on disturbed seedbeds early stages of stand development, the best birch regeneration is obtained by using where mineral soil is moderate shade in the first few years, and then increasing the amount of light as birch exposed or is mixed with seedlings enter into competition with other species. Moderate shade conditions created humus. Buried organic with dense grass and/or herb communities should be avoided, as they also compete layers seem to be very for soil moisture. important to the success of birch regeneration. Soil disturbance that exposes mineral soil or mixes mineral soil and organic soil layers provides the best seedbed for the germination of birch and growth of seedlings. This is one reason that birch researchers in the Lake states think that forest floor seed banks are important. Scarification can stir up buried seed and make mixed organic- mineral soil seedbeds available for germination of dispersed birch seeds. Birch’s preference for mineral soil seedbeds partly explains why there are not extensive stands of paper birch. Suitable seedbeds are typically spotty following major disturbances such as logging and wildfire. Wildfires that expose mineral soil over large areas are the most likely source of seedbeds suitable for the establishment of large, continuous stands of paper birch.

71 After the initial advantage of shading for better birch germination, the opposite is true for seedling growth. Growth of seedlings increases with increased light exposure, but only up to a point. In the laboratory, supporting evidence comes from light saturation curves which show the net photosynthetic rate of 1-year-old birch seedlings was 85% of maximum at a light intensity that was about half of full sunlight. In the field, however, the greatest root growth occurs when seedlings are in full sunlight, and is limited where birch is competing with other species for light as well as soil resources. If managers have sites or circumstances that allow adjustment of shade in early stages of stand development, the best birch regeneration is obtained from moderate shade in the first few years. Once birch is established, it is preferable to increase light availability by brushing encroaching vegetation and removing overstorey shade.

In addition to this light condition anomaly, there is also a seedbed condition anomaly. The best germination and survival on shaded mineral-soil seedbeds is followed by a later stage where best seedling or sapling growth and development is on humus seedbeds (in moderate shade or full sunlight). In these circumstances, the ideal seedbed is where the forest floor, particularly dead wood, has been retained, but is mixed in with exposed mineral soil. Deliberate scarification to achieve the optimal combination of mineral soil, humus, and shade can be achieved through mechanical site preparation, light broadcast burns, and bunch-and-burn, or as a by-product of logging. Where scarification is a by-product of logging, summer logging tends to expose more mineral soil than winter logging.

One way to encourage shade during birch’s germination, but more light during seedling development, is partial cutting or shelterwood cutting followed by later reduction or elimination of the residual stand. Another way is to initially use very small or narrow clearcuts to provide shade along the cut boundaries, with later cutting of new openings adjacent to the original ones to increase the amount of light. Where strip cutting is used, strip width and strip orientation relative to sun direction are other variables that a manager can control. Regeneration of paper birch in areas that have been clear-cut in blocks, strips, or patches requires a choice of openings that are large enough to admit sufficient light for birch regeneration, but small enough to be within the seed dispersal range of residual birch. In Alaska, researchers found that clearcut strips 30 m wide usually had inadequate birch regeneration; narrower strips were more successful. Although clearcuts up to four tree-heights wide have regenerated birches, narrower clearcuts are generally recommended. In the northeastern United States, researchers have recommended narrow east–west strips to provide the best germination and survival of paper birch. In that region, a specific recommendation is to clear-cut about one-third of an area in strips about 15 m wide, oriented in an east–west direction. After about 2 years, the second one-third is cut by removing another 15-m wide strip on the south edge of the first cut. The final one-third is removed as soon as birch regeneration is established on the second cut area. On larger clearcuts where strip Information Sources: methods are not used, retention of 7–12 birch trees per hectare has proven to Bjorkbom 1967; Marquis be effective. 1969; Safford 1983; Leak et al. 1987; Perala Another important factor when evaluating sites for birch regeneration hazards is and Alm 1989, 1990a, b; the non-crop vegetation community that will develop following harvest. Types of vege- Safford et al. 1990; tation management treatments must be planned accordingly. A particular hazard is Hornbeck and Leak 1992; rapid development of dense shrub or herb layers. These hazards can be predicted Simard 1995; J.C. Zasada, from the pre-harvest recording of the site series or plant community, based on local pers. comm., Sep. 1996. experience and a knowledge of the biogeoclimatic subzone or variant.

72 5.1.2 Natural regeneration and planting as ways to establish birch

• The choice between natural regeneration or planting for renewal of paper birch crops is difficult because either choice requires significant commitment to silvicultural expenditures. The natural regeneration approach requires flexibility (and costs) to alter management plans to take advantage of natural regeneration where it occurs. The planting approach is inherently more costly, but simpler and more reliable than natural regeneration.

• Birch container-grown stock can be planted successfully. Planted birch seedlings benefit from thorough site preparation, weed control, and protection from animals. There is some evidence that fertilization and mulching are also useful.

Planting paper birch The choice between natural or assisted regeneration for paper birch is hampered seedlings may be the best by negative opinions on both sides of this choice. Natural regeneration is sufficiently choice where intimate erratic to create an interest in silvicultural assistance, yet large-scale assisted mixtures are prescribed regeneration of birch in British Columbia is not yet a high priority. to regenerate root- diseased sites and where The cheapest and, for some sites, the most reliable method for establishing paper natural regeneration birch stands, natural regeneration can be encouraged through the preparation of suitable fails, but naturally seedbeds, in concert with good seed crops. Suitable seedbeds of mixed mineral-organic regenerated seedlings soil are best prepared through mechanical scarification or by bunch-and-burn. Density are often healthier management is usually necessary to improve growth and yield of such naturally and more prolific. For regenerated stands. Some suggestions for successful regeneration of birch are naturally regenerated summarized in Table 15. birch, selective brushing treatments can be success- Aside from the need for exposed mineral soil, a second prerequisite for natural fully applied to create birch regeneration is close proximity to seed sources, either from reserved seed trees intimate mixtures. or from residual birch along edge zones of strips or patch cuts. There is evidence that canopy manipulation has some regeneration value for the seedling reproduction of birch; openings with a diameter of one quarter to one half of surrounding tree height seem to be the minimum for establishing paper birch because of rapid closure of the canopy. Disturbances that maximize mineral soil exposure, such as mechanical site preparation (for example, discing), broadcast burning, or mechanical bunch-and-burn, are the best ways to maximize birch regeneration. However, these are not always practical or cost-effective steps. In most cases, managers will find it best to plan for birch regeneration in relation to mineral soil exposure as a result of logging practices. This will inevitably result in spotty distribution of birch seedlings. Such irregular distribution of birch regeneration would be undesirable if a fully stocked stand of pure birch were the objective on a productive site. However, in many instances managers may be content simply to maintain a birch component in mixed stands. When that is the goal, then the spotty occurrence of birch may have biodiversity or wildlife values greater than what is offered by large areas of pure birch.

Following harvesting or disturbance, abundant seeding-in of paper birch will normally occur where seed trees are left standing and a suitable seedbed has been created. The sprouting vigour of mature trees cut during harvesting is variable and may depend to some extent on site conditions. For example, following canopy removal in the IDFmw and ICHmw subzones near Salmon Arm, height growth of birch was noted to be initially slow (30 cm/year) but increased rapidly after 2 years. Records indicate that on some other sites initial growth rates can exceed 1 m/year.

73 TABLE 15. Some basic principles for successful natural regeneration of paper birch

1 Plan for natural regeneration well in advance (about 20 years). If information is available on how long birch seed remains viable in the soil/humus seed bank, consider that this is a potential bonus to desired birch regeneration goals.

2 Make maximum use of the vegetative regeneration potential of birch stump sprouts.

3 Allow for flexible harvest timing so that natural regeneration can coincide with a mast year. Time interval since last seed year and observations of flowering in April to June period are indicators of the possibility of a good seed crop in the following autumn.

4 During or just after seedfall, enhance germination by seedbed preparation such as scarification, burning, or the logging process. Control competing vegetation by mechanical or chemical methods to permit uninhibited growth in early years.

5 Remove a proportion of the overstorey after seedfall and before germination to enhance germination requirement for light.

6 During early growth from seedling to sapling, protect natural regeneration of birch from browsing and competition from shrubby vegetation.

7 Time and plan subsequent harvest so that growth of natural regeneration is not impaired.

Information Sources: Perala 1990a, b; Kerr and Evans 1993; Harmer and Kerr 1995.

In addition to crown openings to increase light, some kind of seedbed preparation is needed to provide an optimum soil medium for seed germination and seedling growth, and to control competing vegetation. Seedlings experience high levels of mortality in the first 2 years after germination, and are sensitive to inadequate light and lack of moisture (seedlings may be small and poorly formed in dry locations). While significant mortality of young seedlings can result from girdling by rodents and browsing by deer and hares, the presence of some neighbouring vegetation may help control the extent of browsing by these animals.

Coppicing is not yet commonly practised for birch reproduction in British Columbia. Evidence that there may be a greater incidence of heartrot in coppice-origin stems than in seedling-origin stems, and also greater mechanical weakness in coppice- origin stems at the sprout-stump interface, may be biological reasons for avoiding coppice reproduction of birch. Probably for a variety of reasons, planting birch remains Information Sources: the favoured approach, even though coppice sprouts grow rapidly compared to birch seedlings, and were also the source of many of today’s birch stands. Planting is Bjorkbom 1969, 1972; particularly preferred where a high degree of control is needed for spatial design of Clausen and Garrett broadleaf–conifer mixtures. The time of planting nursery-produced birch seedlings 1969; Marquis 1969; (planting very soon after lifting from the nursery bed versus planting after storage) Hibbs 1982; Perkins needs further testing, particularly in relation to root integrity at the time of planting. As et al. 1988; Perala and with other tree species, characterization of what is an optimal microsite for placement Alm 1990b; Safford of a nursery-produced birch seedling is a major knowledge gap. There is a need for et al. 1990; Simard more information on different stock types and stock production methods for paper and Vyse 1992; Zasada birch. It is already known that there are serious limitations to the use of bareroot birch et al. 1992; M. Carlson, planting stock. Also the range of containerized stock types that is acceptable needs pers. comm., Oct. 1995. further testing, including assessment of various sizes and types of styro containers.

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