Annu. Rev. Ecol. Syst. 1989. 20:1-28 Copyright © 1989 by Annual Reviews Inc. All rights reserved

ENVIRONMENTAL FACTORS AND ECOLOGICAL PROCESSES IN BOREAL FORESTS

Gordon B. Bonan

Earth Resources Branch/Code -623, Laboratory for Terrestrial Physics, NASAl Goddard Space Flight Center, Greenbelt, Maryland 20771

Herman H. Shugart

Department of Environmental Sciences, University of Virginia, Charlottesville, Virgi­ nia 22903

INTRODUCTION

The boreal forest is a broad, circumpolar mixture of cool coniferous and deciduous tree species which covers over 14.7 million km2, or 11%, of the earth's terrestrial surface (Figure 1). At these latitudes, a strong correlation exists between the seasonal dynamics of atmospheric carbon dioxide and the seasonal dynamics of the "greenness" (49) of the earth (145)- A possible causal relation, in which the dynamics of the forests at these latitudes reg­ ulates the atmospheric carbon concentrations, appears to be consistent with the present-day understanding of ecological processes in these ecosystems Annu. Rev. Ecol. Syst. 1989.20:1-28. Downloaded from www.annualreviews.org by Swedish University of Agricultural Sciences on 02/05/14. For personal use only. (31, 46). Along with its familiar role in plant photosynthesis, carbon dioxide is a "greenhouse" gas that markedly affects the heat budget of the earth (23). Thus, the possibility that boreal forests may actively participate in the dynam­ ics of atmospheric carbon dioxide is of considerable significance, especially since the climatic response to elevated atmospheric carbon dioxide con­ centrations seems to be strongly directed to the boreal forests of the world (32, 128). These large-scale forest/environment interactions provide motivation to understand better the environmental factors controlling the structure and

0066-4162/89/1120-0001$02.00 2 HONAN & SHUGART

1100 1000 900E � Continuous Permafrost ELZI Discontinuous Permafrost [ill] Boreal Forest

1300 1100 Annu. Rev. Ecol. Syst. 1989.20:1-28. Downloaded from www.annualreviews.org by Swedish University of Agricultural Sciences on 02/05/14. For personal use only. Figure 1 Circumpolar distribution of the boreal forest. From Van Cleve & Dymess (151)_

function of boreal forest ecosystems. Numerous researchers have examined specific aspects of boreal forests, but no one has formulated a unifying model of the boreal forest, a paradigm to link the pattern of forest vegetation with causal environmental factors. In this review, we develop a qualitative con­ ceptual model of environmental factors and vegetation patterns in the cir­ cumpolar boreal forest. Our model represents a complex interrelationship among climate, solar radiation, soil moisture, soil temperature and per­ mafrost, the forest floor organic layer, nutrient availability, forest fires, insect outbreaks, and forest structure (Figure 2). FOREST STRUCTURE ------i CLOUDINESS ,I - ---) ------NUTRIENT _____ �I IWAILABILITY r------, r----SOLAR iSLOPEI, ASPECT L--:" RADIATION �', ===--4=':=-"':"::4��"':":::':':"::+t--.!:======t======:;-, �------; :..------�----, ---- ttl 1------, ,"======:j' SOIL PHYSICAL: r PROPERTIES , ______-, I ______1 L.. • � , l ·l���:��R�����_�: :::::=t-.._-.__r-..J �==== ru-- l -- U; 25 ELEVATION : en§ -- J >-i L u ur u .f��E-C�P�T-A-T���':... -' � , , ______I ______, L.. J 5 Figure 2 Hypothesized environmental processes controlling forest structure and vegetation patterns in boreal forests. Dashed boxes represent climatic and

Annu. Rev. Ecol. Syst. 1989.20:1-28. Downloaded from www.annualreviews.org edaphic input parameters. Arrows indicate interacting processes. � by Swedish University of Agricultural Sciences on 02/05/14. For personal use only.

w 4 BONAN & SHUGART

CLIMATE

Geographic Patterns

The climate of the boreal forest is characterized by strong seasonal variation with short, moderately warm, and moist summers and long, extremely cold, and dry winters (88, 122). This seasonal variation is most pronounced in the continental climates of interior Alaska and eastern Siberia, where seasonal fluctuation of mean monthly temperature is on the order of 44°C and more than 56°C, respectively (122). These harsh, continental regions are also characterized by a range of up to a 100°C in seasonal temperature extremes (122). Pronounced interannual variation in air temperatures is coupled with this strong seasonal variation. At Fairbanks, Alaska, mean monthly January air temperature between 1971 and 1986 ranged from -35 AOC to -7. 7°C, and averaged -22.1°C with a standard deviation of 7.1 (14). Together, these large seasonal and annual temperature fluctuations form a distinct feature of the continental climate of the boreal forest (88, 122). Where a more oceanic climate prevails, such as in eastern Canada and Scandinavia, these tempera­ ture fluctuations are not as extreme. Throughout the boreal forest, annual rainfall is relatively light. In North America, winters are characteristically dry, and more than half of the annual precipitation falls in the summer (22). In the northwest, where high moun­ tain ranges restrict the inland penetration of moisture-laden air, annual rainfall is less than 38 cm and, in Fort Yukon, Alaska, is as low as 18 cm (122). East of the Rocky Mountains, annual rainfall increases to 38-51 cm in central Canada and to 51-89 em in eastern Canada (122). In contrast to northwestern North America, the northern European boreal forest region west of the Ural Mountains lacks high mountain barriers. Here, under the moderating influence of maritime air from the north Atlan­ tic, the mean annual temperature range is relatively low (122). Though sum­ mers are cooler than in areas east of the Ural Mountains, winters are less

Annu. Rev. Ecol. Syst. 1989.20:1-28. Downloaded from www.annualreviews.org severe (122). East of the Ural Mountains, the continentality of the climate by Swedish University of Agricultural Sciences on 02/05/14. For personal use only. increases. Here, seasonal weather patterns resemble those of northern North America, but the large seasonal contrast between warm, moist summers and cold, dry winters is exaggerated by the larger land area, which insulates most of the interior of the Soviet Union from moderating oceanic influences (122). In Eurasia, as in North America, most of the annual rainfall occurs in the summer (122). Annual rainfall is greater than 51 cm west of the Ural mountains, but less than 51 cm east of the Urals (122). In northeastern Siberia, precipitation averages less than 25 em and is as low as 13 em in BOREAL FOREST ECOLOGY 5

Verkhoyansk, which is well removed from the Arctic Ocean and isolated from the Pacific Ocean by mountains to the south and east (122).

Climate-Vegetation Interactions Long-term average values of many climatic parameters, especially measures of summer temperatures, are coincident with the northern and southern boundaries of the boreal forest (56, 88, 107, 146, 170). Certain air mass characteristics also coincide with these boundaries. Throughout Canada, the taiga-tundra boundary is marked by the modal July position of the front that separates continental Arctic and maritime Pacific air masses (21), though this relationship is not as clear in the Labrador-Quebec peninsula (82). The Arctic air mass is associated with cool summers, long, cold winters, and low annual precipitation. Pacific air is typically warmer, moister, and more unstable than continental Arctic air. To the south, the boreal forest is bounded by the winter position of the Arctic front (21). Similar relationships have been found in the boreal forests of Eurasia (82). Climatic gradients usually are perpendicular to frontal positions, and if climate controls vegetation patterns, one would expect corresponding vegeta­ tion gradients normal to the mean Arctic frontal position. Indeed, radial growth of black spruce, Picea mariana, and white spruce, P. glauca, in central Canada declines sharply as the northern tree line (and hence Arctic front) is approached; isopleths of similar growth rates parallel the location of the Arctic front (101). In central Canada, the similarity of Picea mari­ ana communities also parallels the northern tree line (86). In addition, the occurrence of Picea mariana communities is positively correlated with the frequency of Arctic air mass and negatively correlated with the frequency of Pacific air mass; Picea glauca communities show the converse relation­ ship (87). However, the net energy received at the earth's surface, rather than air temperature or air mass characteristics, may be the critical factor for vegeta­ tion (50, 88). Both total stand biomass and net annual production increase

Annu. Rev. Ecol. Syst. 1989.20:1-28. Downloaded from www.annualreviews.org with increases in annual net radiation (50). Hare & Ritchie (50) used this by Swedish University of Agricultural Sciences on 02/05/14. For personal use only. criterion to propose a zonal division of the boreal forest, and they speculated that the relationship Bryson (21) found between vegetation and air mass characteristics is manifested through the effect of air masses on the radiative balance. Larsen (88) also downplayed the importance of frontal activity, suggesting that it is significant only to the degree that air mass characteristics determine the characteristics of the local energy budget during the growing season. In high-latitude forests, where the growing season is restricted by cold temperatures, plant processes may be more limited by extreme, or anomalous, 6 BONAN & SHUGART

climatic fluctuations than by average weather patterns. For example, though the mean position of the Arctic front over Eurasia is correlated with the taiga-tundra ecotone, its annual location varies by over 100 lat in the west and over 6-70 lat in the east (82). In northern Scandinavia, annual growing season temperature sums can vary by as much as plus or minus 25% of the long-term mean (81). In the Soviet Union, extreme cold spells can kill trees that are normally resistant to frost (77). Cold summer temperatures can restrict bud differentia­ tion in Picea abies (51) and can result in severe growth damage to Pinus sylvestris (114). Experimental evidence suggests that at least in the early years of growth, seedlings of Pinus sylvestris, Picea abies, and Betula pendula follow a flexible regime in which they are adapted to both the long-term average conditions and interannual variation (79-81). Climate-vegetation correlations are only useful if they offer insight into the physiological causes involved. However, the ecological and physiological explanation for climate-vegetation relationships in boreal forests is obscure (88), in part because little is known about the physiology of boreal forest tree species (108). Photosynthetic air temperature optima for boreal forest tree species suggest that, under ordinary growing season climatic conditions, photosynthesis is not limited by air temperature (108, 169). Indeed, summer climatic conditions-long days, relatively warm air temperatures-are ideal for plant growth. Air temperature is, however, limiting in that it restricts photosynthesis to a relatively short growing season. Skre (132, 133) and Sarvas (124-126) emphasized this duration, as measured by temperature sums, in determining the growth and distribution of vegetation. Many factors in addition to climate influence vegetation patterns. In Alas­ ka, where climatic demarcations of the northern tree line typically fail (50, 56, 161), the complex interactions among physiographic patterns, the occur­ rence of permafrost, forest fires, and the oceanic influence along the coast have precluded the development of latitudinal vegetation zones similar to those in Canada, Scandinavia, and the Soviet Union (160, 161). In Canada, Annu. Rev. Ecol. Syst. 1989.20:1-28. Downloaded from www.annualreviews.org

by Swedish University of Agricultural Sciences on 02/05/14. For personal use only. the occurrence of fires during periods of climatic cooling can be an important factor determining the location of the taiga-tundra ecotone (22, 106, 1] 1, 113, 161). Furthermore, marginal plant populations can often surviveclimatic conditions unfavorable for sexual reproduction. Isolated clumps of Picea mariana can be found well north of the current boreal forest in Canada. These individuals, which rarely produce viable seeds and only reproduce by layer­ ing, are apparently relict trees left over from more favorable climatic con­ ditions (39, 88, 112). In the following sections we examine these and other features in detail to see how they interact and influence local and geographic vegetation patterns within the circumpolar boreal forest. BOREAL FOREST ECOLOGY 7

SOLAR RADIATION

Trees in high-latitude environments grow under peculiar light conditions independent of the prevailing climate. Maximum day length varies from 16 hr at the southernedge of the boreal forest to 24 hr at the northern treeline. The annual cycle of some boreal forest tree species appears regulated in part by these light conditions (79-81). The long-day photoperiod ensures that growth will be initiated and completed when frost hazard is lowest. In part, northward migration of plants adapted to shorter photoperiods may be prevented because the short photoperiod required to break or trigger dormancy occurs when frost hazard in the Arctic is very high. The low sun angle in high-latitude regions may greatly affect forest struc­ ture and productivity. For example, the canopy geometry of Pinus pumila appears to be arranged to absorb light maximally at low solar angles (110), and this may interact with sun angles to create important latitudinal patterns of vegetation productivity (142). In addition the low solar angle in the Arctic also ensures that a canopy individual casts a long shadow, shading an area more related to height than to crown radius, with the result that the light profile will be little affected by the death of a single canopy dominant. Only the synchronous death of several dominants will create a gap large enough for light to reach the forest floor. Consequently, one would expect high-latitude forests to exhibit strong cohort-structured regeneration and biomass dynam­ ics. The dynamics of a high-latitude forest in the Soviet Union appears consistent with this notion (127). Moreover, simulation analyses of boreal forest dynamics indicate that the appropriate spatial scale to model forest dynamics is approximately 0. 1 ha, much larger than the canopy area of a dominant tree (14, 15, 92). The low solar elevation angle in the Arctic also accentuates the effect of topography, especially slope and aspect, on site characteristics. North­ facing slopes and level sites receive less solar radiation than do south-facing slopes and therefore tend to be cooler and moister (116, 123, 129, 139, 165). Annu. Rev. Ecol. Syst. 1989.20:1-28. Downloaded from www.annualreviews.org

by Swedish University of Agricultural Sciences on 02/05/14. For personal use only. These site differences increase with increasing slope gradients. The high soil moisture and low soil temperature encountered in many boreal forests are major factors controlling forest productivity and nutrient cycling (152, 153, 156).

SOIL MOISTURE

Vegetation patterns throughout the circumpolar boreal forest landscape reflect moisture characteristics as well as air temperature (146). For example, at the southern edge of the North American boreal forest (100) and in central 8 BONAN & SHUGART

Canada (26), forest communities can be segregated into wet (Picea mariana, Larix laricina), mesic (Betula papyrifera, Populus tremuloides, P. balsami­ fera, Picea glauca, Abies balsamifera), and dry (Pinus banksiana) sites. Soil moisture, along with soil temperature, is one of the primary site characteris­ tics distinguishing vegetation patterns in interior Alaska (152-156, 160, 165, 168). Picea mariana stands occupy the wettest sites with average seasonal forest floor and mineral soil moisture contents of 120-240% and 60-160%, respectively; Picea glauca and the successional hardwoods tend to occur on drier sites. Over much of the perennially frozen north, there is an abundant supply of soil water from seasonal snow melt and soil thawing that .cannot drain owing to underlying permafrost (96, 173). Even in extremely arid regions, the slow thawing of permafrost soils provides sufficient moisture for tree growth (96, 170). However, trees may be subjected to severe water loss and drought damage by exposure to dry winds while their roots are still encased in frozen soil and cannot absorb water (10). One particularly important consequence of soil moisture conditions is forest bogs, which are common throughout the boreal forest landscape. For ex­ ample, up to one third of the taiga zone in western Siberia is forested bog (4). With their high water content, low organic matter decomposition, and high organic matter accumulation, these bogs are important regulators of the water budget and carbon cycle. Bogs form when high soil moisture content reduces aeration and de­ composition of organic matter. Forest cutting, which reduces evapotranspira­ tion and raises the water table, is one means of forming bogs. Debogging is possible when improved drainage increases aeration. In the northern Soviet Union, debogging is not common except in cases of manmade drainage but was widespread in the past under drier climatic conditions (4). Reverse bogging is more common in the southernta iga of West Siberia, where better drainage can result from tectonic movement and a current climate unfavorable to bogging (4).

Annu. Rev. Ecol. Syst. 1989.20:1-28. Downloaded from www.annualreviews.org i by Swedish University of Agricultural Sciences on 02/05/14. For personal use only. Antonovsky et al (4) recognized two mportant feedback loops in bog dynamics: (a) The greater the amount of Sphagnum mosses, the higher the soil acidity and the faster the rate of an impermeable soil horizon formation. This process creates wetter conditions, which promote the growth of moss. (b) The thicker the peat layer, the less nutrients available to trees and the greater the moss growth. This, in tum, promotes greater peat accumulation. Permafrost can also be important in creating the saturated soil conditions required for bog formation. As we show in subsequent sections, surplus moisture and cold soil temperatures lead to enhanced organic matter accumulation, creating wetter, colder soils. These conditions promote moss activity and reduce tree growth and regeneration. BOREAL FOREST ECOLOGY 9

SOIL TEMPERATURE AND PERMAFROST

The cold soil temperatures found in some boreal forests result in reduced organic matter decomposition and restricted nutrient cycling (103, 152, 153, 156, 171). Cold soil temperatures are often associated with the presence of permafrost-the thermal condition of soil when its temperature remains below O°C continuously for at least two years (16, 17, 19). Approximately 20% of the world's land surface area and over 50% of Canada and the Soviet Union are underlain by permafrost (95). This broad circumpolar permafrost region is divided into the discontinuous zone, in which permafrost is spatially sporadic, and the continuous zone, in which permafrost is found everywhere beneath the ground surface (Figure 1).

Regulating Factors The existence of permafrost is the result of both the historical and the current state of the surface energy balance and geothermal heat flow (95). Even if present energy conditions are not conducive to the formation of permafrost, it may still exist as relict permafrost if past conditions have been favorable. However, the current dynamics of the permafrost table depend on the current surface energy balance, which primarily reflects air temperature as modified to a secondary degree by solar radiation, vegetation, snow cover, and soil characteristics (17, 95, 116). The most important climatic factor in the regional distribution of per­ mafrost is air temperature (16, 95, 116). The temperature of the air is most directly related to ground heat losses and gains, and throughout Canada a broad relation exists between mean annual air temperature and the occurrence of permafrost (16). The southern limit of permafrost corresponds with the -1. 1°C mean annual air temperature isotherm. Between this and the -3. 9°C isotherm, permafrost is restrictedto peatlands. Between -3.9°C and -6.7°C, permafrost is discontinuous but widespread. North of -6.7°C, permafrost is virtually continuous.

Annu. Rev. Ecol. Syst. 1989.20:1-28. Downloaded from www.annualreviews.org The solar radiation received at the earth's surface produces significant by Swedish University of Agricultural Sciences on 02/05/14. For personal use only. effects secondary to the broad pattern imposed by air temperature (16, 17, 95). In the continuous zone, permafrost is thicker and the soil active layer is thinner on north-facing than on south-facing slopes (19, 123). In the dis­ continuous zone, permafrost is found on poorly drained, north-facing slopes or level sites, but not on well-drained, south-facing slopes (19, 34, 165, 168). A forest cover influences soil temperature and the occurrence of permafrost because the presence of trees alters wind velocity, reducing the turbulent transfer of heat between the ground and the air (16, 118, 147), and shades the ground surface from solar radiation (93, 129, 139). Thus, in the summer, forested soils are cooler than open areas and thaw later and less deeply (28, 10 HONAN & SHUGART

64, 93, 105, 162), and a dense tree cover can maintain a permafrost table in an otherwise unstable thermal regime (93). However, in the winter, forested soils are warmer relative to open areas (129, 139). The presence of a thick moss-organic layer on the forest floor also signifi­ cantly lowers soil temperatures. In interior Alaska, soil temperatures and depth to permafrost are directly related to the thickness of the forest floor organic layer (36, 152-154). For example, Van Cleve & Viereck (154) found, during growing season, an average 37°C decline in soil heat sum at a depth of 10 cm in the mineral soil, for each centimeter increase in the forest floor thickness. The low bulk density and low thermal conductivity of the organic mat effectively insulate the mineral soil, lowering soil temperatures and maintaining a high permafrost table. With its high reflectivity and low thermal conductivity, a snow cover protects the soil from oscillations in air temperature (118, 129, 139). A snow cover in the fall significantly restricts winter frost penetration; conversely, in the spring, the snow cover acts as an insulating blanket to delay soil thawing (16, 17, 64, 95, 129). In addition, convective heat flow during snow melt may increase the input of heat to the soil and increase depths to which the soil thaws (64, 76, 105). The overall effect of the snow cover on the soil thermal regime is quite noticeable in eastern Canada and western Siberia, where the presence of a thick, early snow cover restricts the occurrence of permafrost (88). The warming and cooling of soil depends on its thermal conductivity and its heat capacity. These thermal properties depend on physical soil properties such as texture, porosity, water content, and the thermal state of the soil (95, 118, 129, 139). The effect of soil moisture on the thermal regime of soils is particularly important in the waterlogged soils found throughout the taiga. Though warm water puddled on the soil surface can increase depths of thawing through convective heat flow (64, 76, 105), high moisture content significantly increases the energy needed to freeze or thaw mineral soil. This is particularly important because the presence of a shallow permafrost table

Annu. Rev. Ecol. Syst. 1989.20:1-28. Downloaded from www.annualreviews.org impedes soil drainage, causing the soil water table to be at or near the soil by Swedish University of Agricultural Sciences on 02/05/14. For personal use only. surface (33, 117, 147, 165, 168).

Effect On Forest Ecology

Low soil temperatures have important consequences for physical site con­ ditions. Cold soils and permafrost impede infiltration (43). Combined with the slow release of water from the seasonal melting of the active soil layer, these can result in high soil moisture content throughout the growing season (33, 117, 165, 168). Soil movement caused by frost heaving can result in haphazardly leaning, "drunken" trees (l0, 180, 181). BOREAL FOREST ECOLOGY 11

Plant metabolism is directly affected by low soil temperatures. The begin­ ning and end of the growing season are closely related to soil thawing and freezing (123). Root elongation in several boreal forest tree species is pro­ moted by an increase in soil temperature (89, 144). Low soil temperatures may inhibit biological activity in trees by increasing root resistance and the viscosity of water, thereby inhibiting water uptake (48, 90, 173). Trees may also be subjected to severe water stress by high evaporation demands when their roots are still encased in frozen soil and cannot obtain water (10). Low soil temperatures also influence forest productivity by restricting nutrient availability. The presence of permafrost restricts the rooting zone of trees and the amount of nutrients available for uptake. Moreover, the slow rate of organic matter decomposition in cold soils results in low nutrient availability because nutrients and biomass are accumulated in the forest floor (102, 103, 130, 138, 147, 152-156). Van Cleve et al (153) documented this effect by heating a Picea mariana forest floor in interior Alaska to approx­ imately 9°C above ambient temperatures for three summers. Heating resulted in a 20% reduction in the forest-floor biomass and increased available nutrient supplies. As a result of this more favorable soil temperature and nutrient regime, tree foliage increased rates of photosynthesis. The influence of soil temperature and permafrost on forest structure and function can be seen in geographic vegetation patterns. In western Soviet Union, the boreal forest is bounded by isopleths of 5°C and 10°C mean thawing season soil temperatures (129). In this region, optimal Larix sibirica and Picea abies root growth occurs on cooler and wetter soils than for Pinus sylvestris (78). Permafrost is thought to be one of the major factors controlling the northern tree line in eastern Soviet Union (83). In Canada, the northern boundary of the forest-tundra ecotone corresponds with the approximate southern limit of continuous permafrost (88). Comparisons of topographic vegetation patterns in the discontinuous per­ mafrost zone of interior Alaska show the effect of soil temperatures on stand productivity and nutrient cycling (Figure 3). With their high growth potentials Annu. Rev. Ecol. Syst. 1989.20:1-28. Downloaded from www.annualreviews.org

by Swedish University of Agricultural Sciences on 02/05/14. For personal use only. and nutrient requirements (27), Picea glauca and the successional hardwoods occupy warm, mesic, permafrost-free south slopes and floodplain sites, where soil temperature and soil moisture result in a high rate of nutrient availability (152-156). In contrast, Picea mariana, with its low growth potential and nutrient requirements (27), occupies the least productive, cold, wet, north­ facing and bottomland sites (165, 168). Cold soil temperatures, encountered in Picea mariana stands underlain by permafrost, reduce organic matter decomposition and nutrient mineralization (150, 152-156). Picea mariana trees may have adapted to these nutrient-poor conditions through high foliage longevity (25-30 years) in which even the oldest needles contribute positively to the carbon balance (55). 12 BONAN & SHUGART

Forest Floor Depth

Annual Production

901 ';"E f 0, s6l250 210 170 130 .:;;-o �O

N-Mineralization

• Aspen

o Birch

'" Black Spruce

'" White Spruce o Poplar

Figure 3 Productivity and nutrient cycling in forest vegetation types near Fairbanks, Alaska, in relation to forest floor moisture content and soil temperature. (A) Forest floor depth. (B) Annual above-ground tree production. (C) N mineralization in the forest floor. From Van Cleve & Yarie Annu. Rev. Ecol. Syst. 1989.20:1-28. Downloaded from www.annualreviews.org

by Swedish University of Agricultural Sciences on 02/05/14. For personal use only. (156).

FOREST FLOOR ORGANIC LAYER

An adequate understanding of the structure and function of boreal forests requires a study of the forest floor organic layer. As was indicated in the previous sections, a thick moss-organic layer on the forest floor is an important structural component of boreal forests, controlling energy flow, nutrient cycling, water relations, and through these, stand productivity and dynamics. BOREAL FOREST ECOLOGY 13

Lichens Throughout North America and Eurasia, lichens are frequently a conspicuous component of the boreal forest floor, growing in open Picea, Pinus, or Larix woodlands where they virtually cover the ground surface between trees (2, 58-60, 72, 115). Rencz & Auclair (115) described a typical Picea mariana­ lichen woodland near Schefferville, Quebec,in the transition region between the boreal forest and the tundra. Aboveground woody biomass was among the lowest reported in boreal forests (18.6 t ha-1), and the ground was covered by a thick lichen mat that contained 20% of the total stand biomass, and over 66% of all the photosynthetic material in the woodland. In these nutrient-poor lichen woodlands, tree growth is slow (annual diameter increment 0.1-0.2 mm), and trees rarely exceed 10 m in height and are typically spaced 3-15 m apart (58-60). The ecology of lichens is primarily controlled by their inability to compete with faster-growing higher plants (72,73). Lichen woodlands are characteris­ tic of high-latitude or dry, sandy, acidic, nutrient-poor soils (2, 72, 73, 84, 91, 139). Extensive lichen cover can also be found in moist, peaty, acidic woodlands underlain by permafrost (72). Apparently, a lichen cover develops only in environments where competition from higher plants is excluded or reduced (72, 73). Light is also a significant factor limiting the growth of lichens. Few lichens are able to survive in the deep shade of a closed forest canopy (61,62, 72,73,84,88,91). A deep snowpack may also be important for protecting lichens from cold winter temperatures (72, 73). Lichens have a crucial role in the moisture and thermal regimes of forest soils. A lichen mat maintains soil moisture at or near field capacity throughout the growing season, reducing moisture stress and allowing growth on soils that otherwise would be too dry to support tree growth (72-75, 119). With its high reflectivity and low thermal conductivity, a lichen mat also acts as an insulator that greatly impedes the flux of heat into the underlying soil. A 12-cm-deep lichen mat can reduce the soil heat flux by almost 50%, signifi­ cantly lowering temperatures throughout the �oil profile (72-75). Annu. Rev. Ecol. Syst. 1989.20:1-28. Downloaded from www.annualreviews.org

by Swedish University of Agricultural Sciences on 02/05/14. For personal use only. Nutrient availability is strongly influenced by lichens. The low soil temper­ atures promoted by a thick lichen mat, coupled with acidic conditions,inhibit organic matter decomposition so that a large proportion of nutrients is tied up in the soil organic matter (6, 102-104). Nitrogen fixation by lichens under natural conditions has been reported in North Sweden (57), North Finland (69), Canada (30, 73), and Alaska (3, 11). This may be an important component of the nitrogen input to the soil, especially in old-growth forests where nitrogen deficiencies are likely. However, slow decomposition rates may preclude major redistribution of fixed nitrogen in organic matter. A thick lichen mat can hinder tree growth through its deleterious effects on 14 BONAN & SHUGART

soil temperature, nutrient availability, and allelopathy (20, 29, 73). Despite these effects, the removal of a lichen mat appears to reduce tree growth significantly (20, 29). In certain instances, a lichen mat apparently enhances tree growth by creating favorable soil moisture conditions despite possible allelopathic effects and low soil temperatures (29, 73).

Mosses In moist, shaded woods, mosses replace lichen as the dominant ground cover (2, 61, 62, 88, 91, 139). A thick moss ground cover composed primarily of feathermosses (Hylocomium splendens, Pleurozium schreberi) or Sphagnum is common throughout the boreal forests of North America (85, 88), Scandi­ navia (138, 139), and the Soviet Union (1, 61, 62, 70, 141). In interior Alaska, the depth of the moss-organic layer averages 20-30 cm in cold, wet Picea mariana stands, but may exceed 50 cm (164). In warmer, drier Picea glauca stands, the depth of the moss-organic mat is on the order of 0-10 cm, and in hardwood stands virtually no moss occurs (155, 165, 168). This moss-organic layer influences forest structure and function in two ways. As seen above, soil temperatures and depth to permafrost are directly related to the thickness of the forest floor organic layer. Mosses also influence nutrient availability. With their high capacity to absorb water, mosses act like sponges, efficiently absorbing and immobilizing the nutrients in precipitation (109, 138, 172). Moreover, cyanobacteria associated with green feathermoss­ es are important nitrogen fixers in interior Alaska (3, 11). However, by promoting cold soil temperatures, the moss-organic layer reduces organic matter decomposition, immobilizing these nutrients until the mosses die and slowly decompose (41, 152-156, 171). In forests with a thick moss layer, the forest floor often contains the largest pool of nutrients (152-156). These nutrients are not available to vascular plants until the mosses die and de­ compose, usually at rates 1-10% of those in vascular plants (l09). Thus, stand structure and function in the taiga are largely controlled by the moss-organic layer. The presence of a moss layer contributes to organic

Annu. Rev. Ecol. Syst. 1989.20:1-28. Downloaded from www.annualreviews.org matter accumulation, decreased soil temperatures, increased soil moisture by Swedish University of Agricultural Sciences on 02/05/14. For personal use only. content, and reduced nutrient availability (130, 138, 147, 152-156, 160). Over time, as the forest develops and nutrients are immobilized in un­ decomposed organic matter, the forest floor becomes the principal nutrient reservoir (Figure 4). Mosses thrive and form a continuous cover where conditions are both moist and shady (88). In cold, wet Picea mariana stands, up to 80-90% of the aboveground biomass may be contained in the moss layer (9), and annual moss production may be twice that of annual foliage production and almost the same as total aboveground tree production (l09, 134). Moss establishment and productivity are apparently promoted by the low temperature, high water BOREAL FOREST ECOLOGY 15

Forest Floor

_ Available Nutrients c=JTotal Nutrients

Net Primary Production ' Available Soil Nutrient en +-' c: rz2LZl Vascular Plants (Overstory) Phosphorus EOJ Non- Vascular Plants Nitrogen .., � L- eu 0.. E o o N o I

- o o OJ (J) .t:! (/) Q) > 11-11 --- . 2 3 4 5 Time

Figure 4 Hypothesized changes in the structure and function of a Picea mariana forest in the uplands of interior Alaska over approximately 150 years following fire. From Van Cleve et al (152).

content, and poor nutrient status of Picea mariana soils (134). Most feath­ ennoss species are intolerant of direct sunlight and are soon killed by expo­ sure to full sunlight (138, 172). In addition, dense litter fall in deciduous forests inhibits moss growth by creating substrates unsuitable for moss es­ tablishment and by shading mosses (l09, 138). The fine needle litter of coniferous forests does not shade mosses or prevent their establishment (109, 138). Water relations and drought resistance fundamentally control the growth

Annu. Rev. Ecol. Syst. 1989.20:1-28. Downloaded from www.annualreviews.org and distribution of mosses (24, 25, 94, 108, 135, 138). Many mosses (e.g. by Swedish University of Agricultural Sciences on 02/05/14. For personal use only. Hylocomium splendens, Pleurozium schreberi, Sphagnum subsecundum) lack roots, a vascular transport system, and a mechanism to store water or reduce water loss (94). Though these mosses absorb water well, they are easily desiccated on warm sunny days (94, 108, 136), and once this external water evaporates, net photosynthesis decreases with increasiJ,lg water stress (25, 108). Trees have a strong effect on moss growth. In forests, the highest pro­ ductivity of mosses often is near the border of the tree crown projection (1, 138, 141). This has been attributed to favorable light and nutrient availability (138), inhibitory effects of litter fall beneath the canopy (141), and in- 16 BONAN & SHUGART

sufficient precipitation under the canopy (1). However, water supply, not low light or litter fall, is probably responsible for limiting moss growth near the trunk of trees (24). Mosses have a reciprocal effect on the ability of tree species to regenerate. For example, Pinus sibirica, common in western Siberia, can successfully regenerate only when the moss layer is sufficiently thick that seeds can be buried by birds (Nucifraga caryocatactes), and thus be protected from con­ sumption by small mammals (71). In most species, however, seedling germination and establishment is either precluded or hindered by a thick moss-organic layer that is easily desiccated, shades seeds, prevents roots from reaching the mineral soil, and engulfs the slower growing tree seedlings (60, 88, 159, 176-179).

Forest Floor Litter Quality Forest floor chemistry interacts with soil temperature and moisture to enhance or restrict stand productivity and nutrient cycling (27, 42, 103, 152, 153, 156). For example, compared to other forest types, the Picea mariana forest floor has the highest concentration of lignin and the lowest concentration of all plant macronutrients (42). This high concentration of decay resistant material and the low concentration of nutrients essential to microbial activity reduce forest floor decomposition and nutrient availability.

FIRE REGIME

Much of the floristic diversity and many mosaic vegetation patterns within the boreal forest are directly attributable to recurring wildfires (54, 121, 159, 160, 167). Moreover, by consuming all or part of the forest floor, fire influences soil moisture, soil temperature, organic matter accumulation, and regeneration and through these controls major ecosystem processes such as nutrient cycling, energy flow, and productivity (38, 96, 98, 130, 131, 148, 149, 167). Annu. Rev. Ecol. Syst. 1989.20:1-28. Downloaded from www.annualreviews.org by Swedish University of Agricultural Sciences on 02/05/14. For personal use only. Fire Cycle The natural fire cycle in the North American boreal forest ranges from an average of 50 to 200 years, to an extreme of 500 years in moist regions of eastern Canada (44, 53, 163, 167). Reported fire frequencies in northern Sweden are similar, ranging from 50 to 270 years and averaging between 110-155 years (40, 175). Macroclimate, especially the moisture balance, determines the regional component of the fire cycle. Low precipitation com­ bined with high summer temperatures makes the Yukon Flats and Porcupine Plateau physiographic regions the most extreme fire climate in Alaska (143, 174). In northwest Canada, the recurrence interval also increases towards the tree line, and this may be related to Bryson's (21) air mass movements (66, BOREAL FOREST ECOLOGY 17

67). The local fire cycle reflects modification of the regional pattern by terrain and site conditions (66, 120). Dry sites tend to have a shorter fire recurrence interval than do wet sites (54, 120, 160). A few major fires that occur in extreme fire years account for the vast majority of forests burned. Sixty to 80% of all fires in northwest Canada and Alaska are less than 5 ha in area (8), and 85% of all fires in Canada between 1961-1967 were 4 ha or smaller (121). Yet in severe years, individual fires can cover 50,000 to 200,000 ha (38). Where detailed fire history records are available, most of the area affectedin a region is burnedby a few large fires in severe fire years (7, 44, 52, 140). There is much disagreement over whether the probability of burning in­ creases with stand age. Where the probability of burning is independent of stand age, the negative exponential distribution describes the fire cycle (68, 157). Many researchers believe that after a short period of low flammability following fire, most boreal forests are equally flammable regardless of age (53, 121). Others consider flammability to increase with age. The Weibull distribution provides an age selection model in which the probability of burningis a power function of the time since the last fire (66, 68). This model also describes the fire cycle in different forest types in northwest Canada (66, 174). Analysis of the current fire cycle in boreal forests is hindered by conflicting

fire management policies and fire detection capabilities. The long-term fire records needed to reconcile these conflicts and to put current data into their proper context are lacking (8). Though the boreal forest is classified as a lightning fire bioclimatic region, the natural lightning fire cycle most likely prevails only in remote areas (54).

Fire Intensity

The fire regime is also characterized by fire intensity, that is, the release of energy from the fire. Most tree species are not fire resistant; only the pines

Annu. Rev. Ecol. Syst. 1989.20:1-28. Downloaded from www.annualreviews.org (e.g. Pinus banksiana, P. sylvestris) have thick enough bark to survive the by Swedish University of Agricultural Sciences on 02/05/14. For personal use only. heat generated by fire (53,54,96) . In general, the fire regime consists of high intensity, short-to-Iong return interval crown fires, or severe surface fires that kill and regenerate entire stands (53, 54, 163). Potential fire intensity, and especially the tendency of a fire to crown, varies among forests owing to differences in the distribution of fuels within

stands. For example, the < fuel arrangement within Picea mariana stands makes them highly susceptible to crown fires. The open, highly flammable ericaceous shrub layer rapidly carries flames from the ground surface to low, dead branches covered with lichens, which provide a ladder into the canopy (54, 163,< 167). Pine stands (e . g . Pinus banksiana, P. sylvestris) also are highly susceptible to crown fires (121,158). In deciduous forests, surface fire 18 BONAN & SHUGART

hazard is high when the litter layer is dry, but crown fires are rare except under extreme weather conditions (121, 158).

Fire Severity The effects of fire on site conditions are a consequence of fire severity, that is, the amount of organic matter consumed in the fire. Wet organic layers are less readily consumed than are dry layers (37, 158), and many fires have little effect on the forest floor because they occur when the ground is wet or frozen (54, 164). Direct soil heating during burning is minimal and has little long-lasting effect on soil temperature (18, 148, 166). Mineral soil and organic matter are poor conductors of heat energy. Moreover, the lower portion of the thick forest floor layer often remains moist during burning (37, 148, 166). Howev­ er, fire drastically alters ground surface energy exchange processes by (a) reducing the thickness of the insulating forest floor layer, (b) blackening the forest floor, and (c) removing the forest canopy, (18, 38). The main thermal effect of fire results from the complete or partial removal of the organic layer (18). Reduction of the forest floor thickness reduces its insulative effect, allowing greater heat flow and resulting in increased soil temperatures that are directly related to the amount of organic matter removed (36, 162, 164, 166). For example, four summers after experimental fires in an interior Alaska Picea mariana forest, depth of thaw where half the forest floor was removed had increased to 85 cm from a pre-burn depth of 26 cm; where the forest floor was completely removed, depth of thaw had increased to 138 cm (36). The long-term effects of fire on the soil thermal regime are poorly un­ derstood. In permafrost soils, a consistent annual increase occurs in the active layer thickness following fire (36, 162, 164, 166). How long it will take the depth of thaw to stabilize or to return to pre-burn levels is unknown. Linell (93) maintained sites free of ground cover for 26 years and found that depth of thaw increased yearly. Van Cleve & Viereck (155) speculated that the per­ Annu. Rev. Ecol. Syst. 1989.20:1-28. Downloaded from www.annualreviews.org

by Swedish University of Agricultural Sciences on 02/05/14. For personal use only. mafrost table may return to its original depth 25-50 years following fire when the forest canopy and moss-organic layer are fully reestablished. Heinselman (54) estimated this time as 10-15 years. Removal of the forest floor organic mat also affects nutrient availability­ directly by releasing or volatilizing nutrient elements accumulated in the forest floor, and indirectly by increasing decomposition rates due to a more favorable soil thermal regime (5, 37, 38, 96, 98, 130, 131, 148, 149, 163, 167). However, the effects of fires on nutrient availability depend on the forest type and the severity of the bum. In the Picea mariana forests of interior Alaska, soil alkalinity and available phosphorus increased following burning, with a maximum in moderately and severely burned areas (37). Total BOREAL FOREST ECOLOGY 19

nitrogen in the forest floor increased in moderately burned areas, but de­ creased in heavily burned areas due to excessive volatilization. In contrast, the nutrient flush following fire in the Picea mariana-lichen woodlands of subarctic Quebec is much less than in other forest types (6, 35, 102).

Regeneration Following Fire Many of the major tree species in the boreal forest depend on recurring fires for maintenance in the landscape. However, no single sequence of revegeta­ tion follows fire (Figure 5). The postfire vegetation is a complex function of climate, pre-bum vegetation type and age, time of bum, fire severity, topography, and the presence or absence of permafrost (159, 160, 163). The viability of tree seeds stored in the soil is generally low (39, 45, 65, 159), and the physical characteristics of tree seeds provide little protection from the high temperatures generated by fire (159). Thus, the availability of reproduction sources is an important condition for successful regeneration following fire (96, 159). Such sources include (54, 120):

1. Highly dispersed propagules: many species persist due to their copious production of light, wind-dispersed seeds (e.g. Betula papyrijera, B. pubes­ cens, B. pendula, Populus tremuloides, P. balsam(fera). 2. Serotinous cones: some species persist following fire due to their ability

Paludification Thermal erosion � No fire /� Balsam Fir , , Long period Fire on fire (Eastern Canada) Annu. Rev. Ecol. Syst. 1989.20:1-28. Downloaded from www.annualreviews.org by Swedish University of Agricultural Sciences on 02/05/14. For personal use only.

Figure 5 Successional relationships among some North American boreal forest types in relation to the occurrence of forest fires. From Viereck (163). 20 BONAN & SHUGART

to store seeds in serotinous or semiserotinous cones in the canopy (e.g. Pinus banksiana, Picea mariana). 3. Fire tolerance: the thick bark of some species enables them to survive the intense heat generated by fires (e.g. Pinus banksiana, P. sylvestris). 4. Vegetative reproduction: many species persist following fire due to their tremendous capacity to root and stump sprout (e.g. Populus tremuloides, P. tremula, Betula papyrifera).

Infrequent good seed crops can severely restrict the availability of seeds following fire (96, 148, 159, 160, 167). However, the hot summers important for good Picea glauca, P. abies, and Pinus sylvestris seed production also create environmental conditions favoring a high fire danger potential (96, 148, 159). This correlation may be particularly important in the far north where irregular seed crops are more common (96). The long-term stability of Picea-moss forests in the 'absence of fire is unknown. Moss-organic layers greater than 5-8 cm thick are thought to preclude successful regeneration (71, 159). For example, in the Picea

mariana-feathermoss communities in the uplands of interior Alaska, success­ ful germination and establishment following fire are promoted by increased fire severity and occur almost exclusively on heavily burned microsites where the mineral soil is exposed (178, 179). However, some species, particularly Picea mariana, can vegetatively reproduce by layering when low branches are covered by moss-organic matter and, as a result, root and produce new individuals. This process is an important means of maintaining stand structure in the absence of fire, when the thick moss-organic layer precludes seedling establishment (12, 176). In the absence of fire, evidence suggests both degradation and stability of Picea-lichen woodlands. A lichen woodland as old as 850--900 years has been found in northern Quebec (111, 113). Sexual reproduction was common in this stand. In some cases, however, few seedlings can grow within a lichen mat (29, 58-60). Strang (137) found decreasing depths of seasonal soil Annu. Rev. Ecol. Syst. 1989.20:1-28. Downloaded from www.annualreviews.org

by Swedish University of Agricultural Sciences on 02/05/14. For personal use only. thawing in open lichen woodlands and postulated an eventual degradation to tundra vegetation. Others found that in the absence of fire, open lichen woodlands may develop into a closed canopy forest with a ground cover dominated by mosses (67, 99).

INSECT OUTBREAKS

Severe outbreaks of defoliating insects that kill or damage forests over large areas are a common part of the boreal forest ecosystem. Picea-Abies forests in eastern Canada are especially vulnerable to repeated spruce budworm (Chor­ istoneurafumiferana) outbreaks, which have been documented as far back as BOREAL FOREST ECOLOGY 21

the early l700s (13). Repeated outbreaks of Siberian silkworm (Dendrolimus sibiricus) and big coniferous long-horned beetle (Monochamus urossovi) are common in some Siberian coniferous forests, destroying three million hect­ ares of forest in West Siberia between 1954-1957 (63). Stand structure and composition predispose the forest to insect outbreaks and affect the duration, severity, and rate of spread. Mature Abies balsamea is the preferred target of spruce budworms, and spruces (Picea mariana, P. glauca, P. rubens) are less vulnerable (97). A prolonged period of early summer drought is also essential for insect outbreaks because cool, wet summers prevent the insects from completing their development (13, 63). Severe insect attacks are thought to increase the hazard of fires. When the forest canopy is removed, increased solar radiation and wind speed rapidly dry out fuel,especially loose bark,on the forest floor. These drier conditions, combined with a greater fuel loads, may increase the danger of fire (47, 52). However, the long-term fire history records needed to test the hypothesis are lacking (47).

SUMMARY

The borcal forest is a mosaic of vegetation typcs that rcflectsa combination of environmental factors unique to high-latitude forests. Climate, solar radia­ tion, soil moisture,soil temperature and the presence of permafrost, the forest floor organic layer, nutrient availability, insect outbreaks, and wildfires interact to contribute to the mosaic pattern of forest types and the wide range in stand productivity characteristic of boreal forests (Figure 1). Our model of boreal forests is on a circumpolar scale,and one should not infer that the full model will apply to all boreal forests. Rather, this review has identified the critical processes and parameters required to understand the ecology of boreal forests. In doing so, it has provided a framework for a circumpolar compari­ son of boreal forests and a mechanistic context for climatic-biogeographic

Annu. Rev. Ecol. Syst. 1989.20:1-28. Downloaded from www.annualreviews.org classifications of boreal forest regions. by Swedish University of Agricultural Sciences on 02/05/14. For personal use only. Bonan (14, 15) quantified this conceptual scheme into a simulation model of boreal forest dynamics. This model simulated local, seasonal patterns of solar radiation, soil moisture, and soil freezing and thawing for different topographies at Fairbanks, Alaska, and annual regional patterns of the same processes throughout the boreal forests of North America, Scandinavia, and the Soviet Union (15a). The model also simulated forest structure and vegeta­ tion patterns in several conifer, hardwood, and mixed-conifer hardwood forests in different bioclimatic regions of northern North America (14, 15). These analyses indicated that though not floristically complex, boreal forests are complex in the environmental factors required to understand boreal ecolo- 22 BONAN & SHUGART

gy and the interactions among these factors. Thus, a multifunctional approach is needed to understand the ecology of boreal forests.

ACKNOWLEDGEMENTS This work originated from a WMO/UNEP/ICSU meeting on the circumpolar boreal forest, International Meteorological Institute, Stockholm, January 1985, and a meeting on the "Impacts of changes in climate and atmospheric chemistry on northern forest ecosystems and their boundaries," International Institute for Applied Systems Analysis, Laxenburg, Austria, August 1987. We are indebted to the people who organized and attended these meetings. Keith Van Cleve, University of Alaska, Ross Wein, University of Alberta, Serge Payette, Laval University, and Colin Prentice, University of Uppsala, provided critical comments and advice on the manuscript. Michael An­ tonovsky and Allen Solomon arranged for the authors to make an extensive visit to IIASA during the summers of 1987 and 1988. Mihail Korzuhin and Mihail Ter-Mikaelian, Goskohydromet Natural Environment and Climate Monitoring Laboratory, Moscow, provided access to Soviet data and kindly translated several Soviet papers. This work was supported by a National Science Foundation grant NSF-BSR-85 10099 and a National Aeronautics and Space Administration grant NASA-NAG-5-1018 to H. H. Shugart and a NRC-NASA research associateship to G. B. Bonan.

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Annu. Rev. Ecol. Syst. 1989.20:1-28. Downloaded from www.annualreviews.org R. 1979. Preliminary results of ex­ aspen and willow after fire on black by Swedish University of Agricultural Sciences on 02/05/14. For personal use only. perimental fires in the black spruce type spruce/feathermoss sites in interior Alas­ of interior Alaska. US For. Servo Res. ka. For. Chron . 63:84--88 Note PNW-332. 27 pp. 179. Zasada, J. C., Norum, R. A., Van 167. Viereck, L. A., Schandelmeier, L. A. Veldhuizen, R. M., Teutsch, C. E. 1980. Effects of fire in Alaska and adja­ 1983. Artifical regeneration of trees cent Canada: a literature review. Tech. and tall shrubs in experimentally burned Rep. 6. US Dept. Interior, Bureau Land upland black spruce/feathennoss stands Manage., Alaska, 124 pp. in Alaska. Can. J. For. Res. 13:903 168. Viereck, L. A., Van Cleve , K., Dyr­ -13 ness, C. T. 1986. Forest ecosystem dis­ 180. Zoltai, S. C. 1975. Tree ring record of tribution in the taiga environment. See soil movements on permafrost. Arct. Ref. 150a, pp. 22-43 Alp. Res. 7:331-40 169. Vowinckel, T., Oechel, W. C., Boll, 181. Zoltai, S. C. 1975. Structure of subarc­ W. G. 1975. The effect of climate on the tic forests on hummocky permafrost ter­ photosynthesis of Picea mariana at the rain in northwestern Canada. Can. J. subarctic tree line. I. Field measure- For. Res. 5:1-9