The Boreal Forest Ecosystem☆

The Boreal Forest Ecosystem☆

The Boreal Forest Ecosystem☆ Donald L DeAngelis, University of Miami, Coral Gables, FL, United States r 2019 Elsevier B.V. All rights reserved. Intro...

90KB Sizes 0 Downloads 34 Views

The Boreal Forest Ecosystem☆ Donald L DeAngelis, University of Miami, Coral Gables, FL, United States r 2019 Elsevier B.V. All rights reserved.

Introduction The boreal biome is the largest of all terrestrial biomes, amounting to roughly 15  106 km2, with estimated storage of about 195 billion (195  109) metric tons of carbon (C) in aboveground living pools, which is about one-third of the total terrestrial carbon. Approximately three times that amount is stored in soil. The boreal forest biome is also referred to as the “taiga” (Russian) for “swamp forest.” Geographically, the boreal forest is located between latitudes 451 and 701 N, and virtually all of it in Canada, Alaska, and Siberia, with portions in European Russia and Fenno-Scandia. The boreal forest is bordered on the north by treeless tundra and on the south by mixed forest. The boreal forest is termed a “biome” by ecologists, a term that refers to a biogeographic unit that is distinguished from other biomes by the structure of its vegetation and dominant plant species. A biome is the largest scale at which ecologists classify vegetation. All parts of a biome tend to be within as the same climatic conditions, but because local conditions differ, a biome may encompass many specific ecosystems (e.g., peatlands, river floodplains, uplands) and plant communities. Despite this diversity within a biome, in referring to the boreal forest we will here use the terms “biome” and “ecosystem type” interchangeably.

Climate and Soils The climate of the boreal forest is continental and, importantly for the growing season, there tend to be between 30 and 150 days of temperatures above 101C. Temperature lows can fall below  251C. Average annual precipitation is 38–50 cm, with the lowest amounts in the northern boreal forest, and greater frequency of precipitation during the summer season. Water is seldom limiting because of the generally flat topography and low rate of evaporation. Permafrost can occur in the northern parts of this zone, the southern limit coinciding roughly with a mean air temperature of  11C and snow depth of about 40 cm. The zone of permafrost generally starts at depths ranging from 1.5 to 3 m in the areas of the boreal forest where it occurs. Its occurrence limits soil processes to an upper active layer and impedes water drainage, leading to waterlogged soils. The soil decomposition rate in the taiga is slow, which leads to the accumulation of peat. Several soil types characterize the boreal forest. The soils of a major part of the boreal forest, lying under a dense coniferous canopy, are heavily podzolized where the soil is permeable. These soils consist largely of spodosols. Intense acid leaching forms a light ash-colored eluvial soil horizon leached of most base-forming cations such as calcium. Thus taiga soils tend to be nutrient poor. Gelisols are common in the north, where permafrost occurs. These are young soils with little profile development. Histosols, which are high in organic matter, form in nonpermafrost wetlands, where decomposition is slowed by hypoxic conditions. These are often referred to as peatlands.

Biodiversity Tree species richness is far smaller than that in the temperate forests to the south, where more than 100 species are typically observed in 2.51  2.51 quadrats in eastern United States. Species richness clearly declines from south to north in the taiga. Whereas 40 or more tree species can be found in the southern taiga in Canada, this declines to 10 or so species near the tundra boundary. Animal species also show strong gradients. Reptile and amphibian species are almost nonexistent above 551. Mammal species richness declines from close to 40 species to about 20 going northward in the boreal forest biome in North America, while bird species decline from about 130 to less than 100.

Forest Structure and Species Because many hardwood trees are both sensitive to low winter temperatures and require a long and warm summer, the true boreal forest begins where the few remaining hardwoods become a minor part of the forest. Four coniferous genera dominate a major ☆

Change History: April 2017. Donald DeAngelis updated sections Introduction, Climate and Soils, Climatic variation and effects on vegetation, Landscapescale vegetation differences, Herbivorous insect outbreaks, Difference in food webs due to climatic differences, Predator-prey cycling, Fire and its effects, Conservation and global issues Summary. This is an update of D.L. DeAngelis, Boreal Forest, In Encyclopedia of Ecology, edited by Sven Erik Jørgensen and Brian D. Fath, Academic Press, Oxford, 2008, pp. 493–495.

Encyclopedia of Ecology, 2nd edition, Volume 2




Ecosystems: The Boreal Forest Ecosystem

part of the taiga; Picea (spruce), Abies (fir), Pinus (pine), and Larix (larch). The hardwoods, which largely occur in dwarf form, include Alnus (alders), Populus (poplars), Betula (birches), and Salix (willows). The hardwoods tend to be early successional species following disturbances such as fires or erosion/deposition processes on riverbanks, which are eventually shaded out by slowergrowing spruces and firs. Much of the main boreal forest is dominated by a few spruce species. These form a dense canopy in the central and southern taiga, with a ground cover of dwarf shrubs, such as cranberries and bilberries, and mosses and lichens. In northern Siberia, huge areas are covered almost solely by larch, and the canopy is much less dense. Pine species, which can withstand a range of harsh conditions, grow in light, sandy soils and other dry areas. As the boreal forest-tundra boundary is approached, conifers thin out to a woodland with lichen and moss dominating the ground. Trees become more and more stunted. The standing stock of biomass of the boreal forest ranges is estimated at 200 (range 60–400) metric tons per hectare (t ha1). This compares with an estimate of 350 t ha1 for the temperate deciduous forest and 10 t ha1 for the tundra ecosystems. The boreal forest differs from the temperate forest in having a much higher percentage of its total biomass as photosynthetic foliage (7% vs. 1%). It differs from the tundra in having a lower percentage of root biomass (22% vs. 75%).

Climatic Variation and Effects on Vegetation There is a great deal of climatic variation within the boreal biome. Winter temperatures in the boreal forest of western North America are about 15–201C colder than in northwestern Europe (Fennoscandia) with half as much precipitation. Across Russia, the climate changes from relatively mild and wet in the Baltic region to extreme continental climate in northeastern Siberia, with mean annual temperatures of  101C and low precipitation. The differences in climate between areas of forest at the continental scale bring about differences in vegetation. Typical areas of boreal forest of upland western North America are dominated by white spruce (Picea glauca), with lesser amounts of aspens and poplars (Populus spp.) in the canopy. In northwestern Europe, Scots pine (Pinus sylvestris), Norway spruce (Picea abies), and birches (Betulus spp.) make up most of the overstory layer. But the main differences are in the understory structure. In North America, there is a relatively tall shrub layer (0.6–2 m high) of willow (Salix spp.) and birches, whereas in northwestern Europe the tall shrub layer is absent. Instead there is a layer of dwarf shrubs (0–0.5 m), mostly ericaceous, such as bilberry (Vaccinium spp.). The difference has been attributed to greater tolerance to low winter temperatures of willow and birch shrubs, while smaller shrubs such as bilberry are poor at surviving cold temperatures without a deep snowpack for insulation. The extreme cold and dry conditions of eastern Siberia result in the deciduous needle-leaf conifer, larch (Larix spp.) being the most common component of the forest. The deciduous trait allows larches to avoid needle dessication from the extreme winter temperatures.

Landscape-Scale Vegetation Differences The taiga of interior Alaska is a particular example showing the variability of vegetation communities at the landscape scale. Welldrained south-facing slopes are dominated by communities of white spruce and hardwoods such as birches and aspens, which are adapted to warmer conditions. Along a transect down gentle north-facing slopes, closed black spruce (Picea mariana) forest gives way to more open black spruce-Sphagnum muskeg with sedge tussocks dominating on areas of stream valley underlain with permafrost. It is also possible for alternative stable states of community to occur in some areas on the same site, depending on the history of that area. This is true of the white spruce and hardwood type community and black spruce community. These communities, both of which can potentially occupy the same sites in the taiga zone, are characterized by different nutrient cycling efficiencies. Black spruce ecosystems have mechanisms that conserve nutrients. Because the litter of black spruce is low in nutrients, it is slow to decompose; thus thick layers of organic matter accumulate on the forest floor under black spruce. This insulates the soil and thus reduces summer soil temperature. Rates of mineralization become even slower on these stands because of the low summer soil temperatures. If a fire disturbance occurs, it may burn off enough of the litter, so that summer soil temperatures are warmer and nutrient cycling is higher. Under these circumstances, white spruce, which needs greater nutrient availability, may be able to invade and dominate. However, any disturbance that reduces nutrient availability to the white spruce may allow some black spruce trees to invade. The black spruces will contribute more low quality (nutrient poor) litter to the ground, causing the insulation factor to increase again, and reducing soil summer temperatures. Eventually, the black spruces may dominate again.

Animals Animal life in the boreal forest is far less diverse than in most temperate zone ecosystems. A number of bird species are adapted to being residents of the taiga. Grouses such as the capercaillie of the Old World, are adapted to year-round life in the taiga, as are some owls, woodpeckers, tits, nuthatches, crossbills, and crows. Small mammal herbivores of the boreal forest include the squirrels, chipmunks, voles, and snowshoe hares. These provide food for a small number of predators species, including the red fox (Vulpes vulpes) and members of the weasel family. The moose (Alces alces) (called elk in the Old World) has a wide geographic distribution in the taiga. They are prey for wolves (Canis lupus) and occasionally the brown bear (Ursus arctos).

Ecosystems: The Boreal Forest Ecosystem


One component of the taiga fauna, conspicuous for its frequent devastating effects on thousands of hectares of forest, is that of phytophagous insects. Populations of these insects, which include pine sawflies (Neodiprion spp.), spruce budworms (Choristoneura fumiferana (Clemens)), bark beetles, and many others that attack conifers, are capable of escaping natural enemies and building up to huge population densities. The large monospecific stands of the boreal forest may be especially vulnerable.

Herbivorous Insect Outbreaks The boreal forest is known for the occurrence of severe outbreaks of herbivorous outbreaks insects like the spruce budworm. Patterns of outbreaks may be cyclic or irregular (irruptive). Environmental factors can affect outbreaks, and outbreaks of some species of insect have increased in recent times; perhaps due to logging practices and fire suppression, leading to greater densities of tree species preferred by particular insects; for example, there were nine outbreaks of eastern spruce budworm during the nineteenth century, whereas during the first 80 years of the 20th century there were 21, and they were more widespread. Weather is an important factor in herbivorous insect outbreaks. An example is the spruce budworm, which feeds preferentially on foliage of white spruce and balsam fir (Abies balsamea). The caterpillars of this moth do not feed immediately after emergence from the egg, but are dispersed by wind and hibernate over winter. When they emerge in spring they start to feed on old fir needles; that is, they are senescence feeders, feeding on needles that are breaking down and releasing nutrients, that is, amino acids, in high concentrations during nutrient translocation. Outbreaks tend to occur when there are large numbers of old trees, where it is likely that many of the needles are old and not very vigorous, so they tend to break down quickly. But the presence of a large stand of old trees is not sufficient in itself to lead to a spruce budworm outbreak. The period of 1948 to 1958 in eastern Canada was a time of summer droughts interspersed with wetter than normal winters. This combination of drying of roots in the summer and waterlogging them in the winter caused dieback of crowns, with rapid aging of leaves and release of high concentrations of amino acids being translocated out of the dying leaves. The extra amino acid in the diets of the spruce budworm caterpillars was enough to increase caterpillar survival, which led to population explosions that predators and parasitoids could not control. DDT was sprayed to stop the outbreaks, but that just made things worse by saving the old stands trees and letting them get older and even more susceptible to outbreaks. Insect herbivores are estimated to cause more timber losses than fires in the boreal zone. The spruce budworm is the most destructive boreal forest insect which caused as average annual loss during the period of 1982–1987 of 27.3  106 ha of forest in eastern North America. Among other defoliator insect pest of the boreal forest, the forest tent caterpillar (Malacosoma disstria) caused 2.4  106 ha, and the jack pine budworm (Choristoneura pinus) a 2.2  106 ha annual loss during the same period. Insect damage causes forests to become more susceptible to fire. However, the high numbers of insects during the warm months is a main explanation for the large numbers of birds that migrate from the south to breed in the taiga, especially large numbers of species of warblers and thrushes.

Difference in Food Webs Due to Climatic Differences The differences in vegetation between western North America and northwestern Europe also lead to differences in the food webs. The tall shrubs of the former provide winter forage for snowshoe hares (Lepus americanus) in North American, while the voles (e.g., bank vole (Myodes and Microtus spp.)), which are key rodents in Fennoscandia, are better adapted to foraging on the shorter dwarf shrubs that are beneath the snow surface in winter. The snowshoe hare is the prey of relatively specialist predators like the lynx (Lynx canadensis), coyote (Canis latrans), and great horned owl (Bubo virginianis), while mustelids such as the weasel (Mustela nivalis) and stoat (Mustela ermine) are adapted to feeding on the voles and thus being key predators in northwestern Europe boreal forest. In addition, the snow characteristics differ in the two regions, with deeper and softer snow in western North America, which could have favored the Canadian lynx (Lynx Canadensis) as a specialist predator on the snowshoe hare, in contrast with the wetter snow, with hard surface, in northern Europe. The generalist red fox (Vulpes vulpes) is better suited to the latter hard-packed snow conditions, and so is an important generalist in northwestern Europe but not western North America. The moose (Alces alces)-wolf (Canis lupus) predator–prey relationship is important in both systems.

Predator–Prey Cycling The interaction between a specialist predator and its prey can lead to population cycles, and this occurs between both the snowshoe hare and the Canadian lynx and voles and mustelids. The population cycle of the snowshoe hare is the best studied and most famous. Hare populations exhibit 9–11 year fluctuations in abundance that can be explained in terms of interactions with the lynx, although not all aspects of the cycling are understood. Fire may be an instigator of the cycle, as fires are followed a few decades later by very high levels of edible deciduous vegetation.

Ecosystem Dynamics In keeping with its position between much warmer climate of the temperate zone and colder climate of the tundra, the boreal forest's indices of production are intermediate between those two ecosystem types. Annual net primary production in the boreal


Ecosystems: The Boreal Forest Ecosystem

forest has been estimated at 7.5 (range 4–20) metric tons per hectare (t ha1 year1). This compares with 11.5 t ha1 year1 for temperate forest and 1.5 t ha1 year1 for tundra ecosystems. Mean boreal forest litterfall is estimated to be 7.5 t ha1 year1, compared with 11.5 and 1.5 t ha1 year1 for the temperature forest and tundra. Because low temperatures slow decomposition, the rate of litterfall decay in the boreal forest, 0.21 year1, is also intermediate between 0.77 and 0.03 for the temperate forest and tundra. This means that it takes roughly 3  (1/0.21) ¼ 14 years for 95% of a pulse of litter to decompose.

Fire and Its Effects Fire is an inherent factor in the ecosystem dynamics of the boreal forest. Lightning-caused fires occur on a given area at intervals of 20–100 years in drier areas to 200 þ years in wetter areas such as floodplains. Because nutrients tend to be tied up in slowly decomposing organic matter, fire may be important maintaining tree growth by releasing pulses of nutrients periodically. Many taiga plant species have adaptations to fires, such as serotinous cones and early sexual maturity of some conifers, and resprouting capacity of hardwood trees and many herbs and shrubs. Fires also reset the successional cycle, allowing shade intolerant species like birch and aspen to invade. Fires destroy the highly flammable late successional evergreen forests that are dominated by the spruces (white spruce and black spruce), creating the habitat required by early successional deciduous trees and shrubs. The dominant early successional deciduous species include the trees quaking aspen (Populus tremuloides) and Alaska paper birch (Betula neoalaskana) and an assemblage of willows (Salix spp.). As post-fire succession proceeds, the vegetation becomes progressively dominated by evergreens such as spruces, and ericaceous shrubs (e.g., Ledum spp.), as well as green alder (Alnus viridis subsp. fruticosa). Fires create a mosaic of patches of deciduous and evergreen trees and shrubs created by fire and the subsequent post-fire succession. Fire is important to the boreal forest. In boreal North America, fire is essential to the existence of most browsing mammal populations, and especially snowshoe hare populations. This is because fire creates most of the habitat mosaic required by these herbivores. Fire destroys late successional evergreen forests dominated by the slowly growing spruces Picea mariana and P. glauca, which are effectively defended against browsing by resins that contain toxic lipid-soluble secondary metabolites such as the monoterpene camphor that deters feeding by snowshoe hares. However, although spruce is a comparatively poor winter-food for boreal browsers, dense spruce thickets provide the protective cover that these herbivores use to evade their predators. The recently burned patches within an unburned spruce forest matrix are colonized by the more rapidly growing and comparatively poorly defended early successional deciduous species such as the willows (Salix spp.), quaking aspen (Populus tremuloides) and birches that in are the preferred winter-foods of most North American boreal browsing mammals. Thus, in boreal North America most of the optimal habitat for browsing mammals occurs at the edge of burns where browsers have ready access to both good predator escape cover (spruce forest) and good food (fast growing deciduous species in recently burned patches). For this reason in boreal North America the abundance of browsing mammals is often greatest in a landscape that contains patches of poorly defended early successional deciduous woody species within a late successional spruce matrix. And where the abundance of these browsing mammals is greatest, the intensity of their selective browsing on the recruitment of rapidly growing deciduous tree species such as B. neoalaskana and B. papyrifera is generally most intense.

Conservation and Global Issues The boreal forest represents the single largest pool of living biomass on the terrestrial surface. It contains more than 30% of the total terrestrial pool, and it is therefore critically important in global carbon dynamics. Much of the carbon is stored in the ground layer. Currently, the taiga is thought to act as a net sink of carbon, with an estimated 0.54 billion metric tons stored per year. However, global climate change, in the form of higher temperatures, may cause significant changes in the carbon dynamics by increasing decomposition rates faster than photosynthetic rates. Fire frequencies may also increase with temperature, as precipitation is not expected to rise, which will further increase the release of carbon stored in the ground layer. According to some studies, the boreal forest will be a net contributor to CO2 in the atmosphere under the projected climate changes. Climate-induced changes in the boreal forest would also have an impact of migrant birds that use the region for reproduction. Changes in tree species composition may challenge the capacity of birds to adapt, as has already the increasing fragmentation of the forest due to clear-cutting in many areas within the biome.

See also: Aquatic Ecology: Deep-Sea Ecology. Behavioral Ecology: Mating Systems. Ecological Data Analysis and Modelling: Modeling Dispersal Processes for Ecological Systems. Ecosystems: Riparian Wetlands. Evolutionary Ecology: Association. General Ecology: Biomass ; Carrying Capacity; Temperature Regulation. Human Ecology and Sustainability: Political Ecology

Further Reading Bonan, G.B., Shugart, H.H., 1989. Environmental factors and ecological processes in boreal forests. Annual Review of Ecology and Systematics 20, 1–28.

Ecosystems: The Boreal Forest Ecosystem


Danell, K., Lundberg, P., Niemalä, P., 1996. Species richness in mammalian herbivores: Patterns in the boreal zone. Ecography 19, 404–409. Henry, J.D., 2003. Canada's boreal forest. Washington, DC: Smithsonian. Hunter Jr., M.L., 1992. Paleoecology, landscape ecology, and conservation of neotropical migrant passerines in boreal forests. In: Hagan III, J.M., Johnston, D.W. (Eds.), Ecology and conservation of neotropical migrant landbirds. Washington, DC: Smithsonian. Knystautus, A., 1987. The natural history of the USSR. New York: McGraw-Hill. Krebs, C.J., Boutin, S., Boonstra, R., 2001. Ecosystem dynamics of the boreal Forest: The Kluane project. New York: Oxford University Press. Larsen, J.A., 1980. The boreal ecosystem. New York: Academic Press. McCullough, D.G., Werner, R.A., Neumann, D., 1998. Fire and insects in northern and boreal forest ecosystems of North America. Annual Review of Entomology 43, 107–127. Oechel, W. C., and Lawrence, W. T. (1985). Taiga. In: Chabot, B. F., and Mooney, H. A. (eds.). Physiological ecology of North American plant communities, pp. 66–94. New York: Chapman and Hall. Van Cleve, K., C. T. Dyrness, L. A. Viereck, J. Fox, F. S. Chapin III, and W. Oechel. 1983. Taiga ecosystems in interior Alaska. Bioscience 33:39–44.