Importance and relationship of boreal forests to climate

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Credit Andre Nantel


Published: September 30, 2014, 10:47 am
Author: International Arctic Science Committee
Topic Editor: Sidney Draggan
Topics:

[1]

This is Section 14.2 of the Arctic Climate Impact Assessment.

Lead Author: Glenn P. Juday; Contributing Authors: Valerie Barber, Paul Duffy, Hans Linderholm, Scott Rupp, Steve Sparrow, Eugene Vaganov, John Yarie; Consulting Authors: Edward Berg, Rosanne D’Arrigo, Olafur Eggertsson,V.V. Furyaev, Edward H. Hogg, Satu Huttunen, Gordon Jacoby, V.Ya. Kaplunov, Seppo Kellomaki, A.V. Kirdyanov, Carol E. Lewis, Sune Linder, M.M. Naurzbaev, F.I. Pleshikov, Ulf T. Runesson,Yu.V. Savva, O.V. Sidorova,V.D. Stakanov, N.M.Tchebakova, E.N.Valendik, E.F.Vedrova, Martin Wilmking.

Global importance (14.2.1)

The boreal region (Terrestrial biome) covers about 17% of the terrestrial area of the earth[1], with a broad zone of forest in a continuous distribution across the Eurasian and North American landmasses. The boreal forest (Arctic boreal forest environments) is defined as a belt of forest south of the tundra (Arctic tundra and polar desert ecosystems) characterized by a small number of coniferous species including spruce, larch, pine, and fir and a limited number of broad-leaved species, primarily birch and poplar. At the landscape scale, conifers dominate the boreal forest, although broad-leaved trees can be locally dominant. Forest and woodland in the arctic nations (excluding Denmark), most of which is boreal forest, represent about 1.5 billion hectares (ha) of the total global forest area of 3.8 billion ha (in 2000), or about 38% of global forest area[2] (Table 14.1). Russian forests (excluding woodlands), the vast majority of which are boreal, represent 22.4% of the global total, by far the largest proportion of any nation in the world (Table 14.1). Two of the remaining three countries with the largest percentage of global forest area (Canada and the United States) are also arctic nations.

Boreal forest, Canada (Source: Saikat Basu, own work)

Table 14.1. Total forest area in arctic nations and percentage of global forest area[3] (US Forest Service).

Total forest area (106 ha)

% of global forest area

Russia

851.4

22.4[8]

Canada

44.6

6.4[9]

United States (all)

302.4

8.0[10]

United States (Alaska boreal only)

35

0.9[[[11]]]

Finland

21.9

0.6[12]

Sweden

27.1

0.7[13]

Norway

8.8

0.2[14]

Iceland

0.034

<0.001[15]

Arctic nation total

1456.2

38.3

World total

3800

100

^"forest" category[4]; ^"boreal forest" category[5]

This chapter focuses on the northern portion of the boreal forest, but many aspects of the topic must be considered from a broader perspective. The boreal forest (Arctic boreal forest environments) contains trees growing at the highest latitude on earth, and along its northern margin it merges into the circumpolar tundra. The boreal region is the northernmost part of the world where agricultural crops are produced regularly on a significant scale and where a settled agricultural way of life has historical continuity.

The [[boreal forest (Arctic boreal forest environments)]s] of North America and Eurasia share some plan] and animal species and display a number of other similarities. The boreal forests of Russia and especially Siberia are often referred to as the taiga (East Siberian taiga), an indigenous term meaning "little sticks". The term taiga is equivalent to the true boreal forest.

The boreal forest is both affected by and contributes to climate change: both topics are examined in this chapter. Globally, the existence of large areas of boreal forest cover has a significant effect on the radiative balance of the planet[6]. The rough-textured, dark surface of land covered with boreal forest canopy intercepts and absorbs a high proportion of solar radiation, converting it to heat[7]. In contrast, the smooth, snow-covered surface of the tundra (Arctic tundra and polar desert ecosystems) is highly reflective. In high-latitude regions where snow covers the ground for half of the year or more, the albedo effect of tundra versus boreal forest cover is magnified. Future expansion of the forest into present-day tundra regions resulting from a warming climate would thus amplify the warming further.

Another important influence of the global boreal forest (Arctic boreal forest environments) on climate is its influence on levels of atmospheric (Atmosphere layers) carbon dioxide (CO2) and other greenhouse gases (GHGs). Boreal forests take up CO2 through photosynthesis, and store carbon in live and dead plant matter, including substantial long-term accumulations in large tree boles and in soil (Soil carbon sequestration fact sheet). Forests release CO2 to the atmosphere through decomposition of dead organic matter, live plant and animal respiration, and combustion that takes place during fires. Both natural (e.g., fire) and anthropogenic (Anthropogenic carbon dioxide emissions) (e.g., timber removal) disturbances are important influences on the boreal forest. An ecosystem disturbance is defined as a change in state or condition that disrupts the way in which the system has been functioning (photosynthesis, water regulation, etc.), causing it to reinitiate successional development. Disturbances vary by cause, rate, intensity, extent, timing, frequency, and duration.

Management-related factors influence carbon uptake and storage in the form of tree mass. These management practices include the rearrangement of forest age classes by timber harvest or suppression of wildfires, selection of tree species, fertilization, and thinning regimes. The combined effect of all management actions can either enhance or reduce carbon uptake and storage. For example, across the Russian boreal region, for many years after logging the forests that regrow take up less atmospheric CO2 than nearby old-growth forests[8]. In the Boreal Cordilleran ecozone of Canada (see [9] for definition), it is estimated that total suppression of natural disturbances and their complete replacement by harvesting for maximum sustainable yield would increase carbon storage in soils and wood products over a period of a century or two[10]. Direct climate effects that increase or decrease tree growth in unmanaged natural forests also influence short-term uptake of atmospheric carbon.

The boreal forest (Arctic boreal forest environments) and northern tundra (Arctic tundra and polar desert ecosystems) together contain 40% of global reactive (readily decomposable to CO2, methane, water, and mineral nutrients) soil carbon, an amount similar to the amount of carbon held in the atmosphere (Atmosphere layers)[11]. The extensive boreal forest plains of northeast Europe, western Siberia, and central and eastern North America that are within or immediately south of the discontinuous permafrost (Permafrost in the Arctic) region occupy the zone of maximum carbon storage in soil organic matter on the earth. Climate change, interacting with human use and management of boreal forest, northern agricultural, and tundra [[ecosystem]s], would enhance the decomposition of carbon stored in soil organic matter and its subsequent release into the atmosphere, thus compounding climate change caused by anthropogenic (Anthropogenic carbon dioxide emissions) GHG emissions.

The boreal forest is one of the most intact major vegetation regions of the earth, but in some areas boreal forest has been extensively converted to other land uses or severely damaged by air pollution (e.g., in Iceland and particular areas of Russia, respectively). Boreal forests in Finland, Sweden, Norway, and parts of Canada are generally intensively managed for timber production, and in such intensively managed stands, tree age structure, tree species, and spacing are controlled (Section 14.3.4 (Importance and relationship of boreal forests to climate)). However, huge areas of central and eastern Siberia and northwestern North America represent the most extensive remaining areas of natural forest on the planet[12]. Not all natural boreal forests consist of older trees: large areas are burned or subject to insect-caused tree mortality annually. Climatic factors, especially prolonged periods of warm weather, often create the conditions that result in fire and insect disturbances in boreal forests. The boreal forest is subject to rapid changes causing long-term consequences as a result of these climate-related effects.

The boreal forest is the breeding zone for a huge influx of migratory forest birds that perform many important roles (e.g., insect consumption, seed dispersal) in the boreal region and in other forests of the world during migration and winter residence in the south. Climate affects the population level of these migratory birds and their food resources. Climate-associated processes also determine the amount and quality of forest habitat available to migratory birds.

From 1981 to 1999, three regions of the world primarily affected the terrestrial carbon sink, which takes up and stores atmospheric carbon equivalent to 15 to 30% of annual global emissions from fossil fuel combustion and industrial activities[13]. The majority of the net terrestrial carbon change came from biomass carbon gains in the Eurasian boreal region and North American temperate forests, and carbon losses from some Canadian boreal forests (Terrestrial biome). Some of the terrestrial biomass change was a response to direct and indirect climate effects. However, human use and management of the boreal forest was an important factor as well, and could be a significant future contributor to human management of the carbon cycle. Certain forest biomass carbon sinks can be used to meet national commitments to reduce GHG emissions under the Kyoto Protocol of the Framework Convention on Climate Change. Land and resource managers in the Arctic and boreal regions are interested in potential "carbon cropping", which might involve payments from organizations wishing to sustain or enhance carbon storage[14]. Mechanisms to place values on the various carbon transfers are not fully in place. However, if effective, exchangeable systems of placing values on transfers of carbon are adopted at an international level, boreal forests could potentially generate a flow of wealth into arctic and subarctic regions from other parts of the world for boreal forest and land management treatments, offsets for emissions elsewhere, or policies designed to store or retain carbon.

Uncertainties remain about the influence of the boreal forest (Arctic boreal forest environments) on each of the key processes that determine global carbon balance. For example, the uptake of atmospheric CO2 by tree and other plant growth may either increase or decrease with increasing temperature, depending on the species, the geographic region where the growth occurs, the range of the temperature increase, and other climate factors such as precipitation that are likely to change in a changing climate. However, there has been substantial recent progress in understanding the response of elements of the boreal system to temperature. Across the boreal regions, a first generation of studies, models, databases, and measurements have provided a significantly better understanding of one of the most extensive and important vegetation types on earth. Continued and expanded data collection from research and management activities will provide a reasonable basis for determining the net contribution of the boreal forest to GHG balance and climate, the further changes a warming climate would induce in the boreal forest, and the agricultural]] and forest management opportunities available to the region in the future.

Arctic importance (14.2.2)

Trees occur on only a small proportion of the land surface within the Arctic as defined in this chapter. Even so, forests and woodlands are important on a regional basis within the Arctic for several reasons. Where trees do occur, they serve as indicators of more productive terrestrial ecosystems with longer growing seasons than treeless tundra (Arctic tundra and polar desert ecosystems). Trees, even when present in small numbers on the arctic landscape, offer resources to arctic residents for a variety of uses. Finally, some areas of full-canopy forest within the Arctic are generally the most productive natural systems within the political jurisdiction where they occur (e.g., the galley forests along rivers that extend into the tundra in the northern Yukon and Northwest Territories in Canada). Specific reasons for the importance of boreal forest and agriculture in the Arctic include the following:

  • Portions of the boreal forest devoted to forest products production are major contributors to the national economies of some arctic nations. Although the current zone of optimum climate for boreal forest growth is in the middle or southern boreal region, nearly all scenarios of climate change place the climatically optimum growth region within the present-day Arctic within a century or so.
  • Residents of the boreal region depend on the products and resources of the forest for a variety of ways of life, including traditional ways of life.
  • The major rivers of the boreal region transport large volumes of wood into the Arctic[15], and this wood resource supports ecosystems that decompose the wood and feed organisms in rivers, oceans, and beaches. Climate change is very likely to affect all the processes in this system, including tree growth, erosion, river transport, and wood decay.
  • Wood transported into the Arctic was an important resource for people in a naturally treeless environment during prehistoric times[16] and is still a useful and valued resource for many arctic residents today.
  • The boreal forest collects, modifies, and distributes much of the freshwater that enters the Arctic Basin (see Sections 6.8 and 8.2 (Importance and relationship of boreal forests to climate)), and changes in boreal forests resulting from climate change would certainly affect many of these important functions.
  • Portions of the boreal forest region have experienced some of the greatest temperature increases reported during the 20th century, and the responses of the forest system and the societal consequences in the region provide lessons that may be useful to other regions that could eventually experience similar change.
  • Recent temperature increases in the boreal region have increased the frequency of occurrence of critical temperature thresholds for the production of agricultural crops currently grown in the region. Possible future temperature increases almost certainly would increase the land area on which crops could be produced successfully, and are very likely to increase the variety of agricultural crops that could be grown.

Climatic features (14.2.3)

The boreal region is often assumed to be a zone of homogenous climate, but climate across the region is actually surprisingly diverse. During the long summer days, interior continental locations under persistent high-pressure systems experience hot weather that facilitates extensive forest fires frequently exceeding 100,000 ha. In maritime portions of the boreal region affected by air masses that originate over the North Atlantic, North Pacific, or Arctic Oceans, summer daily maximum [[temperature]s] are on average cooler than interior locations and seldom reach the high temperatures experienced at locations further inland.

Precipitation (Precipitation and evapotranspiration in the Arctic) is abundant in the boreal zone of most of the Nordic countries, western Russia, and certain coastal and mountain regions of western North America. By contrast, in the topographically complex landscapes of Alaska, northwestern Canada, and central and northeastern Siberia, precipitation sometimes limits forest growth so that natural grasslands are part of the landscape.

Precipitation in the boreal region of western North America is influenced by storms in the southern Bering Sea and North Pacific Ocean and reaches a distinct maximum in late summer. In other parts of the boreal forest region, precipitation is more evenly distributed throughout the year or exhibits a winter maximum. East-central Siberia experiences low winter snow depths because the strong Siberian High suppresses precipitation. The boreal landscapes of far eastern Siberia and western North America are mountainous, whereas the topography of most of central and western Siberia and eastern Canada is characterized by low, smooth hills and level terrain. The mountainous boreal regions are characterized by sharply varying local climates[17] and aspect-controlled differences in forest types[18]. As a result, the forest–tundra boundary is much more irregular there than on the plains of the central portions of the continents (Fig. 14.1). All of this regional climatic variation must be taken into account as a fundamental backdrop when considering climate change and ecological response in boreal forests.

Fig. 14.1. Present-day boreal forest distribution, using a simplified formation system representing a gradient of decreasing productivity and species diversity from south to north[19].The area depicted in orange represents a broad zone where the ratio of precipitation to evapotranspiration is nearly one: under a scenario of doubled atmospheric CO2, this area is projected to become too dry to support closed-canopy boreal forest, shifting instead to aspen “parkland”, a woodland formation[20]. Numbered locations are sites analyzed in this chapter (see Method of analysis,Table 14.2).

In Eurasia and North America, both the northern and the southern boundaries of the boreal zone are not aligned at the same latitude east to west. The Icelandic and Aleutian Lows deflect storm tracks and advect relatively mild air masses northward as they approach the western margins of Eurasia and North America, respectively. As a result the boreal forest belt is located considerably farther north in both the Nordic countries and western North America than in the center of the continents (Fig. 14.1). In contrast, cold polar air flowing southward follows a persistent path along the eastern portion of both continents, and consequently the boreal forest belt reaches its southernmost limits there. In the center of the Eurasian landmass, the latitudinal position of the boreal forest depends mainly on the degree of continentality of the climate. The most northerly forests in the world occur along the Lower Khatanga River in central Siberia (72° 32’ N), where the climate is more continental than either western or eastern Siberia. The warmer summer temperature in northernmost central Siberia is the critical factor that allows trees to survive there despite extreme winter cold.

Climate variability (14.2.4)

Fig. 14.2. Opposing high-latitude temperature trends from treering reconstructions of mean annual temperature with a strong signal from eastern and central North America (data adapted from [21]) versus warm-season (Apr–Aug) temperature in western North America (observed and reconstructed from tree rings[22]). Anomalies are calculated from the 1671–1973 mean.

Essentially all of the boreal forest in Alaska is north of 60° N, and practically all the boreal forest of eastern Canada is south of 60° N (Fig. 14.1). The boreal forest region is particularly prone to climatic variability because minor variations in key features of the atmospheric circulation can either intensify the advection of warm air into this naturally cold region, or enhance the distribution of cold air southward through the region. There is some evidence that the climate system in the far north operates in a way that positive (western continent) and negative (eastern continent) anomalies operate in synchrony with each other. It appears that the intensification of meridional air mass movement is especially effective in warming the western margin of the North American Arctic while cooling the eastern margin of the continent (Fig. 14.2), leading to east–west temperature anomalies in the arctic and boreal regions (see also Section 2.6.2.1, e.g., Fig. 2.7 (Importance and relationship of boreal forests to climate), and Section 6.7.2 (Importance and relationship of boreal forests to climate)).

Periods of major climate change, including alternating glacial and interglacial conditions, have repeatedly and drastically affected the northern regions of the planet (Section 2.7 (Importance and relationship of boreal forests to climate)). During the late Pleistocene (Geologic time) and several previous glaciations, the present-day boreal region was mostly covered with glacial ice and forest organisms were largely displaced south of the current limits of the region[23] ([[Section 2.7 (Importance and relationship of boreal forests to climate)]2]). The relatively small unglaciated portion of the present-day boreal region was almost entirely treeless and contained assemblages of species unlike any found today[24].

Present-day boreal forest (Arctic boreal forest environments) vegetation characteristically has a large ecological amplitude (i.e., the ability to survive across a wide range of environmental conditions). The paleoenvironmental record and modern instrumental measurements demonstrate major shifts in temperature regimes in this region even during the time that forest has been present (Sections 2.7 and 14.6 (Importance and relationship of boreal forests to climate)), and the presence of this large ecological amplitude in the trees indicates that this wide range of possible temperatures is a consistent enough feature of the environment that it has required an adaptive response. Rapid and large changes in weather over the short term send signals through boreal systems that initiate vital processes such as infrequent periodic tree reproduction[25]. On the one hand, this high level of natural climate variability suggests that during periods of climate change, the effects are more likely to be detectable at an earlier time in the boreal region than in many other parts of the earth. On the other hand, a long and persistent history of climate variability in the region suggests that organisms in the boreal forests of today may be among those better adapted to climate change because they have been filtered by many climate fluctuations in the past.

Because [[ecosystem]s] quite unlike those of today have existed in the region in the past[26], major climate change in the future is likely to produce ecosystems unknown today. The emergence of novel ecosystems (from the human perspective) is partly the result of individualistic species responses to changes in the environment. Each species has its own environmental requirements, tolerances, and thresholds, so that some species that co-occur today may not in the future, or existing sets of species may be joined by additional species (Sections 7.3 and 7.6 (Importance and relationship of boreal forests to climate)). Because of this property of individualistic responses to change in the environment, conservation efforts must be informed by monitoring the status of key species on a continuing basis. Conservation measures, such as modified harvest limits or fire management, must account for rapidly changing environmental conditions or changes in species [[population]s] not anticipated in management planning assumptions or outside historical experience in order to meet a goal of sustainability (Chapter 11 (Importance and relationship of boreal forests to climate)).

The simultaneous pattern of temperature and precipitation anomalies can have important ecological impacts in forests of the north. Wildfire in the boreal forest is the product of short- to medium-term warm and dry conditions, usually associated with high-pressure dominance during the long days of summer[27]. Alternating periods of warm and dry versus cool and moist summer climate in central Alaska regulate the growth and reproduction of white spruce (Picea glauca), ultimately providing a mechanism to synchronize the production of seed crops to periods immediately following major forest fires[28]. If future climate change alters not just the mean of climate parameters, but also the pattern of alternating warm/dry and cool/moist conditions, the resulting climate pattern could interfere with the reproductive success of one of the most widely distributed and dominant North American conifers. This example indicates the potential for subtle influences to be major factors in climate change effects.

Unique influences on climate (14.2.5)

The Boreal Ecosystem–Atmosphere Study (BOREAS) was a large-scale, international interdisciplinary experiment in the northern boreal forests of Canada that began in 1993. Its goal was to understand how boreal forests interact with the atmosphere, how much CO2 they were capable of storing, and how climate change will affect them.

Albedo measurements from BOREAS are among the lowest ever measured over vegetated regions, and indicate that the boreal forest (especially forest dominated by black spruce – Picea mariana) absorbs nearly 91% of incident solar radiation[29]. In terms of water and energy balance, BOREAS found that the boreal ecosystem often behaves like an arid landscape, particularly early in the growing season. Even though the moss layer is moist for most of the summer, nutrient-poor soils and limiting climatic conditions result in low photosynthetic rates, leading to low evapotranspiration (Precipitation and evapotranspiration in the Arctic). As a result, relatively little of the available moisture is transferred to the atmosphere. Much of the precipitation penetrates through the moss layer into the soils, which are permeable, then encounters the underlying semi-impermeable layer and runs off. Most of the incoming solar radiation is intercepted by the vegetation canopy, which exerts strong control over transpiration water losses, rather than by the moist underlying moss/soil surface. As a result, much of the available surface energy is dissipated as sensible heat.

The BOREAS experiment also found that coniferous vegetation in particular follows a very conservative water-use strategy. Stomatal closure drastically reduces transpiration when the foliage is exposed to dry air, even if soil moisture is freely available. This feedback mechanism acts to keep the surface evapotranspiration rate at a steady and surprisingly low level (less than 2 millimeters/day [mm/d] over the season). The low evapotranspiration rates coupled with high available energy during the growing season can lead to high sensible heat fluxes and the development of deep (3,000 meter [m]) planetary boundary layers, particularly during the spring and early summer. These planetary boundary layers are often characterized by intense mechanical and sensible heat-driven turbulence.

Chapter 14: Forests, Land Management, and Agriculture (Importance and relationship of boreal forests to climate)
14.1. Introduction (Importance and relationship of boreal forests to climate)
14.2. The boreal forest: importance and relationship to climate
14.3. Land tenure and management in the boreal region
14.4. Use and evaluation of the ACIA scenarios
14.5. Agriculture (Agriculture in the Arctic)
14.6. Tree rings and past climate
14.7. Direct climate effects on tree growth
14.8. Climate change and insects as a forest disturbance
14.9. Climate change and fire
14.10. Climate change in relation to carbon uptake and carbon storage
14.11. Climate change and forest distribution
14.12. Effects of ultraviolet-B on forest vegetation
14.13. Critical research needs

Image by Peter Wilton (Wikimedia Commons)

References

Citation

Committee, I. (2014). Importance and relationship of boreal forests to climate. Retrieved from http://editors.eol.org/eoearth/wiki/Importance_and_relationship_of_boreal_forests_to_climate
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  23. ↑ Wright, H.E. Jr., 1983. Introduction. In: H.E. Wright Jr. (ed.). Late-Quaternary Environments of the United States,Vol. 2,The Holocene, pp. xi-xvii. University of Minnesota Press.–Wright, H.E. Jr. and C. W. Barnosky, 1984. Introduction to the English Edition. In: A. A.Velichko (ed.). Late-Quaternary Environments of the Soviet Union, pp. xiii-xxii. University of Minnesota Press. 
  24. ↑ Anderson, P.M. and L.B. Brubaker, 1994.Vegetation history of Northcentral Alaska: a mapped summary of late-quaternary pollen data. Quaternary Science Reviews, 13:71–92.
  25. ↑ Juday, G.P.,V. Barber, S. Rupp, J. Zasada and M. W. Wilmking, 2003. A 200-year perspective of climate variability and the response of white spruce in Interior Alaska. In: D. Greenland, D. Goodin and R. Smith (eds.). Climate Variability and Ecosystem Response at Long-Term Ecological Research (LTER) Sites, pp. 226–250. Oxford University Press.
  26. ↑ Anderson, P.M. and L.B. Brubaker, 1994.Vegetation history of Northcentral Alaska: a mapped summary of late-quaternary pollen data. Quaternary Science Reviews, 13:71–92.–Wright, H.E. Jr., 1983. Introduction. In: H.E. Wright Jr. (ed.). Late-Quaternary Environments of the United States,Vol. 2,The Holocene, pp. xi-xvii. University of Minnesota Press.–Wright, H.E. Jr. and C. W. Barnosky, 1984. Introduction to the English Edition. In: A. A.Velichko (ed.). Late-Quaternary Environments of the Soviet Union, pp. xiii-xxii. University of Minnesota Press. 
  27. ↑ Johnson, E. A., 1992. Fire and Vegetation Dynamics: Studies from the North American Boreal Forest. Cambridge University Press, 129p.
  28. ↑ Barber,V., G. Juday and B. Finney, 2000. Reduced growth of Alaska white spruce in the twentieth century from temperature-induced drought stress. Nature, 405:668–672.–Juday, G.P.,V. Barber, S. Rupp, J. Zasada and M. W. Wilmking, 2003. A 200-year perspective of climate variability and the response of white spruce in Interior Alaska. In: D. Greenland, D. Goodin and R. Smith (eds.). Climate Variability and Ecosystem Response at Long-Term Ecological Research (LTER) Sites, pp. 226–250. Oxford University Press. 
  29. ↑ Hall, F.G., P.J. Sellers and D.L. Williams, 1996. Initial results from the boreal ecosystem-atmosphere experiment, BOREAS. Silva Fennica, 30(2–3):109–121.
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