This is Chapter 14 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.
The boreal region covers about 17% of global land area, and the arctic nations together contain about 31% of the global forest (non-boreal and boreal). The boreal forest is affected by and also contributes to climate change through its influence on the carbon cycle and albedo. Boreal forests influence global levels of atmospheric carbon dioxide and other greenhouse gases by taking up carbon dioxide in growth, storing carbon in live and dead plant matter, and releasing carbon through decomposition of dead organic matter, live plant and animal respiration, and combustion during fire. Human management influences on carbon uptake and storage include the rearrangement of forest age classes through timber harvest or wildfire suppression, selection of tree species, fertilization, and thinning regimes. The combined effect of all management actions can either enhance or reduce carbon uptake and storage.
Agriculture has existed in the Arctic as defined in this chapter for well over a millennium, and today consists of a mixture of commercial agriculture on several thousand farms and widespread subsistence agriculture. Potatoes and forage are characteristic crops of the cooler areas, and grains and oilseed crops are restricted to areas with the warmest growing seasons. The main livestock are dairy cattle and sheep, which have been declining, and diversified livestock such as bison or other native animals, which have generally been increasing in commercial operations. The five ACIA-designated models all project rising temperatures that are very likely to enable crop production to advance northward throughout the century, with some crops now suitable only for the warmer parts of the boreal region becoming suitable as far north as the Arctic Circle. The average annual yield of farms is likely to increase at the lower levels of warming due to climate suitability for higher-yielding crop varieties and lower probabilities of low temperatures limiting growth. However, in the warmest areas, increased heat units during the growing season are very likely to cause a slight decrease in yields since warmer temperatures can speed crop development and thereby reduce the amount of time organic matter accumulates. Under the ACIA-designated model projections, water deficits are very likely to increase or appear in most of the boreal region. By the end of the 21st century, unless irrigation is practiced, water stress is very likely to reduce crop yields. Water limitation is very likely to become more important than temperature limitations for many crops in much of the region. Overall, negative effects are unlikely to be stronger than positive effects. Lack of infrastructure is likely to remain a major limiting factor for commercial agricultural development in the boreal region in the near future. Even under model-projected levels of climate change, government policies regarding agriculture and trade will still have a very large, and perhaps decisive, influence on the occurrence and rate of agricultural development in the north.
Understanding the condition or character of the forest resource system that climate change affects is crucial in assessing forests and land management. Russia has made commitments to management of carbon stocks that are of global interest because of the amounts involved. Fire and insect disturbance at very large scales have generated resource management challenges in Canada. A large proportion of Alaska is managed as strict nature reserves and as resource lands for biodiversity and ecosystem services. Large forest disturbances associated with climate change have occurred in Alaska, disrupting ecosystems and imposing direct costs, but the large area of reserves improves the ultimate prospects of species surviving potential future climate change. In highly managed forests of Finland, Sweden, and Norway, forests are generally managed effectively and are increasing in volume, but the prospect of climate change puts at risk human expectations of specific future resource returns. In Iceland, temperature increases have improved tree growth at a time of a large afforestation program designed to increase forest land cover and sequester carbon.
About 6000 years BP (the end of the postglacial thermal maximum), radial growth of larch trees on the Taymir Peninsula of Russia surpassed the average of the last two millennia by 1.5 to 1.6 times.Tree growth and warm season temperature have irregularly decreased in northernmost Eurasia and North America from the end of the postglacial thermal maximum through the end of the 20th century. Long-term tree-ring chronologies from Russia, Scandinavia, and North America record the widespread occurrence of a Medieval Warm Period about 1000 years BP, a colder Little Ice Age ending about 150 years ago, and more recent warming. Recent decades were the warmest in a millennium or more at some locations.Temperature and tree growth records generally change at the same time and in the same direction across much of the Arctic and subarctic. However, intensified air-mass circulation associated with a warmer climate has introduced a stronger flow of warm air into specific regions of the Arctic and enhanced the return flow of cold air out of the Arctic in other regions. Temperature and tree-growth trends are correlated but opposite in sign in these contrasting regions.
Between 9000 and 7000 years BP, trees occurred in at least small groups in what is now treeless tundra nearly to the arctic coastline throughout northern Russia. Around 6000 years BP, the northern treeline on the Taymir Peninsula (currently the farthest north in the world) was at least 150 km further north than at present. During the period of maximum forest advance, mean July temperature in northern Russia is estimated to have been 2.5 to 7.0 ºC higher than the modern mean.This record of past forest advance suggests that there is a solid basis for projecting similar treeline change under climate change producing similar temperature increases. It also suggests that the components of ecosystems present today have the capacity to respond and adjust to such climate fluctuations. The greatest retreat of forest and expansion of tundra occurred between 4000 and 3000 years BP. In northeast Canada, the black spruce forest limit has remained stable for the past 2000 to 3000 years. In recent decades, milder winters have permitted stems that were restricted to snow height by cold and snow abrasion to emerge in upright form, and future climate projected by the ACIA-designated models would permit viable seed production, which is likely to result in infilling of the patchy forest–tundra border and possibly begin seed rain onto the tundra. In the Polar Ural Mountains, larch reproduction is associated with warm weather, and newly established trees have measurably expanded forest cover during the 20th century, although there is a time lag between climate warming and upslope treeline movement.
Across the boreal forest, warmer temperatures in the last several decades have either improved or decreased tree growth, depending on species, site type, and region. Some tree-growth declines are large in magnitude and have been detected at different points across a wide area, although the total extent has not been delineated. Temperature-induced drought stress has been identified as the cause of reduced growth in some areas, but other declines are not currently explained. Reduced growth in years with high temperatures is common in treeline white spruce in western North America, suggesting reduced potential for treeline movement under a warming climate.Tree growth is increasing in some locations, generally where moisture and nutrients are not limiting, such as in the boreal regions of Europe and eastern North America. The five ACIA-designated models project climates that empirical relationships suggest are very unlikely to allow the growth of commercially valuable white spruce types and widespread black spruce types in major parts of Alaska and probably western boreal Canada.The models project climates that are very likely to increase forest growth significantly on the Taymir Peninsula. The upper range of the model projections represents climates that may cross ecological thresholds, and it is possible that novel ecosystems could result, as during major periods of global climate change in the past.
Large-scale forest fires and outbreaks of tree-killing insects are characteristic of the boreal forest, are triggered by warm weather, and promote many important ecological processes. On a global basis, atmospheric carbon equal to 15 to 30% of annual emissions from fossil fuels and industrial activities is taken up annually and stored in the terrestrial carbon sink. Between 1981 and 1999, it is estimated that the three major factors affecting the terrestrial carbon sink were biomass carbon gains in the Eurasian boreal region and North American temperate forests, and losses in areas of the Canadian boreal forest. Particular characteristics of forest disturbance by fire and insects, such as rate, timing, and pattern of disturbance, are crucial factors in determining the net uptake or release of carbon by forests. The evidence necessary to establish a specific climate change effect on disturbance includes a greater frequency of fire or insect outbreaks, more extensive areas of tree mortality, and more intense disturbance resulting in higher average levels of tree death or severity of burning. Some elements of the record of recent boreal forest disturbance are consistent with this profile of climate change influence, especially forest fires in some parts of Russia, Canada, and Alaska and insect disturbances in North America.
Carbon uptake and release at the stand level in boreal forests is strongly influenced by the interaction of nitrogen, water, and temperature influences on forest litter quality and decomposition. Warmer forest-soil temperatures that occur following the death of a forest canopy due to disturbance increase the rate of organic litter breakdown, and thus the release of elements for new plant growth (carbon uptake). The most likely mechanism for significant short-term change in boreal carbon cycling as a result of climate change is the control of species composition caused by disturbance regimes. Successional outcomes from disturbance have different effects on carbon cycling especially because of the higher level and availability of nutrient elements (and thus decomposition) in organic litter from broadleaf trees compared to conifers. Net global land-use and landcover change, especially aggregate increases or decreases in the area of forest land, may be the most important factor influencing the terrestrial sink of carbon.When water and nitrogen remain available at the higher growth rates typical of enhanced carbon dioxide environments, further carbon uptake occurs. Broadleaf litter produced under elevated carbon dioxide conditions is lower in quality (less easily decomposed) than regular litter because of lower nitrogen concentration, but quality of conifer litter in elevated carbon dioxide environments may not be as affected.
Different crop species and even varieties of the same species can exhibit substantial variability in sensitivity to ultraviolet-B (UV-B) radiation. In susceptible plants, UV-B radiation causes gross disruption of photosynthesis, and may inhibit plant cell division. Determining the magnitude of the effect of elevated UV-B radiation levels is difficult, because interactions with other environmental factors, such as temperature and water supply, affect crop reactions and overall growth. Damage by UV-B radiation is likely to accumulate over the years in trees. Evergreens receive a uniquely high UV radiation dose in the late winter, early spring, and at the beginning of the growing season because they retain vulnerable leaf structures during this period of maximum seasonal UV-B radiation exposure, which is amplified by reflectance from snow cover. Exposure to enhanced levels of UV-B radiation induces changes in the anatomy of needles on mature Scots pine similar to characteristics that enhance drought resistance. UV-B radiation plays an important role in the formation of secondary chemicals in birch trees at higher latitudes. Secondary plant chemicals released by birch exposed to increased UV-B radiation levels might stimulate its herbivore resistance.
Chapter 14: Forests, Land Management, and Agriculture
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
14.6. 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
- ^ AMAP, 1997. Arctic Pollution Issues: A State of the Environment Report. Arctic Monitoring and Assessment Programme, Oslo, 188p.