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Kirschbaum, M. & Fischlin, A., 1996. Climate change impacts on forests.
In: Watson, R., Zinyowera, M.C. & Moss, R.H. (eds.), Climate change
1995 - Impacts, adaptations and mitigation of climate change:
scientific-technical analysis. Contribution of Working Group II to
the Second Assessment Report of the Intergovernmental Panel of Climate
Change (IPCC). Cambridge University Press, Cambridge a.o., pp. 95-129.
Executive Summary:
Forests are highly sensitive to climate change. This has been shown by observations from the past, experimental studies and simulation models based on current ecophysiological and ecological understanding. In particular, the following was concluded (for levels of confidence: (L) - Low; (M) - Medium; (H) - High. See this volume, Summary for Policy Makers):
• Sustained increases of as little as 1°C in mean annual air temperature can be sufficient to cause changes in the growth and regeneration capacity of many tree species. In several regions, this can significantly alter the function and composition of forests; in others it can force forest cover to disappear completely. (M)
• Suitable habitats for many species or forest types are likely to shift faster with climate change than the maximum natural rate at which many species can migrate and establish. Consequently, slow-growing species, such as late successional species, or those with restricted seed dispersal, will be replaced by faster growing, highly adaptable or more mobile species. (H)
• Forests are particularly vulnerable to extremes of water availability (either drought or water-logging) and will decline rapidly if conditions move towards one of the extremes. (H)
• Forced by a 2xCO2 climate, global models project that a substantial fraction of the existing forests will experience climatic conditions under which they do not currently exist; eventually, large forested areas will have to change from the current to new major vegetation types (H). Averaged over all zones, the models predict that 33% of the currently forested area could be affected by such changes; in the boreal zone, one model projects it to be as high as 65% (M). Yet, it is currently not possible to predict transient forest responses at a regional to global scale (H).
• Although net primary productivity may increase, the standing biomass of forests may not increase because of a) more frequent outbreaks and extended ranges of pests and pathogens, and b) increasing frequency and intensity of fires. (M).
• Mature forests are a large store of terrestrial carbon. Because the maximum rate at which carbon can be lost is greater than the rate at which it can be gained, large amounts of carbon may transiently be released into the atmosphere as forests change in response to a changing climate and before new forests replace the former vegetation. The loss of above-ground carbon alone has been estimated to be 0.1 - 3.4 Pg yr-1 or a total of 10 - 240 Pg. (M)
The following regional assessments were primarily based on climatic change scenarios for 2050 as provided by Greco et al. (1994):
Tropical forests:
• Tropical forests are likely to be more affected by changes in land use than by climatic change as long as deforestation continues at its current high rate. (H)
• Any degradation of tropical forests will lead to an irreversible loss in biodiversity, whether it be caused by climatic or land-use changes (H).
• CO2-fertilization may have its greatest effect in the tropics and may lead to a gain in net carbon storage in undisturbed forests, especially in the absence of nutrient limitations. (M)
• Tropical forests are likely to be more affected by changes in soil water availability (caused by the combined effects of changes in temperature and rainfall) rather than temperature per se, which may accelerate forest loss in many areas where water availability is currently already marginal. In other areas, increasing precipitation may be more than adequate to meet increased evaporative demand, or may even lead to erosion. (M)
Temperate forests:
• Compared with other latitudinal zones, the potential area for temperate forests is projected to change the least, but many existing forests will still undergo significant changes in their species composition. (H)
• Water availability will change in many regions, and in some regions where water supply is already marginal, forests may be lost in response to increased summer droughts. (M)
• While warming and elevated CO2 are likely to increase net primary productivity of many forests, net carbon storage may not increase because of the associated stimulation of soil organic matter decomposition by soil warming. (M)
• Temperate forests are currently a carbon sink, mainly because of regrowth started in many regions from the 19th century. However, these forests could become a source if they degrade due to climatic change or other causes such as air pollution. (M)
• Most temperate forests are located in developed countries with resources to reduce the impacts of climatic change on their forests through integrated fire, pest and disease management and/or encouraging reforestations. (M)
Boreal forests:
• As warming is expected to be particularly large at high latitudes, and as boreal forests are more strongly affected by temperature than forests in other latitudinal zones, climatic change is likely to have its greatest impact on boreal forests. (H)
• Northern treelines are likely to advance slowly into regions currently occupied by tundra. (H)
• Increased fire frequency and pest outbreaks are likely to decrease the average age, biomass and carbon store, with greatest impact at the southern boundary, where the boreal coniferous forest is likely to give way to temperate zone pioneer species or grasslands. (M)
• The net primary productivity of forests not limited by water availability is likely to increase in response to warming, partly mediated by increased nitrogen mineralization. However, there may be a net loss of carbon from the ecosystem because of associated increases in soil organic matter decomposition. (M)
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