Forests, which are characterized by their composition of tree species, are among the important ecosystems of the environment. They are greatly affected by climate, soil, fire, industry, tourism, and pipeline/railway/road construction. Consequently, the expected impact of global warming on forests would vary from region to region.
In Temperate Asia, temperature seems to be the primary environmental parameter affecting the distribution of forests because sufficient precipitation exists for almost all forest types. Exceptions are in regions with heavy snow accumulation, windy mountain crests and slopes, or windy coastal zones.
In particular, the amount of snow accumulation seems to be an important parameter for forest growth. Accumulated snow protects trees from severe cold, but its weight can damage the branches and trunks. In Japan, large differences in snow accumulation between the Japan Sea side and the Pacific side are strongly reflected in the corresponding forest types.
The amount of snowfall may decrease as a result of global warming, resulting in spring and summer drought. Uncertainty remains, however, because changes in snowfall amounts depend on changes in the frequency of snow and freezing precipitation in the winter in the mountain regions; the wind speed and the difference between the air temperature and the sea-surface temperature over the Japan Sea in winter, which determine the amount of snow precipitation over mountain regions on the Japan Sea side; the ratio of snowfall to rainfall in winter; the dates of onset and termination of snowfall and the maximum depth of snow accumulation during winter; and the density of accumulated snow.
Almost all forest trees would grow better under a warmer climate, but increases in diseases, insect damages, and other meteorological hazards (e.g., severe storm damage) also may result in a shortened life for forest trees (Tsunekawa et al., 1996a). When the original species in a forest decline, other species from neighboring forest zones more suitable for warmer conditions would grow. If there were an afforested region or broad cultivated area between natural forests, such changes would not occur. The impact of warming would occur noticeably on a time scale of decades to centuries, in areas where the distribution of plant communities is continuous.
Forests of the Hokuriku region on the Japan Sea side of central Japan are likely to respond to climatic change (Kojima, 1996). Of the four vegetation belts, the Pinus pumila zone-the highest belt in elevation-would be most seriously affected. It may remain only in small areas at higher elevations or even become fragmented by the upward advance of plant species adapted to a warmer climate. Uncertainties from the viewpoint of vegetation are the rate of geographical expansion of plant species, the time lag between climate change and soil change, and phenological (seasonal) and environmental adaptation of plant species.
Catastrophic scenarios include water deficit as a result of decreasing precipitation and/or increasing evapotranspiration due to warming; elevated minimum temperatures, resulting in the absence of low-temperature stimulation needed for germination, flowering, and fruition; and discord in the seasonal rhythm of air temperature and daytime length, which controls the phenology of plant growth for plants propagated to the north.
According to a study on potential vegetation distribution shifts in China (Tsunekawa et al., 1996b), the northward shift rate of the deciduous genus Queras is projected to be 7 km/year-14-24 times faster than the migration rate recorded over the past 13,000 years in Europe. Difficulty in adaptation would be expected for such a rapid shift.
Forest and grassland fire is expected to occur more frequently and in broader areas in northern parts of Temperate Asia as a result of global warming and deforestation activities. In May 1987, one of the largest forest fires of the past 30 years occurred in China and Russia. In the Daxinganlin mountain range of northeast China, it was reported that the total area burned was 1,010 x 103 ha, at 729 sites. The most serious wildfire in the forests and grasslands in Mongolia occurred in March 1996. The burned-out area was estimated to be about 8,000 x 103 ha. Although the occurrence of forest fires varies greatly each year, such serious cases should be taken into consideration in global warming impact studies.
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