Because of the likely shifts of phytoclimatic boundaries in a warmer world (based on GCM estimates for Europe of 3±1.5°C in the next century), there will be changes in the distributions of Mediterranean and boreal grassland species. Changes in individual species will depend on the nature of the climate change in a given area. Because the ranges of many species are determined primarily by soil moisture patterns, changes in the absolute amounts of precipitation and the seasonal distributions of precipitation will be important in determining vegetation responses. Some grassland species also may respond to increasing CO2 concentrations-with the greatest responses among species characteristic of productive, undisturbed habitats and the lowest responses among species adapted to high environmental stress or rates of physical disturbance (Hunt et al., 1991, 1993). Experiments with alpine grassland species suggest, however, that any effects will be apparent only if CO2 increases are accompanied by climatic warming or enhanced nitrogen supply; increases in CO2 concentrations alone have little effect (Schäppi and Körner, 1996).
In Mediterranean countries, displacement of grass and dwarf shrub steppes will occur at the expense of existing sclerophyllous shrubland. Extension of shrubland as a result of agricultural release is expected-with a parallel rise in wildfire episodes, loss of water through enhanced evapotranspiration, and a decrease in livestock grazing (with concomitant increases in game and other wildlife). Furthermore, shifts in carbon storage from soil to biomass are likely to occur.
In northern Europe, there is likely to be a reduction in the number and extent of mires and tundra and permafrost areas as forests expand into the tundra zone. In addition, changes in the concentration of atmospheric gases may alter competitive relationships in the plant community. A series of complex responses can be expected because of the interaction of different environmental factors. For example, an increase in temperature is likely to result in an increase in nutrient availability because of the greater mineralization of soil organic matter by soil microorganisms (Anderson, 1991; Bonan and Van Cleve, 1992). As with forests, the responses will be species-specific (Baxter et al., 1994; Parsons et al., 1994, 1995).
Most of the major noncoastal wetland areas in Europe are confined to northern Scotland, Fennoscandia, and northern Russia (Hartig et al., 1997). A changing climate is likely to have a significant impact on peat formation and ecological function in such regions. An increase in temperature by 1-2°C accompanied by decreases in soil moisture would lead to an estimated 25% reduction in peat formation.
Tundra peatlands will be extremely vulnerable because higher temperatures will result in thawing of the permafrost layer in areas with discontinuous permafrost, as well as an enlargement of the active layer in continuous permafrost. This shift will have considerable implications because permafrost is a key factor in maintaining high water levels in these systems. Further, it is unlikely that new permafrost areas will develop. As a result, hydrology and landscape patterns will be affected, leading to lowered water tables in some areas and the overflow of flooded thaw lakes in others. Such melting may shift bogs on permafrost back to fens, from which they originated after the warm mid-Holocene period (5000-6000 years BP).
A rapid rate of climatic change-implied by climate model simulations for northern latitudes-may cause degradation of the southern boundaries of wetlands and peatlands much faster than the northward expansion potential of their northern boundaries. Such an imbalance between biomass loss on the one hand and biomass increase on the other is likely to have implications for the carbon cycle, whereby the wetlands could undergo a reversal from sinks to sources of carbon. There also would be consequences for methane fluxes in these regions. A change in the total methane flux from northern wetlands can be expected if the areal extent of wetlands changes, the duration of the biologically active period is modified, or the production or oxidation of methane per unit area changes. Because of uncertainties in the changes in water regimes as projected by climate models, however, it is difficult to obtain reliable quantitative estimates for shifts in methane fluxes to the atmosphere.
Many peatlands may be subject to increasing pressure from afforestation operations as a result of changes in land capability classifications (Proe et al., 1996). At the same time, degradation of peatlands is likely to increase the conservation value of the remaining intact areas, thereby creating the potential for more intense land-use conflicts.
Water draining from peatlands also is likely to be sensitive to climatic change, particularly in the form of summer droughts. Such droughts could increase autotrophy in the streams, leading to increases in the biofilm biomasses present in the water. Changes in nutrient content also may occur, with increases in inorganic nutrients and decreases in organic nutrients (Freeman et al., 1994).
Studies indicate that wetlands in semi-arid regions of southern Europe can be very sensitive to climate warming; such warming has severe effects on their hydrological and ecological functions. They also are likely to be adversely affected by increased water demands. Biological reserves such as the Cota Donana in Spain are likely to come under increasing pressure.
Other reports in this collection