干旱区生态环境知识资源中心

Patchy wetlands are commonly found in the polar desert at localities where ample water supply is available during the thawed season. These wetlands can occur on a wide range of topography, and they support more luxuriant vegetation than their surrounding areas. Snowbank wetlands are characterized by a zonation of vegetation, with increasingly rich plant coverage away from the snowbed edge. Ground-water wetlands usually occupy topographic concavities or depressions, with exfiltration of subsurface water to sustain a high water table and to provide an enrichment of nutrients. Riverine wetlands experience prolonged flooding during the peak snowmelt season, and their vascular plant cover is low owing to inundation by the cold, turbulent stream water. Lakes and ponds are associated with topographic depressions, receiving surface- and ground-water inputs from catchment areas larger than the surface extent of the water bodies. Their evaporation rates are high, but the open water season is curtailed by ice cover. Lake outflow wetlands are supported by spillage and seepage from lakes, and they maintain a relatively lush vegetation cover. Coastal wetlands form narrow strips enclosed by raised beaches, with high salinity that limits plant diversity. Strong interactions exist among the water and heat fluxes to the wetland, the vegetation, and the hydrogeomorphology. Locally enhanced water supply and shallow permafrost enable frequent saturation, augmenting nutrient enrichment and encouraging vegetation development. Slow vegetation decay in a cold environment facilitates the formation of peat. The saturated soil conditions give rise to large ground-ice content in the winter, requiring much latent heat for ground thaw in subsequent summers. This, together with the insulating properties of the organic layer, creates shallow frost tables during the summer and produces a positive feedback that maintains high water tables in the wetlands to favor plant growth.