WATER LIMITATIONS AND PLANT COMMUNITY-DEVELOPMENT IN A POLAR DESERT

Polar deserts of the High Arctic contain vast areas of minimal plant c over and low primary productivity. Significant development of polar de sert plant communities is largely restricted to areas with considerabl e cover of a cryptogamic soil crust, which develops in sites exposed t o continued surface runoff from melting snow for some of the short gro wing season. Thus, soil drought and plant water stress have often been assumed to be major constraints to plant community development in pol ar deserts. To examine this issue, water availability and plant water relations of common herbaceous perennial plants were studied over thre e growing seasons in a typically barren (''noncrusted'') site and a si te with a well-developed cryptogamic crust and vascular plant communit y (''crusted''). Soil water content was consistently higher in the cru sted site than the noncrusted site through all growing seasons. These differences had limited biological relevance because subsurface soils at both sites remained effectively saturated (soil water potential > - 0.1 MPa) through each growing season, despite low amounts of precipita tion that varied nearly twofold from year to year. However, the surfac e soils (0.5-2.0 cm in depth), especially in noncrusted sites, dry con siderably in some years. There were no significant differences in plan t water potential and midday values of whole-plant transpiration and w ater vapor conductance for adult plants growing in crusted vs. noncrus ted sites. Water stress for established plants was minor in both sites . Greater plant community development in crusted areas of this polar d esert does not result from a reduction in plant water stress by a grea ter supply of meltwater through the growing season. instead, surface m eltwaters probably benefit vascular plants indirectly by facilitating the growth, development, and nitrogen fixation activities of cryptogam ic organisms in the soil crust. The presence and activity of these cry ptogams favor vascular plant success through increased nutrient availa bility, soil organic matter, surface temperatures, reduced soil cryotu rbation, and more favorable sites for germination and seedling establi shment.