WATER LIMITATIONS AND PLANT COMMUNITY-DEVELOPMENT IN A POLAR DESERT

Polar deserts of the High Arctic contain vast areas of minimal plant cover and low primary productivity. Significant development of polar desert plant communities is largely restricted to areas with considerable cover of a cryptogamic soil crust, which develops in sites exposed to continued surface runoff from melting snow for some of the short growing season. Thus, soil drought and plant water stress have often been assumed to be major constraints to plant community development in polar deserts. To examine this issue, water availability and plant water relations of common herbaceous perennial plants were studied over three growing seasons in a typically barren (''noncrusted'') site and a site with a well-developed cryptogamic crust and vascular plant community (''crusted''). Soil water content was consistently higher in the crusted 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 precipitation that varied nearly twofold from year to year. However, the surface soils (0.5-2.0 cm in depth), especially in noncrusted sites, dry considerably in some years. There were no significant differences in plant water potential and midday values of whole-plant transpiration and water vapor conductance for adult plants growing in crusted vs. noncrusted sites. Water stress for established plants was minor in both sites. Greater plant community development in crusted areas of this polar desert does not result from a reduction in plant water stress by a greater supply of meltwater through the growing season. instead, surface meltwaters probably benefit vascular plants indirectly by facilitating the growth, development, and nitrogen fixation activities of cryptogamic organisms in the soil crust. The presence and activity of these cryptogams favor vascular plant success through increased nutrient availability, soil organic matter, surface temperatures, reduced soil cryoturbation, and more favorable sites for germination and seedling establishment.