No abstract

JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org. This content downloaded from 128.235.251.160 on TueAbstract. 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.

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Precise control and modulation of fluorescent sunlamps are necessary for ecologically valid simulation of solar UV‐B (280‐320 nm) radiation enhancement which would correspond to ozone reduction. A control system is described which allows lamp emittance to be modulated over a large dynamic range (50:1), permits stable lamp operation and starting at low temperatures, and provides a sensitive feedback loop to compensate for both atmospheric conditions, such as cloud cover, as well as changes in radiant emittance from the lamps resulting from factors such as temperature and lamp age.

Evidence regarding the interaction of ultraviolet‐B (UV‐B, 280‐320 nm) radiation and plant competition in terrestrial ecosystems is examined. The competitive interactions of some species pairs were affected even by ambient solar UV‐B radiation (as exists without ozone depletion), when compared to control pairs grown without UV‐B. Also, the total shoot biomass of these species pairs was depressed under ambient UV‐B. Relatively large increases in UV‐B radiation (approximating a 40% ozone layer reduction when weighted with the generalized plant action spectrum) altered the competitive interactions of some species pairs grown in pots under field conditions, but did not affect the total shoot biomass production of those pairs. Recent field experiments have examined the competitive interactions of wheat (Triticum aestivum L. cv. Bannock) and wild oat (Avena fatua L.) under a simulated increased UV‐B regime resulting from a 16% ozone layer reduction when weighted with the generalized plant action spectrum. This increase in UV‐B altered the competitive interactions of these two species without affecting the total shoot biomass production of the species pair. The manner in which increased UV‐B affected the relative competitive abilities of the two species was highly dependent upon the environmental conditions during the early life stages of the plants. The implications of these results for both agricultural and natural plant communities are discussed.