Vascular plant reproduction, establishment, and growth and the effects of cryptogamic crusts within a polar desert ecosystem, Devon Island, N.W.T., Canada

Abstract

Most of the ice-free lands within the Canadian High Arctic are classified as polar desert (44%) or semidesert (49%). Much of this desert landscape supports no more than 6-10 vascular species that provide 1-3% cover and cryptogamic organisms that occupy 0-5% cover on the soil surface. The barrenness of these lands results from a short growing season and low summer temperatures that limit plant growth and the production of viable seeds. Limited areas have a 50-80% cover of cryptogamic crust and an 8-12% cover of vascular plants. These are areas in which surfaces remain moist for considerable periods in summer from snowflush waters. Where such cryptogamic soil crusts develop, they play a central role in soil development and nitrogen fixation. The two major components of this study were (i) an examination of fundamental reproductive, establishment, and growth characteristics of polar desert plants in the field and (ii) the relationship of these characteristics to environmental conditions in areas with and without cryptogamic crusts. Summer conditions during the study ranged from unusually warm (1991; 252 degree-days) to unusually cool (1992; 123 degree-days) with two average years (1994 and 1995; 172 and 166 degree-days, respectively). Differences in reproduction and establishment among these summers included (i) higher germination ability of seeds produced in the warm summer (1991) compared with more average summers (1994 and 1995) and (ii) significant seedling occurrence only in the average summers of 1994 and 1995. Seedling densities were much higher at crusted (206 seedlings/m2) than noncrusted sites (26 seedlings/m2). In both sites, root elongation of seedlings and older plants were significantly greater than shoot elongation, yet in established plants, shoot biomass was much greater than root biomass (root/shoot ratios from 0.1 to 0.3). Reproductive attributes varied among the species examined. Saxifraga caespitosa L. produced much smaller seeds then Draba corymbosa R. Br. ex DC. and Papaver radicatum Rottb., but the largest seeds of S. caespitosa (produced in the warm summer of 1991) had much higher germination rates than any seeds of the other species. Based on an analysis of population age structures, seedling survivorship was low for all species but was especially low for S. caespitosa, despite its higher germination rates. All species were slow to germinate, with isolated seeds under ideal controlled conditions requiring a minimum of 20-30 days at a 20°C day and 15°C night. Seed germination in excised soil blocks under comparable conditions averaged 36-48 days. We believe these polar desert plant species lack special attributes to uniquely exploit the environment of these very stressful locations. Rather, they are simply adept at surviving the rigors experienced there. The same species grow much larger, flower and fruit more abundantly, and produce more viable seeds in high arctic environments more favorable than polar deserts, such as lowland polar oases (e.g., Truelove Lowland, Devon Island, Canada). Within the harsh polar desert landscape of the High Arctic, the presence of a cryptogamic crust that retains surface moisture, prevents soil churning, and includes nitrogen-fixing organisms provides a more favorable habitat for plant reproduction, establishment, and growth.Key words: High Arctic, plant reproduction, plant establishment, plant growth, cryptogamic crust, polar desert, Devon Island.