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Die Rolle der Schneebedeckung für die Kryptogamen-Vegetation in der maritimen Antarktis (Potter-Halbinsel, King George Island) = The role of snowcover on the cryptogamic vegetation in the maritime Antarctic (Potter Peninsula, King George Island)
Winkler, J.B. (2000). Die Rolle der Schneebedeckung für die Kryptogamen-Vegetation in der maritimen Antarktis (Potter-Halbinsel, King George Island) = The role of snowcover on the cryptogamic vegetation in the maritime Antarctic (Potter Peninsula, King George Island). Ber. Polarforsch. Meeresforsch. 371: 1-151
In: Berichte zur Polar- und Meeresforschung = Reports on Polar and Marine Research. Alfred-Wegener-Institut für Polar- und Meeresforschung: Bremerhaven. ISSN 1618-3193, more
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    Marine

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  • Winkler, J.B.

Abstract
    The extent to which seasonal snow cover determines vegetation pattern was investigated along a transect at Potter Peninsula (King George Island, maritime Antarctic). The transect extended southwards from a depression to a knoll with a change in elevation of 4.5 m. Special sensors were developed that automatically measured small changes in snow depth by means of infra-red light beams. Snow depth was recorded continuously from March 1996 to November 1997 at seven points along a 15m segment of the transect. The suitability of this system under the prevalent climatic conditions is discussed. A steep gradient in snow cover occurred along the transect. In contrast to the depression, where snow lay for long periods, snow-free episodes were frequent on the windswept knoll, even during the winter. Major differences in snow depth occurred along the transect for the months September and October in 1996 and 1997. The vegetation composition at 25 points along the transect was described using estimates of percentage cover and species frequency. It was shown that the number of species decreased with increasing duration of snow cover. The sampling points were grouped into 5 clusters with respect to their floristic similarity using multivariate analysis techniques (cluster analysis and non-parametric multidimensional scaling). These clusters mirror the gradient in snow cover extent. In the depression, where snow lay for nearly six months, the pleurocarpous moss Sanionia uncinata dominates the vegetation. In contrast, fruticose lichens such as Himantormia lugubris and Usnea aurantiacoatra occurred only on the more elevated, windswept part of the transect where snow lay for less than 120 days per year. As well as measurements of snow depth, microclimatic conditions were recorded from 1996 to 1997, inclusive, for three cryptogams chosen as typical for sites with contrasting snow cover along the transect. The three investigated habitats were almost identical for temperature and photosynthetic active photon flux density (PPFD) during the snow free period in the austral summer and autumn. However, during winter and spring their microclimatic conditions differed greatly. Incoming radiation was reduced drastically by deep snow cover at the site of the moss Sanionia uncinata, in the depression. Thallus temperature was constant at approximately 0°C during spring, possibly due to melt water that originated higher up the slope. The fruticose lichens Himantormia lugubris, in the middle of the slope, and Usnea aurantiaco-atra, at the knoll were not insulated by a deep snow-cover during winter. As a consequence they endured colder temperatures than the moss in the depression and were exposed to recurrent thaw-cycles. In spring radiation did not limit photosynthesis at the two lichen sites. Being poikilohydrous, water availability is a factor of great importance for the primary production of lichens and, in the Antarctic, snow is an important water source. Thalli of the fruticose lichens Himantormia lugubris, Usnea antarctica and U. aurantiaco-atra became almost saturated within a few hours through water uptake from snow under natural light and temperature conditions. After this initial phase, thallus water content rose only slightly with increasing duration of snow cover. Differences in snow type and cover had little effect on thallus water content. Within two to six hours thallus water content had become sufficient to activate CO2-gas exchange. At air temperatures above the freezing point enough liquid water was present under the snow to activate photosynthesis of the cyanolichen Leptogium puberulum. Only the crustose lichen Lecidea sciatrapha showed a depression in net photosynthesis due to the high water contents that are reached beneath snow. Models of photosynthetic capacity were developed for the fruticose lichen Himantormia lugubris, which was snow-free most of the time, and the chionophilous crustose lichen Lecidea sciatrapha. Both species are well adapted to the temperature conditions that prevail in their habitats. Light compensation points, exceeding 50 µmol m-2 s-1 PPFD, were comparatively high. By combining the photosynthesis models with the records of microclimatic conditions from the three investigated sites it was found that high respiratory losses could be expected for both lichens if they are covered by deep snow until summer. The carbon loss, due to low light, during the period of snow cover could not be compensated for during the time when the site was snow free. This remains true even if water contents were always optimal for photosynthesis. Therefore, snow cover in spring might well be a harmful factor for these cryptogams.

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