Vascular plant distribution patterns at the low temperature limits of plant life - the alpine-nival ecotone of Mount Schrankogel (Tyrol, Austria)
Pauli, Harald; Gottfried, Michael; Grabherr, Georg
Compared with typical alpine plant communities, the generally open and scattered plant assemblages of the subnival and the nival belts are only marginally treated in the extensive ecological literature on the Alps. Although a few authors have provided general comments about plant assemblages within the uppermost vegetation belts, no detailed account on vegetation patterns in relation to environmental gradients and factors has been published so far. Such an account, however, is needed to understand the impact of climate change on alpine vegetation. This paper deals with the characteristic vascular plant assemblages and species distributions within the subnival and nival belts. The analysis is based on almost 1000 vegetation samples from Schrankogel (3497 m, Stubaier Alpen, Tyrol), a typical nival peak of the Eastern Alps dominated by a characteristic siliceous vegetation. All samples were taken from permanently marked 1x1 m quadrats established for long-term monitoring of climate-related vegetation changes. The quadrats - arranged along transects - were distributed over the south-west to south-east exposed slope-system, beyond the more closed upper alpine swards, in order to cover the full variety of the subnival and nival plant assemblages. The vegetation classification revealed 14 vascular plant assemblages belonging to two alliances: Caricion curvulae and Androsacion alpinae. Only three of the 14 units could be assigned to well-documented associations: the uppermost alpine Caricetum curvulae, the subnival-centred Androsacetum alpinae, and a subnival variant of the mostly upper alpine Sieversio-Oxyrietum digynae. The other 11 assemblages are characteristic of the successive impoverishment and disaggregation of upper alpine swards and of well-developed subnival Androsacion alpinae-coomunities. Despite the low species and community diversity, well-differentiated ecological distribution patterns could be indicated. The habitat characteristics "solid rock" and particularly "scree" proved to differentiate the habitat preferences of the diagnostic species of subnival plant assemblages. The "scree-avoiding" sward-forming graminoids, but also the pioneer species of swards grow preferably in rocky habitats; character species of the Androsacion alpinae prefer mixed rock and scree habitats, whereas the chionophilous Sieversio-Oxyrietum-species mostly occur in habitats with high scree-cover. The rock-preferring plants seem to be sensitive to disturbance (from debris fall and substrate movements) as well as to long-lasting snow cover. Therefore, possible climate-induced plant migrations will be highly dependent on topographically determined gradients. The potential pathway of upward-migrating alpine grassland species will be stable rocky ridges. On the other hand, chionophilous plants at the scree sites may be affected severely by reduced snow cover, induced by climate warming.