Introduction

The climate of Europe has changed in the past century. An increase in mean annual air temperature of +0.90°C could be observed between 1901 and 2005 (Jones & Moberg 2003). For the period 1977–2000, trends are even higher for Europe's mountain regions (Böhm et al. 2001). Beniston (2005) showed that for the alpine region minimum temperatures have increased up to 2 °C during the twentieth century, whereas the snow cover period has been reduced (IPCC 2007). The alpine and nival (uppermost altitudinal zone of the Alps above the closed alpine grassland) zones (e.g., Grabherr 1997) of high mountain ecosystems are considered to be particularly sensitive to warming (Diaz & Bradley 1997; Haeberli & Beniston 1998) as these ecosystems are determined by low temperature conditions. This life zone offers ideal conditions to study climate change effects because (1) direct human impact is very low, (2) its ecological systems are comparatively simple, at least in the upper elevation levels, and (3) its systems are dominated by abiotic, climate-related ecological factors. The importance of biotic factors such as competition decreases with altitude (Körner 1994; Callaway et al. 2002). Since high mountain plants have proven to respond sensitively to climate change (Grabherr et al. 1994, 2001), great efforts were made to establish the large-scale monitoring network GLORIA (Global Observation Research Initiative in Alpine environments) (Pauli et al. 2003).