- Natural avalanche disturbance shapes plant diversity and species composition - 735 Journal of Vegetation Science 18: 735-742, 2007 © IAVS; Opulus Press Uppsala. Abstract Background: Disturbances by avalanches have created unique habitats for animals and plants in subalpine ecosystems worldwide, but at the same time avalanches can pose a major threat to humans. Thus, avalanches are suppressed by means of avalanche barriers to protect settlements and infrastructures in populated areas of the European Alps. As a consequence, the disturbance regime in avalanche tracks has fundamentally changed. Methods: In the present study we address ecological conse- quences of avalanche suppression on plant diversity. We ana- lysed plant diversity and species composition in recent and old avalanche tracks with and without avalanche suppression and in undisturbed adjacent forests at high and low elevations. Results: The number of species was higher in both active and inactive avalanche tracks as compared to undisturbed subalpine forest. The species composition indicated a wider range of eco- logical niches in active than in inactive avalanche tracks. The vegetation from active tracks showed lower indicator values for temperature and nitrogen availability. The proportion of alpine species was lower in formerly active tracks. Conclusions: The conditions that exist in active avalanche tracks increase plant diversity in relation to undisturbed forest. In the few decades following avalanche suppression, species composition changes in tracks from which avalanches have been excluded. Continued suppression of avalanche disturbance may lead to a decline in plant and habitat diversity. Avalanche disturbance can exert an important influence on the biodiversity of subalpine forests and provide important habitats. Anthropo- genic changes in the natural regime of avalanche disturbance are likely to contribute significantly to future landscape changes in subalpine forests. Keywords: Alps; Avalanche; Diversity; Indicator value; Snow; Switzerland. Nomenclature: Lauber & Wagner (2000). Abbreviation: FDR = False Discovery Rate. Natural avalanche disturbance shapes plant diversity and species composition in subalpine forest belt Rixen, Christian 1* ; Haag, Susanne 1,2 ; Kulakowski, Dominik 1,3 & Bebi, Peter 1,4 1 WSL Swiss Federal Institute for Forest, Snow and Landscape Research – Unit Ecosystem Boundaries – Alpine Ecosystems, Flüelastr. 11, 7260 Davos Dorf, Switzerland; 2 E-mail susanne_haag@yahoo.com; 3 Department of Geography, University of Colorado, Boulder, CO, USA; E-mail dominik.kulakowski@colorado.edu; 4 E-mail bebi@slf.ch; * Corresponding author; Fax +41 814170110; E-mail rixen@slf.ch; Introduction Disturbances are crucial driving forces for processes and biodiversity in many ecosystems (White 1979; At- tiwill 1994; Frelich 2002) and they have been found to often increase diversity. Such disturbances may be exerted e.g. by animal activity (Fox 1981; Martinsen et al. 1990). In forest ecosystems, diversity has been found to increase by high fire frequency (e.g. Glitzenstein et al. 2003), windthrow (e.g. Hiura 1995) or beetle outbreak (e.g. Stone & Wolfe 1996). As important as disturbance itself is the suppression of disturbance by humans to protect settlements and infrastructure, which can also affect diversity. Such anthropogenic suppression of disturbances has caused changes in structure and processes of ecosystems. For example, suppression of fires has changed the stand structure of forests, and has resulted in accumulation of fuels that may have increased the hazard of exceptionally large fires in certain forest types (Gedalof et al. 2005). An important but hardly investigated force shaping ecosystems in mountain forests is disturbance through avalanches. Avalanches can be frequent and occur several times during winter (Laternser & Schneebeli 2002). The resulting disturbance reduces tree growth and density and creates open habitat in otherwise closed subalpine forest (Major 1977; Butler 1979). Avalanche tracks contribute largely to the landscape complexity of the European Alps and can cover, depending on the topography, more than one third of the subalpine forest belt (Haag 2006). They are a preferred habitat for brown bear, caribou and wol- verine in North America (Mace et al. 1996; Krajick 1998) and have a characteristic vegetation (Ellenberg 1988; Erschbamer 1989). Likewise, European grouse diversity is highest in uneven-aged and diverse forests (Swenson & Angelstam 1993) and is likely to profit from habitat created by avalanche disturbance. Although studies into other disturbances such as windthrow indicate that dominance reduction of trees contributes significantly to a high species diversity in the subalpine belt (Wohlgemuth