Avalanches creating high structural and floristic diversity in mountain mixed forests in the Alps

On January 18, 1986 a snow avalanche fell down from the steep slope of Mount Watzmann, Berchtesgaden National Park, Germany, into a mountain mixed forest reaching an elevation of only 610 m asl. The trees were bent to the ground, but they survived, forming a dense canopy pressed to the ground. Although on the landscape scale the avalanche marked a strong visual impact we hypothesized that the species composition of the forest ground layer would not change significantly. To test this we established a set of permanent plots. In the following years new avalanches over and adjacent to the former avalanche track occurred. The established permanent plots allow us to evaluate a second hypothesis, namely that not only structural diversity has been enhanced by the avalanches, but also a sequence of different succession pathways initiated which all together will keep diversity at a high level in the affected area. Our analysis of forest patches which were affected only by the first avalanche shows that, although a strong change in forest structure has taken place, the species composition here remained to a large extent unchanged. This suggests that beneath the dense crown canopy built up by the bent trees the microclimate shows little change, mechanical soil disturbances are rare, and therefore species of open land have no chance to establish. In contrast, if the avalanche uproots and kills the trees a clear change in species composition is the result. On our permanent plots we were able to identify eight different succession pathways initiated by the different disturbance events, causing at various points of time a set of different development phases of the forest ecosystem on the avalanche track.