Growing trees decrease the frequency of avalanche release in an alpine afforestation in the Swiss Alps

Forests mitigate the release of snow avalanches by precluding a continuous weak layer and by stabilizing the snowpack. Key factors influencing protective function against snow avalanches include forest density, tree height, tree species, and the size of forest gaps. Field observations highlight tree height as crucial for preventing avalanche release in young forest stands. The effective tree height should range from 1.5 to 2 times the snow height. Despite individual observations, systematic analysis of tree growth, snow height and avalanche activity is lacking. Here, we address this knowledge gap by providing the first quantitative validation of the 2:1 tree-to-snow height ratio, using a unique long-term dataset from the Stillberg afforestation near Davos, Switzerland. Over the past 49 years, we observed a decline in avalanche activity linked to an increasing tree height. The spatial distribution of avalanche-prone areas shifted from widespread releases to localized avalanches in gullies with higher snow accumulations and lower tree densities. Our analysis confirmed that trees at least twice as tall as the snow height reduce avalanches, as shown by the decline in avalanche frequency after 2000 when this tree-to-snow height ratio was exceeded. Even though a greater tree-to-snow height ratio effectively hindered avalanche formation, some snow avalanches still occurred under certain conditions, particularly in gullies with sparse or smaller trees. By integrating long-term data on snow height, tree height, and avalanche release, our study introduces a multi-dimensional framework for understanding avalanche occurrence in a growing afforestation. These results highlight the long-term effectiveness of tree afforestation in reducing avalanche activity at Stillberg, with the potential for additional protection through temporary measures in vulnerable gullies. This emphasizes the need for understanding site-specific vulnerabilities to guide informed forest management strategies for effective avalanche mitigation.