When trees can protect against avalanches

A question of height: SLF research shows when protection forests really work - 50 years of data from Stillberg.

• Tree height plays a key role in avalanche protection: A protection forest only effectively prevents avalanches once the trees are twice as high as the snowpack.
• Tree species influences snow retention: Evergreen conifers such as spruce or Swiss stone pine hold back more snow than larch trees.
• Important practical implications: The results of the SLF study will help plan robust protection forests in the face of climate change.

Forests protect against avalanches, but not every protection forest is equally effective. Thanks to almost 50 years of observations at the Stillberg afforestation site , SLF researchers have gained insight into how the avalanche protective function of afforestation changes over time and from which point it effectively prevents avalanches. "Trees only really prevent avalanches once they are at least twice as high as the snowpack," explains Peter Bebi, head of the Mountain Ecosystems research group. Through their work, he and his team are improving the scientific basis for the rules of thumb that are applied in practice.

Tree species also plays a crucial role. "Evergreen conifers hold back more snow," says Bebi. This is important because the more snow that remains on the treetops, the lower and more irregular the snowpack remains. This means that weak layers that trigger avalanches are unlikely to form. A forest made up only of larch trees is therefore less effective than a forest with Swiss stone pines or spruce trees.

The long-term project at Stillberg celebrates its 50th anniversary this September. At the start of the project, researchers planted around 92,000 seedlings of Swiss stone pine, mountain pine and larch on this steep slope, which is at an incline of around 38 degrees, above the Dischma Valley near Davos. It is probably the world’s oldest and most significant long-term experiment above the treeline. In the years that followed, scientists closely monitored how the forest developed, regularly measuring the trees and, in winter, the snowpack, and observing a total of 214 avalanches in the area. Until the 1990s, avalanches frequently occurred on the Stillberg site. Then came the turning point: more and more trees were now at least twice as high as the snowpack. "After that, there were significantly fewer avalanches, almost all’of them in individual channels where most of the trees had already died early on," says Bebi.

Taking as a basis the long-term results from Stillberg, those responsible for forestry or natural hazards can better plan where a reliable protective function can be expected in the future and how avalanche protection can be improved using suitable afforestation techniques. "In the beginning, we hardly expected that such a sound protective function would be achieved today in most of the former starting zones above the former treeline," summarises Bebi. A starting zone is the area where an avalanche is triggered. "Only in the upper part of individual channels, where avalanches damaged young trees or where these trees died prematurely due to sustained snow cover, would individual additional timber control structures have yielded even better results."

Climate change will also play a role in future projects. Larch trees in particular have benefited from warmer temperatures in recent years and show that avalanche protection forests can also be effective at higher altitudes in the future. It would be even more useful if other tree species were to follow suit. However, there are greater risks to the long-term preservation of the protective function if only one tree species predominates or if the trees are all the same age. Bebi therefore recommends specifically promoting the diversity of tree species and forest structures in mountain forests, including at higher altitudes.

To mark its milestone anniversary, researchers and students are visiting the field site every day this summer. Tree by tree, they are measuring values such as height and trunk circumference and recording information on injuries, frost damage and fungal infestation. They last did this in 2015. "Since then, the afforestation has developed remarkably - it’s increasingly taking on the character of a stand," says Bebi. The current data helps provide a better understanding of the decades-long journey from planting to a completely effective avalanche protection forest.

Growing trees decrease the frequency of avalanche release in an alpine afforestation in the Swiss Alps - the study is freely accessible.

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