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At the top: The forest line

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Forest lines are among the most striking transitions between habitats. But changes in land use and also climate change are causing them to shift in many places. We are investigating what this means for protection against natural hazards and other environmental benefits of mountain regions.


Upper forest lines are transitional areas between subalpine forests and alpine grasslands. On a large scale, upper tree lines and forest lines above all serve as heat boundaries. Only when average temperatures of at least 5 to 7°C prevail during the growing season can the metabolic processes that successfully control tree growth take place. If the growing season is too short or too cold, the trees do not invest enough energy in stem growth. On a small scale, in addition to being affected by temperature, actual land use and changes in land use, tree survival and growth are also influenced by snow movements, frosts in early summer, wind, snowpack, desiccation and competing vegetation.

Using long-term observations and ecological experiments, we are exploring how the upper forest line shifts under the influence of various factors and what impact these changes have on protection against natural hazards and other environmental benefits.

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Extracting the tree core in order to determine the age of the young trees in the Polar Urals. Image: F. Hagedorn/WSL
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The treeline in the Ural Mountains has shifted around 50 metres upward over the last 100 years. Images: F. Hagedorn/WSL
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Expedition to the treeline of the Ural Mountains in winter. The winter conditions, particularly the stripping of needles by snow crystals, can be critical for the trees' survival. Image: F. Hagedorn/WSL

Climate change in the Urals and the Lötschental

Our research shows that the treeline in Switzerland is determined primarily by human use. In the Russian Urals, far from civilisation, the impact of climate change is becoming immediately apparent. In collaboration with ETH Zurich and fellow researchers from Russia and Germany, we are examining the shifting treeline in the Southern and Polar Urals. The treeline in this region has already moved around 50 metres upward since the beginning of the 20th century.


Climatologists are predicting a temperature increase for Switzerland of around 3°C over the next 100 years. This is equivalent to the current temperature difference between the treeline and the valley floor, making the Lötschental an ideal area for studying the effects of climate change. We have been monitoring the growth of two ecologically different species, spruce and larch, along the south and north-facing valley slopes since 2011.