How Forests Protect Against Avalanches: Why Tree Height and Species Matter
Trees as Natural Avalanche Barriers
Forests have long been known to reduce avalanche risks in mountain regions—but not all trees offer equal protection. A 50-year study from the Stillberg afforestation site near Davos, Switzerland, has revealed how tree height, species, and forest development shape their effectiveness in preventing avalanches.Peter Bebi, head of the Mountain Ecosystems research group at SLF, explains: “Trees only really prevent avalanches once they are at least twice as high as the snowpack.” His team’s findings provide a scientific foundation for practical avalanche safety guidelines.
Why Tree Species Matter
Not all trees function the same way when it comes to snow stability. Evergreen conifers like Swiss stone pine and spruce catch and hold more snow on their branches, preventing uniform snowpack formation and reducing weak layers that can trigger avalanches. By contrast, larch forests—whose needles fall in winter—are less effective in holding snow.This difference highlights the importance of planting a mix of species when designing forests intended for avalanche protection.
Insights from the World’s Oldest Avalanche Study Site
Established in 1972, the Stillberg project saw 92,000 seedlings of Swiss stone pine, mountain pine, and larch planted on a steep 38-degree slope above the Dischma Valley. Researchers have since monitored both forest growth and avalanche activity, recording 214 avalanches over five decades.Until the 1990s, avalanches were frequent in the area. But as more trees grew tall enough to surpass the snowpack height, the number of avalanches dropped dramatically. Today, only isolated events occur in channels where tree cover remains sparse or where trees died prematurely.
Guiding Future Avalanche Protection
The Stillberg results now serve as a blueprint for forestry and hazard management. By understanding how long it takes for afforestation projects to deliver effective protection, planners can better design forest strategies in avalanche-prone regions.Climate change also plays a role. Warmer conditions have recently benefited larch trees, allowing them to thrive at higher elevations, which could expand the natural defense capacity of forests in the future.
“In the beginning, we hardly expected such reliable protection,” says Bebi. “But today, the majority of former starting zones are stabilized by forest growth. That’s a remarkable achievement.”
