Fig. 1 - A powder avalanche rushes down a mountain forest. Photo: Michael Bründl (SLF/WSL)
It’s winter. After strong snow fall, avalanches are rumbling towards the valley. With their enormous power they break trees as if they were wooden match sticks; whatever is in their way is torn down. A few months later, the last snow patches are melting and flowers and grasses are flowering in the avalanche track. The question remains: How do avalanches affect nature?
A study with the WSL-Institute for Snow and Avalanche Research SLF, Davos demonstrates that the destructiveness of avalanches also has positive aspects for nature. It creates environmental conditions which enable an entire range of plant species to survive. Large, dominant trees are cracked by avalanches.
Small-scale change of the environmental conditions
As a consequence, much more light hits the soil compared to the neighboring forest and more water and nutrients are available. And the mechanic stress on small plants due to the avalanche is minor: the snow cove protects them or they are still flexible enough to bend down due to the snow weight.
In a small area there are the most diverse environmental conditions, as the mechanic stress is greatest in the track center since smaller avalanches don’t cover the entire track, and as at some spots snow is ripped away whereas at other spots snow is deposited. Accordingly, the biological diversity is high. Many different plant species and communities find suitable conditions; the "typical avalanche plant species" does not exist. Thus, over 80% of the 141 observed species grew in less than 5% of the investigated plots, although the undisturbed vegetation outside of the avalanche tracks consisted of a larch-spruce forest throughout the entire monitoring area (Dischmatal near Davos GR).
Fig. 2 - Avalanche track in the Bernese Alps. Photo: Thomas Reich (WSL)
Fig. 3 - Avalanche track in the Bernese Alps. Photo: Thomas Reich (WSL)
Three times more species than in the neighboring forest
In comparison to earlier studies, the SLF study investigates many tracks with differing frequency of comedowns. The more frequent avalanches come down in a track, the richer and more diverse is the vegetation. Avalanche tracks with a comedown every year host approximately three times more species than in the neighboring forest.
Surprisingly, not only pioneer plants take advantage. They are only represented strongly if during the last years the forest was destroyed. Competitive and perennial species prefer the other avalanche tracks. This shows that despite frequent disturbances relatively stable conditions establish.
Fig. 4 - Not only plants benefit from the increased supply of light and heat, but also numerous animals such as butterflies or reptiles. The picture shows a pregnant viviparous lizard. Photo: Thomas Reich (WSL)