In a mountainous country like Switzerland, it is not only good things that come from above. As much as 6 to 8% of the country's subsurface is unstable, mainly in the Alps and the Alpine foothills. In these areas, heavy precipitation causes the earth to move, particularly on steep slopes. Soil on hillsides may become so saturated with water that it actually liquefies, triggering hillslope debris flows that pose a major hazard to buildings, roads and railway lines. Shallow landslides contain less water than hillslope debris flows and occur when the top layer of earth slips away.

Climate change will only magnify these risks, with shallow landslides and hillslope debris flows in particular likely to become more frequent if heavy rainfall increases as predicted. Forest partially mitigates these risks, which is why steep mountainsides have been reafforested for the past century and more. However, we are now learning – among other things from the over 750 landslides recorded by WSL researchers in the landslide database since 1997 –that the type and condition of the trees matters. The landslide data has recently been made available online.

Spruces easily swept away

The latest evidence concerning forest's impact on landslides has been summarised by a group of researchers in a special issue of the Swiss Forestry Journal (Schweiz. Zeitschrift für Forstwesen, SZF). The most notable finding is that the condition of the forest is key, with very dense woodland and windthrow areas offering less protection from landslides than forest with a multi-layered stand structure. However, on very steep slopes, forests cease to be effective at all. Indeed, at gradients of over 38°, more landslides per area occur in forest than in the open. Very steep forests tend to be less well maintained and located mainly in more elevated and inhospitable areas with a high prevalence of spruce, a species vulnerable to both windthrow and bark beetle infestation. Spruces have shallow roots and are therefore easily swept away on steep slopes.

Using large-scale data sets on vegetation and natural events, WSL and SLF researchers have worked out which type of forest offers the greatest protection. They show that reforestation and natural disturbances such as storm Lothar, which hit 20 years ago this December, as well as careful forest management, can all significantly influence a forest's protective function. "The most effective forests at preventing landslides are those with as many different structures as possible, both underground and above ground," explains Christian Rickli. This includes a good mix of tree species and trees with a varied age structure.