Hillslope processes, eco-engineering and protective forests
Rainfall events in mountain regions are frequently associated with numerous hillslope processes, which may threaten life and property. It is widely recognised that vegetation can considerably improve the stability of the slopes. The positive effects of plants are not only important in influencing and controlling hillslope processes, such as shallow landslides and erosion, but also in stabilising and revegetating landslide scars and artificial escarpments. Although there has been extensive research on the stabilising effects of plants on soil and slopes, still many questions remain about understanding these effects. The present issue of "Forest, Snow and Landscape Research" contains recent studies carried out at the Swiss Federal Institute WSL on the effects of vegetation and tree roots on slope stability, and on the long-term behaviour of different technical and biological measures used to stabilise slopes. It was found that among technical constructions gabions were fully functional after about 25 years but the timber of log crib walls had considerably deteriorated. On the other hand such accompanying biological measures as alder plantings were found to have resisted several heavy thunderstorms even though the actual slope gradient was about 5° steeper than the calculated allowable angle of internal friction. This increase in the angle of internal friction of about 5° was shown experimentally using triaxial compression tests. The dimensions of landslides in forests were not found to be significantly different from those in open land. However, in forests, they occurred with lower densities and on steeper slopes, which may be important to consider when compiling hazard maps. Revegetation measures were found to increase soil aggregate stability by substantially accelerating vegetation development and promoting soil formation processes, such as the accumulation of fine soil particles, organic matter and mycorrhizal propagules. Measurements of root tensile strength indicate that the root system of protection forests loses most of its soil-stabilising function within 15 to 20 years of forest disturbances such as windthrow or bark beetle outbreak. It can be assumed that this period of time is not long enough for new tree generations to have grown enough to stabilise the soil effectually, particularly at high altitudes. Finally, another aspect exploited in this issue is the application of new technologies like laser-scanning to create a realistic three-dimensional model of a complex root system. This is an important step towards a better quantification of tree anchorage and stability. The contributions should be particularly relevant to foresters, eco-engineers and specialists involved in hazard mitigation.