Significance of tree root decomposition for shallow landslides

Tree-root systems can prevent shallow landslides. In layers permeated by roots, the soil shows greater stability as roots are able to absorb forces. When protective forests die off extensively as a consequence of a bark beetle outbreak or of another disturbance (e.g. storms or fires), their protective power on the slope stability decreases with the decomposition of the roots of the dead trees. By determining the relation between the tensile strength of roots and the tree’s time of death, the decrease in stability as a consequence of root decomposition can be estimated. To this end, we measured the tensile strength of roots from: i) freshly felled, living spruces (Picea abies), ii) spruces felled eight years previously, and iii) spruces that had died 10 and 12 years previously in a bark beetle outbreak. Tensile strength decreased continuously with the number of years after death. The results of this study show that within 15 to 20 years of tree death, the root system of protection forests loses most of its soil-stabilising function. It can be assumed that, particularly at high altitudes, this period of time is not long enough for new generations of trees to have grown enough to have the same stabilising effect on the soil.