Abstract
Shallow slope failure and shallow erosion are common forms of erosion that occur all over the world. They cause economic and environmental damage, due to the loss of fertile soil and its transport into aquatic ecosystems. In some cases, shallow slope failure constitutes a potential hazard for human wellbeing, e.g. when roads, or pipelines are affected. Vegetation is broadly used for control of erosion on slopes. Commonly used theoretical frameworks for vegetation-based slope stability assessment were originally developed for woody vegetation. Nevertheless, shallow slope stability often relies on herbaceous vegetation. Field observation reveals that herbaceous vegetation seems to follow only partly the same slope stabilization mechanisms as woody vegetation. Therefore, the stabilization potential of herbaceous vegetation may be often underestimated. Here, a new slope stabilization mechanism based on the surface-mat effect is presented that aims at explaining the stabilization dynamics found on densely, yet, shallow-rooted slopes with herbaceous vegetation. Instead of the effects of deep reaching vertical roots, the mechanism is based on the horizontal stabilizing effect of the dense root mat. Thus, this surface-mat effect relies on the horizontal tensile strength of the topsoil. Through re-distributing forces of erosion hotspots to more stable sections of the slope, the surface-mat effect can prevent erosion events. In order to measure the surface-mat effect, we also present a new measurement method that allows quantifying the horizontal tensile strength of vegetated topsoil under field conditions. We conducted two studies exploring the influence of vegetation, management, soil physical and soil chemical factors on the surface-mat effect. The first study was conducted on montane grasslands with intensive management near the towns Sarnthein and Predazzo in the north Italian Alps (1000 m a.s.l.). The second study was conducted on sub-alpine grasslands with extensive management on the mountains Raschötz (2125 m a.s.l.) and Zendleserkofel (2275 m a.s.l.) in South Tyrol, northern Italy. Generally, the surface-mat effect decreased with depth and depended on the species assemblage and management. Sub-alpine grasslands were 3 times stronger than montane grasslands. Dispersed vegetation, forbs and g asses were associated with low tensile strength. However, some forb and grass species (e.g. Gentiana punctata L., Phleum rhaeticum Schröter) were associated with high tensile strength. Hence, forbs and grasses demonstrated a species specific effect on slope stability. Reinforcement potential did not depend on the root type. Clonal structures (e.g. rhizomes) contributed to slope stabilization as well. Both, very high and low nutrient content were associated with low tensile strength. Soil organic carbon positively affected tensile strength. Soil water content had no significant effect on the surface-mat effect. Thus, the functionality of the surface-mat effect persists when the risk for shallow slope failure due to hydrological changes is high. Therefore, it provides a promising opportunity for erosion control. In conclusion, vegetation management plays an important role for the control of shallow slope failure. Consequent changes in biotic and abiotic factors have a direct influence on shallow slope stability. Adapted vegetation management could thus be a viable integrated measure for shallow slope failure and shallow erosion control.