Geology and vegetation control landsliding on forest-managed slopes in scarplands

Abstract

Abstract. Landslides are important agents of sediment transport, cause hazards and are key agents for the evolution of scarplands. To analyse geologic and vegetation control on landsliding, we investigated three landslides in the Franconian scarplands. We used geomorphic mapping, soil analysis, electrical resistivity and a mechanical stability model to quantify the stability state of the landslides. Furthermore, we mapped tree distribution, quantified rooted area and root tensile strength to assess the influence of vegetation on shallow landsliding. Our results show that landslides are deep-seated incorporating rotational and translational movement with sliding along a geologic boundary between permeable Rhätolias sandstone and impermeable Feuerletten clays. Despite low slope angles, landslides could be reactivated when high pore pressures could develop due to geologic conditions. In contrast, shallow landsliding is controlled by vegetation. Our results show that rooted area is more important than species dependent root tensile strength and limited to the upper 0.5 m of the surface due to geologically controlled unfavourable soil conditions. Due to low slope inclination, root cohesion can stabilize landslide toes or slopes undercut by forest roads, independent of potential soil cohesion, when tree density is sufficient dense. Forest management currently adapts forests to climate change by diversifying tree species and introducing European beech, which would increase slope stability when sufficient rooted area is reached. Forestry activities should aim to keep a certain tree density to enable sufficient root cohesion that prevent landslide activity between harvesting or adaption periods. In summary, geological conditions in scarplands favour landslide activity and influence vegetation control on landslide activity, which suggest a combined forest and hazard management should be applied to prevent future landsliding.