Heavy rainfall periods initiate not only floods and debris flows, but may also trigger shallow landslides on both scree and vegetated slopes. This has had serious consequences in recent years in Switzerland, causing considerable damage to infrastructure, ecosystem, goods and services, even loss of lives. Although vegetation provides significant improvement in stabilizing steep slopes, conventional slope stability analyses generally neglect this entirely. Moreover, mycorrhizal fungi associated with plants contribute to stability, too, by promoting plant, and particularly root growth, and by supporting soil aggregate formation, which results in a further increase in apparent cohesion. It is intended to quantify the effect of biological interventions on the stability of soil and slopes in this study. Specimens consisting of combinations of a tree (Alnus incana), legume (Trifolium pratense), and grass (Poa pratensis) were prepared to investigate the root reinforcement effect. Direct shear tests were conducted in an inclinable large-scale direct shear apparatus on specimens with and without roots, following a six month growth period. The results of some preliminary direct shear tests are presented.