Abstract
The use of vegetation to stabilize slopes with regard to shallow landslides is an ecological and economic alternative to traditional solutions of civil engineering. This study aims at quantifying the effect of soil water content on the reinforcement of soil by plant roots and determines how soil water content influences the failure modes of roots during shear. Direct shear box tests were performed on three species used in hillslope reforestation programs in Yunnan, China. Reinforcement was quantified and divided into a hydric component corresponding to the effect of water removal and a mechanical component reflecting the influence of roots on apparent cohesion and friction angle. It was shown that the hydric component of reinforcement can be of the same order as the mechanical component and that root system architectural traits influence the type of mechanical reinforcement (cohesive or frictional). We also demonstrate the potential development of matric suction effects on the shear strength of soil.