The impact of tree vegetation on the subsoil is complex and its influence on slope stability can be both positive and negative. Vegetation, for life processes, needs water, which is retained during precipitation within the crown, and retrieved from soil through the root system. These processes reduce the soil moisture content and pore water pressure [1-3], and as a consequence an increase in shear strength occurs, which is beneficial from a geotechnical point of view. In an extreme case, excessive intensity of water transpiration Pol. AbstractOur work aimed to determine the spatial distribution of the root cohesion of the roots of 8-year-old black locust and black poplar trees. The scope of our research and analyses included determining characteristics of root systems of the studied tree species by profiling the walls of a trench with a width and a depth of 1.0 m at a distance of 0.5 and 1.0 m from the trunks. Laboratory tests comprised determining the tensile strength of the selected root classes. A modified Schwarz model (RDM) was used to describe the horizontal distribution of roots in the soil. The increase in shear strength of the root-reinforced soil was determined by a strain bundle model in which the value of the force mobilized by the roots is described by the Weibull survival function (RBMw). The results of the root system measurements have shown that the black locust is characterized by a greater number of roots, while the roots of black poplar are thicker, which makes the relative surface of its roots larger. Calculations of root cohesion using the modified bundle model, taking into account the root system displacement, showed that the maximum value for the black locust was 9.4 and 6.4 kPa, and for the black poplar 6.4 and 6.2 kPa respectively at a distance of 0.5 and 1.0 m from the trunk. It was also shown that the optimal spacing of the trees of these species, necessary to achieve effective reinforcement of the soil, was approximately 4 m.