This study compared the resistance of tree root systems to shallow landslides using an in�]situ direct shear test with the pullout resistance of tree roots. We established three study sites in 40�]45�]year�]old alder or pine forests in Bibai, Hokkaido. We cut the trunks near the ground surface and molded soil blocks containing the root systems by excavating around the soil blocks. We put a shear box on the soil blocks and pulled the shear box horizontally, manually, using a chain block, and measured the shear force using a load cell. We also prepared soil blocks without roots and measured them in a similar manner to obtain the soil yield criterion for each study site. We regarded deviation between each measured point with roots and the yield criterion line on the normal stress�]shear force plane as the reinforced shear strength due to the roots. We measured the pullout resistance of roots of trees growing near the site of the direct shear test and obtained root diameter�]pullout resistance curves. The reinforced shear strength of roots in the direct shear test increased with the cross�]sectional area of the roots in the shear plane. We compared the reinforced shear strength of roots using a model that infers it from the pullout resistance, angle of root deformation, and angle of internal friction with the reinforced shear strength of roots determined from the direct shear test. The strength with the direct shear test was only half or so of the strength with the model. We believe that the model overestimated the reinforced shear strength due to the roots.

Key words�Froot, in�]situ direct shear test, shallow landslide, pull�]out resistance, shear strength