Variously aged Leucaena leucocephalain debris flow area of Jiangjia gully were chosen and the fine root systems were dug out from the surface to the bottom layers at different distances from the stem in three directions.Basing on the analysis of fine root vertical distributions and composition

the effect of fine roots on the soil shear strength were estimated according to reinforcement theory.The result show that fine roots exhibited an obvious accumulation trait in the topsoil layer

which reduced with depth.Approximately 80% of the fine root materials were found in the layer between 0—120cm depths.Fine roots were mainly composed of smaller ones(D≤1mm)and determined the development of root distribution.The 10-year stands had more fine roots(D≤1mm)than 5-year ones

and the 15-year stands had obviously more intermediate roots(1mm≤D≤2mm)comparing to the 10-year stands.Root tensile strength decreased significantly with increasing diameter

following apower law function.The soil shear strength enhanced by fine roots was highly variable among depths and ages

largely depended on the smaller roots(D≤1mm).