The typical trees(one year old Platy cladus orientalis

Pinus tabulaef or mis Carr

A certruncatum Bunge and Robinia pseudoacacia)on the Loess Plateau were planted in self-designed soil box with adjustable gradient three years ago.The experiments were carried out by a simulated rainfall in laborat ory.The effects of forest vegetation canopy and root on the charact eristics of overland flow and sediment particle composition were analyzed.The results showed that:(1)Runoff yield was large for all plots.The average overland flow depth of each slope was less than 1mm.Fr of bareland slope was greater than1.0

as supercritical flow

for others it was smaller than 1.0

as subcritical flow.After the canopy layer was removed and the topsoil was scrapped off

overland flow of Robini a pseudoacaci a slope and A certruncatum Bunge slope was close to the critical flow

the function of reducing sediment yield and runoff by Robinia pseudoacacia and A certruncatum Bunge was significantly weaker

while the function of Platy cladusorient al is and Pinust abulaeformis Carr slope became stronger.(2)Clay loss ratio of the slope covered with vegetation canopy was 1.40%~2.00% higher than that of bare land slope.The proportion of sediment particles with diameter bigger than 0.05mm was 1.04%~ 1.26% lower than the bareland.But there was no apparent coarsening soil layer on soil surface.The obvious rough bedding texture layer appeared on bare slope.(3)After removing the forest canopy layer and the disturbed topsoil

the average particle size of sediment particles was larger than that covered by canopy but smaller than the bare land slope.And the percentage of the lost clay content with diameter smaller than 0.001mm fluctuated with the time past.It is suggested that forest root systems could increase antierosion capability of forest soil.