In order to quantify the impact of the abandonment of forest management practices on the runoff of surface-derived suspended sediment at the catchment scale, we compared the relative contributions of surface soil erosion among Japanese cypress plantation catchments with different stand densities and surface coverage in a 4·9-ha nested catchment. Source fingerprinting of suspended sediments was conducted using the fallout radionuclides Cs-137 and Pb-210ex. Eroded material from hillslope plots having different forest conditions and stream bank material were used as surface and subsurface sediment sources, respectively. The nested watershed monitoring of erosional behaviour and particulate runoff of radionuclides demonstrated decreases in the activities of radionuclides at downstream sampling sites. Hence, the scaling effect of catchments and forest conditions can affect the development of the concentration of surface-derived material in stream water. An indicator of the forest condition, particularly the surface coverage, light condition of the forest floor was quantified. At the catchment of a thinned plantation with dense understory vegetation, all suspended sediment was calculated to be derived only from subsurface erosion. In contrast, at the catchment of a slightly thinned plantation, the highest proportion of surface erosion (67·8 ± 3·7%) was estimated. Therefore, the forest condition, particularly the light condition at the forest floor, which is related to the ground surface coverage, affects the magnitude of surface soil erosion and the particulate runoff of surface soil. It is suggested that forest management can affect the overland flow generation, stability of the surface soil and associated elements, and downstream water quality. Copyright © 2010 John Wiley & Sons, Ltd.