Transport Rates of Surface Materials on Steep Forested Slopes Induced by Raindrop Splash Erosion

We used small (25 cm) sediment traps to evaluate the properties of surface material transport on steep slopes under humid temperate conditions in the Shikoku district of Japan. We sampled ten stands of Chamaecyparis obtusa (hinoki), Cryptomeria japonica (sugi), Pinus densiflora (red pine), and deciduous hardwood forest. Transport rates were estimated by dividing the amount of mass transported by the mass of rainfall during a given period. These rates varied widely among the ten stands: 0.0065–0.31 g m−1 mm−1 (a difference of two orders of magnitude) for fine earth, 0.00017–0.97 (four orders) for gravel, and 0.020–0.24 (one order) for litter. The transport rates in young hinoki stands with a low floor cover percentage (FCP) showed no seasonal changes. The transport rates changed by one or two orders of magnitude in hardwood forests with clear seasonal changes in FCP, as well as in juvenile and middle-aged hinoki and sugi stands, where understory floor cover increased and decreased seasonally. Rainfall intensity, however, showed no positive effects on transport rates, even in stands that had changing transport rates. The rates were mostly determined by forest type. We demonstrate that transport rate is an inherent property of a forest that reflects floor cover conditions and indicates the potential of surface material transport.