Factors controlling sediment production from a yarding road and its temporal pattern in an abandoned clear-cut plantation forest in the Shirasu (pyroclastic flow deposits) area, Kagoshima Prefecture, Japan

摘要

To understand the temporal pattern in sediment production from a yarding road, we investigated sediment yield and grain size composition, and examined the relevant physical factors in an abandoned clear-cut plantation forest, based on the 3-year onsite monitoring in Shirasu area, Kagoshima Prefecture, Japan. Under a given amount and intensity of rainfall, although sediment yield and grain size of the sediment increased with increasing rainfall, they remarkably decreased each year; that is, the annualsediment yield from the yarding road in the first, second and third years were 6.1, 2.7, and1.4 mm, respectively. With the elapsed year, the proportion of gravel and coarse sand gradually decreased, whereas the proportion of fine sand and silt-clay gradually increased. Dry density of surface layers decreased with increasing vegetation coverage at the survey site each year. Additionally, surface grain size at the rill and inter-rill points gradually increased with the elapsed year. The results indicatedthat the annual decrease in both sediment yield and its grain size are associated with an increase in soil porosity (i.e., increase in infiltration capacity) due to root development and increased intensity of ground resistance to overland flows and sediment transport following both vegetation growth and surface armouring.