Abstract
Soil movement and physical properties of surface soils were measured at 263 points in the evergreen broad-leaved Aya forest in southern Kyushu, in order to clarify the factors affecting the establishment and survival of seedlings. The mass of soil movement was found to be different in each microtopographical unit. Depositional poits were observed on crest slopes where the angle of slope was gentle, whereas the headmost wall and head hollow, where the angle of slope was steep, demonstrated heavy soil denudation. This denudation is thought to be caused by surface runoff during the heavy rains. Temporal changes in soil movement, the variations in accumulation and denudation, were large in the head hollow and the footslope and depended on the amount of precipitation. In the crest slope, the upper sideslope and the headmost wall, erosion was found to occur from spring to autumn, while deposition occurred from autumn to spring. Porosity in the low suction range, which increases permeability, showed high values on the crest slopes and the upper sideslopes. Porosity in the high suction range, which increases water retention, showed high values on the upper sideslopes and the head hollow. The soil water content in situ was found to be in proportion to the level of high suction range porosity. On the other hand, the soil water content in situ was in inverse proportion to the level of low suction range porosity. From the relationship between physical properties of surface soils and erosion rates, it is clear that the soils which have high level of low suction range porosity show less erosion rate.
