Evaluation of forest surface soil on preventive function of forest against landslides, 1: Change in quality of forest surface soil by irradiating artificial sunrays

During the heavy rainfall which may cause many landslides, forest surface soil is playing many important roles such as infiltration, storage and drainage of water to prevent the occurrence of landslide. Therefore, the relationship between the water behavior and the change of physical properties in forest surface soil was investigated. Soil samples were collected in about 30 years old stand of hinoki, Chamaecyparis obtusa S. and in about 80 years old stand of sugi, Cryptomeria japonica D.Don. On the assumption that the forest was clearly cut, these soil samples were exposed to the change in quality through the irradiation of artificial sunrays by using the weather meter. Comparing the volume of each pore size of surface soil before and after the irradiation, the quantity of larger pore size, which is related to the movement of gravitational water, increased after the irradiation and that of smaller pore size which is related to the soil retention storage, decreased. The degree of aggregation of surface soil also decreased after the irradiation. These influences were more remarkable in stand of sugi than hinoki. If the physical properties of surface soil change through the clear cutting of forest, it is estimated that more infiltrated water would rapidly move downward and cause the decrease of soil shear resistance related to the occurrence of landslide.