Abstract
To understand why moist beech forest developed on top of the western Chugoku Mountains, vegetation and soil studies were carried out in the beech forest on Mt. Garyu. It was identified as a Lindero umbellatae-Fagetum crenatae association which was subdivided into three subassociations and seven variants. The vegetation units were distributed according to the microgeomorphic units that resulted from the water movement and surface geogenetical processes in the area. Six soil types, classified into three soil units were identified in the area. Since the pyroclastic materials that cover the gentle slopes up to the flat top of Mt. Garyu have not been eroded distinctly, they regulate water movement in the soils. This provides the wetmesic environment for the plant communities. It can be called the plain summit effect. The role of pyroclastic materials on the development and divergence of wetmesic beech forest can be estimated in areas affected by the eruptions of numerous volcanoes throughout Japan.
