森林構造が示す斜面崩壊の歴史 : 東広島市キャンパス・ががら山の事例

Variations in tree species composition, tree density, and basal area were studied in relation to microtopography along the hills of Mt. Gagara within the campus of Hiroshima University, Higashi-Hiroshima, where landslides have occurred frequently. Ten sampling plots with a size of 100 m2 (10 m×10 m) were positioned in upper and lower parts of the hills. Tree density was significantly higher in the lower parts than in the upper parts of the hills (p < 0.001). The density of trees with smaller diameters at breast height (DBH; < 13 cm ) was significantly higher in the lower parts than in the upper parts of the hills (p < 0.001), whereas the density of large- or middlesized trees (DBH ≧ 13 cm) was signifi cantly higher in the upper parts of the hills. The species composition in the lower hills mostly consisted of early successional tree species (e.g., Eurya japonica, Pieris japonica, Ilex pedunculosa, and Rhododendron reticulatum), whereas the upper parts of the hills rarely or never included such species but mainly consisted of late successional tree species (e.g., Symplocos lucida, and Quercus glauca). These trends indicate that forest vegetation on the lower parts of the hills has been disturbed by frequently occurring erosion and landslides, which have resulted in suppressed maturity of forest structure and species succession.