2004 Volume 57 Issue 4 Pages 3-14
In order to understand the relationship between plant communities and surface failures on mountain slopes, Hiroshima city and its surrounding area's hillsides slope of granite mountain ranges have been surveyed. The surveyed areas are the same in terms of climate, topography, geology, altitude, inclination of the slope and the condition of the soil, but plant communities are different. The hillside slope is covered with soil of weathered rock whose thickness is more than 50 centimeters and surface failures occurred at the upper sideslope that is generally known as the stable topography. In the surveyed regions, natural evergreen broad-leaved forest, substitution forest of summer-green tree, substitution forest of Japanese red pine, and an artificial hinoki forest are distributed throughout the area. The vegetation of the place where surface failures occurred on the slope consisted of Japanese red pine forest whose forest floor is covered by a fern dominantly and young hinoki forest. The dominance of the fern indicates that diseases or pests damaged the Japanese red pine. The common features of the forest where surface failures occurred on the slope are: dominance by the fern in forest floor, less than 70 percent vegetation cover in the canopy layer, more than 60 percent vegetation cover in field stratum, less than 30 percent mingling of vegetation with the evergreen broad-leaved species and a tree height of less than 10 meters. On the other hand, the common forest features on the slope where surface failures have not occurred are: more than 70 percent vegetation cover in the canopy layer, less than 30 percent vegetation cover in field stratum, more than 30 percent mingling with evergreen broad-leaved species and tree height of more than 14 meters. When the forest grows on the slope, although the slope basically becomes stable, the frequency of surface failures increases when the canopy layer becomes depressed. This is shown by the extreme increase of vegetation cover by the fern. The vegetation cover of the canopy layer is closely related with the hydrological balance of soil layer mainly caused by evapotranspiration. Surface failures occur due to the unstabilization of surface soil by water saturation of seepage water and movement of underground water. However, the forest that formed the perfect canopy layer has strong influence over evaporation : it can alleviate the seepage of rainfall and it regulates the amount of water contained in the soil. Because of the reasons above, it is inferred that the canopy layer functions to stabilize the soil layer. In contrast, on the slope that Japanese red pine forest whose forest floor is covered by the fern dominantly and young hinoki forests distributed, the function of hydrological balance to the soil by vegetation decreases and the instability of the soil layer occurs earlier, resulting in surface failures.