Impact of Forest Harvesting on Slope Stability in Managed Forest Catchment, Nara Prefecture, Japan

摘要

Forest harvesting that induces the root decay of logged trees and the root recovery of replanted trees affects stability of slopes over time; these factors influence the occurrence of shallow landslides. Landslides cause loss of life and property; thus, the occurrence of landslides is considered to be one of the most detrimental effects of forest harvesting. Although many studies have been carried out to evaluate the effect of forest harvesting on the occurrence of landslides, the impact of harvesting differs in each study due to dissimilar site conditions (i.e., geology, physiography, and climate). Thus, field data that can neglect the influence of these complex conditions is necessary for testing simulation model predictions of the effects of forest harvesting on landslide occurrences. Using aerial photograph investigations and field surveys, we examined the occurrence of landslides with regard to forest harvesting in the Sanko catchment (Nara prefecture, central Japan), where rotational management yields a mosaic of different forest ages over the years. In the Sanko catchment, geology and slope gradient are rather uniformly distributed, allowing the evaluation of the forest harvesting effect by ignoring other factors. The trends in the landslide frequency in the managed forests correspond to changes in slope stability; this is explained by root strength decay and recovery. The impact of landslide occurrence can be summarized to be strongest for 1-10 yr after forest harvesting and smaller impacts continue up to 25 yr after harvesting. In the Sanko catchment, the influence of forest harvesting on landslide occurrence is more evident in steep slopes as compared to gentler slopes. Only smaller landslides (< 100 m~3) whose slide surface might be shallower than root depth are accelerated by forest harvesting, while the effect of forest harvesting is not obvious for larger landslides (> 100 m~3). These characteristics of landslide frequency relevant to forest harvesting can be demonstrated by landslide models based on infinite slope stability.