Effect of Forest Harvesting on Hydrogeomorphic Processes in Steep Terrain of Central Japan

Timber harvesting activities affect various hydrogeomorphic processes in forest terrain, including enhancement of surface erosion, increases in landslides and debris flows, and changes in hillslope or catchment hydrology. Thus, the influence of the timing of harvesting on these processes needs to be assessed to protect aquatic ecosystems and develop improved strategies for disaster prevention. We examined the effects of forest harvesting on the frequency of landslides and debris flows in the Sanko catchment (central Japan) using nine aerial photo periods covering 1964 to 2003. We also investigated the spatial distribution of sediment storage (i.e., landslide sediment and channel deposits), and sediment transport rate in the Sanko catchment. Aerial photographs showed a mosaic of different forest ages attributable to the rotational timber harvesting in this area since 1912. Trends of new landslides and debris flows correspond to changes in slope stability explained by root strength decay and recovery. The impact of forest harvesting on occurrence of landslides is large in steep terrain (i.e., slope gradients > 30°). Occurrences of mass movement substantially vary volume of sediment storage that links sediment supply from hillslopes and sediment yield downstream. However, volumes and spatial distribution of sediment deposits are not clearly related to forest age because of the influence of other factors (e.g., rainfall magnitude, local topography). Bedload transport rate is highly affected by sediment storage volume, whereas suspended sediment is affected by both storage volume and area of bare land along channels formed by debris flows and bank erosion. Thus, the effect of harvesting on hydrogeomorphic processes cannot be estimated simply by the elapsed time after timber removal; topography, rainfall magnitude, type of processes, and linkages with other processes need to be considered.