Long-term effects of thinning on runoff change from coniferous forest plantations in headwater catchments

Forest management is crucially important for the sustain- ability of water sources in watersheds because forests are important water-related ecosystem components. Thinning is one technique that affects runoff change. For this study, we examined thinning effects on long-term runoff, based on monthly runoff and rainfall data for a coniferous forest plantation in the Hitachi Ohta watershed, Japan. Paired catchment experiments were used to evaluate water yield. Monitoring was done pre- and post-thinning during 2006- 2021. In 2009, we removed 50% of the trees in catchment HV by thinning management, whereas catchment HB was left as a control catchment. The mean monthly runoff and runoff coefficient of catchment HV were higher than those of catchment HB throughout the post-thinning periods. Fur- thermore, 105.71 mm/year and 142.97 mm/year increases were found, respectively, in yearly water yields for the first (2010-2012) and latter (2018-2021) examination periods. About 12 years after thinning, the thinning effects are expected to continue. Results show that long-term study is necessary to assess forest management in Japan.