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

Forest management is crucially important for the sustainability 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. Furthermore, 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.