Suspended-sediment responses after strip thinning in headwater catchments

Abstract

We examined changes in suspended-sediment yields (SSY) after a 50 % strip thinning in headwater streams draining a Japanese cedar (Cryptomeria japonica) and cypress (Chamaecyparis obtusa) plantation forest. We applied a paired-catchment analysis to treated (KT: 17.1 ha) and control (KC: 8.9 ha) catchments. Annual suspended-sediment yield (SSYan) in the prethinning period in the KT and KC catchments was 110.0 and 142.1 kg/ha per year. For the postthinning period, SSYan in the KT catchment became 5055.6 kg/ha per year, whereas that in the KC catchment increased 893.2 kg/ha per year. The paired-catchment analysis revealed that SSY in the KT catchment increased 17.0-fold compared with the prethinning period. However, the dominant hysteresis pattern remained clockwise in both pre- and postthinning periods. Sequences of large storm events in the postthinning period elevated SSY in both catchments. Increase in suspended sediment in the KT catchment was associated with the combined impacts of thinning and sequences of storm events during the period of thinning operation.