Suspended-sediment responses after strip thinning in headwater catchments

摘要

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 (K (T) : 17.1 ha) and control (K (C) : 8.9 ha) catchments. Annual suspended-sediment yield (SSYan) in the prethinning period in the K (T) and K (C) catchments was 110.0 and 142.1 kg/ha per year. For the postthinning period, SSYan in the K (T) catchment became 5055.6 kg/ha per year, whereas that in the K (C) catchment increased 893.2 kg/ha per year. The paired-catchment analysis revealed that SSY in the K (T) 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 K (T) catchment was associated with the combined impacts of thinning and sequences of storm events during the period of thinning operation.