Sap flow dynamics and transpiration in a Japanese cedar forest: effects of thinning

Abstract

Although decreases in forest transpiration by thinning have been confirmed, the duration of decreased transpiration post-thinning is unknown in Japan. We evaluated transpiration based on outer (0-2 cm depth) and inner (>2cm depth) sap flux density measurements in a Japanese cedar forest for two years before and three years post-thinning during which 38% of cedar trees were cut, equating to 43% of the basal area. Transpiration decreased to 71% of the pre-thinning condition in the first year, while in second and third years, it was 100% and 107%, respectively. The recovery of transpiration was attributed to remarkable increases of inner sap flow, strongly suggesting enhancement of transpiration activity in the lower part of crowns caused by improvement of solar radiation conditions due to thinning. This work has demonstrated that inclusion of both inner and outer sap flux densities is necessary to fully account for changes in transpiration due to thinning.