Effects of forest thinning on sap flow dynamics and transpiration in a Japanese cedar forest

Historically, forest thinning in Japan was conducted to obtain high-quality timber from plantations. Today, in contrast, thinning is also motivated by forest water balance and climate change considerations. It is in this context that the present study examines the effects of thinning on the ecophysiological responses of remaining trees, which are inadequately understood, especially in relation to changes in the magnitude and duration of transpiration. Sap flux densities were measured in both outer and inner sapwood to obtain stand-scale transpiration for two years in the pre-thinning state and three years post-thinning. The effects of thinning on transpiration were quantitatively evaluated based on canopy conductance models. The larger increases in outer sap flux density were found in the first year after the treatment, while those in inner sap flux density were detected in the second and third years. The remaining trees required a few of years to adjust to improved light conditions of the lower crown, resulting in a delayed response of inner sap flux density. As a result of this lag, transpiration was reduced to 71 % of the pre-thinning condition in the first year, but transpiration recovered to the pre-thinning levels in the second and third years due to compensating contributions from inner sap flow. In terms of more accurately chronicling the thinning effect, the distribution of sap flux density with respect to its radial pattern, is necessary. Such measurements are key to more comprehensively examining the ecophysiological response of forest plantations to thinning and, ultimately, its effect on the forest water balance.