Changes in canopy transpiration of Japanese cypress and Japanese cedar plantations because of selective thinning

Canopy transpiration (E_stand) is a main component of forest water cycle, and forest management including thinning affects canopy transpiration E_stand, yet few studies have examined the changes in E_stand. In this study, we investigated the changes in E_stand after the thinning of Chamaecyparis obtusa and Cryptomeria japonica stands by 50% of their stems. The E_stand was estimated using sap flow measurements by Granier-type thermal dissipation probes. The mean stand sap flow density (J_S) after thinning was as high as that before thinning in both plots. Under the same conditions of microclimate, E_stand after thinning decreased by 44.0 and 21.2% for the Ch. obtusa plot and the Cr. japonica plot, respectively. The thinning reduced the sapwood area by 44.5% in the Ch. obtusa plot and 34.2% in the Cr. japonica plot. The decreases in E_stand were similar to the decline in sapwood area, which indicates that the primary cause of the changes in E_stand was the reduction of sapwood area. The reference canopy conductance (G_cref) after thinning was 52.7% and 75.3% lower for Ch. obtusa and Cr. japonica, respectively, whereas stomatal sensitivity to the vapour pressure deficit was similar in both periods. Thus, the changes in canopy conductance after thinning were associated with changes in G_cref, resulting in a decrease of E_stand.