Circumferential sap flow variation in the trunks of Japanese cedar and cypress trees growing on a steep slope

In this study, we conducted sap flow measurements in Japanese cedar and cypress trees growing on a steep slope to examine circumferential variation. Sap flow measurements were conducted for upper and lower slope aspects and in four directions (north, east, south, and west). We also measured the width of the tree crown to examine the effect of sunlight. Japanese cedar and cypress growing at this site extended their crowns toward the lower slope. Individual trees displayed circumferential variation in sap flux density (Fd). For Japanese cedar and cypress, the maximum daily Fd were 1.92 and 3.80 times as large as the minimum, respectively. However, the circumferential variation in Fd did not appear to be dependent on direction or slope aspect. These results suggest that large errors are produced when circumferential variation in Fd is ignored during the estimation of whole tree transpiration. Therefore, it is necessary to use sensors to capture circumferential variation in Fd, but sensors can be inserted randomly without the need to consider the shape of the tree crown or the direction of the tree trunk.