Azimuthal variations of sap flux density within Japanese cypress xylem trunks and their effects on tree transpiration estimates

Abstract

Sap flow techniques are practical tools for estimating tree transpiration. Though many previous studies using sap flow techniques did not consider azimuthal variations of sap flux density (F_d) on xylem trunk to estimate tree transpiration, a few studies reported that ignoring the azimuthal variations in F_d could cause large errors in tree transpiration estimates for some tree species. Therefore, examining azimuthal variations in F_d for major plantation tree species is critical for estimating tree transpiration. Using the thermal dissipation method, we examined azimuthal variations in F_d in six trees of Japanese cypress Chamaecyparis obtusa (Sieb. et Zucc.) Endl., which is one of the most common plantation tree species in Japan. We recorded considerable variations among F_d at four different azimuthal directions. The F_d value for one aspect was more than 100% larger than those for the other aspects. We calculated differences between tree transpiration estimates based on F_d for one to three azimuthal directions and those based on F_d for four aspects. The differences relative to tree transpiration estimates based on F_d for four aspects were typically 30, 20, and 10% in accordance with the F_d for one, two, and three measurement aspects, respectively. This finding indicates that ignoring azimuthal variations could cause large errors in tree transpiration estimates for Japanese cypress.