Calibration of thermal dissipation sap flow probes for Japanese cedar trees

Canopy transpiration in forests has been often observed using the thermal dissipation method (i.e., Granier-type sensors), which is a common sap flux technique. In this method, the recorded temperature difference between the two probes in a stem has been converted into sap flux using an equation described by Granier (1987) without additional fitting of parameters by users. Recently, several studies pointed out that the use of this equation underestimated the sap flux density for some species. This study examined applicability of this equation to Japanese cedar trees, which is the dominant species in Japan. Thirteen segments of Japanese cedar with the diameter of about 12 cm were sampled in four sites of Japan and Taiwan with different climate conditions. For each segment, we inserted four pairs of sensors and generated sap flow in a stem using a pump. Sap flux density based on the equation by Granier (1987) was underestimated for all segments. We revised the equation to be applied to Japanese cedar trees. If the underestimations were confirmed, it would be better to use not the original equation but the revised equation for canopy transpiration estimates of Japanese cedar trees.

Granier, A. (1987) Evaluation of transpiration in a Douglas-fir stand by means of sap flow measurements. Tree Physiology. 3: 309-320.