Calibration of sap flow techniques using the root-ball weighing method in Japanese cedar trees

AbstractSection Key message Sap flow techniques are calibrated by root-ball weighing method evaluating actual transpiration without stem cutting. Our results are similar with cut-segment experiments implying limited effects of stem cutting on calibrations. AbstractSection Abstract In recent years, an increasing number of studies have reported underestimations using sap flow techniques. These studies used cut stem segments and artificial sap flux density generated by water columns or vacuum pumps, which suggests the possible differences between the laboratory calibration results and living trees. Here we used the root-ball weighing method to calibrate the sap flow techniques of the thermal dissipation (TD) and the heat ratio (HR) methods for two young Japanese cedar trees (Cryptomeria japonica). The root systems were dug out and wrapped by an impermeable sheet, then the daily amount of transpiration was measured by the daily weight loss of each tree (including the root ball) using a large electrical balance. Although transpiration was reduced by cutting the roots during root-ball creation, its effect on the calibration performance was negligible. The root-ball weighing method could measure the daily amount of transpiration for > 3 months with suitable water supplies. Given that the wound effect was carefully estimated, the HR method measured the sap flux density with a slight underestimation of 5.8%. The TD method underestimated the sap flux density by 61.4%, which can be reasonably corrected using the newly established calibration curve. Moreover, it was confirmed that new calibration curve was consistent with previous calibration curves reported for conifer species, thus implying that the previous calibrations with cut segments are likely applicable for living trees. Further root-ball experiments are necessary to provide comprehensive understanding of intra- and interspecific differences in the calibration of sap flow techniques.