The balance and potometric sap flow calibration approaches that rely on transpirational pull yield inconsistent estimates of transpiration

Abstract

Calibrating sap flow sensors enhances the accurate estimation of whole-tree transpiration is crucial for understanding forest water use and managing water resources, however calibration approaches that rely on different directional forces to move water through tree stems (push force vs transpirational pull) can result in different calibration coefficients. It remains unclear whether coefficients differ between approaches that use the same directional force. In this study, we compared transpiration estimates obtained from the balance and potometric sap flow calibration approaches using thermal dissipation (TD) sap flow sensors deployed in the same five white birch (Betula platyphylla) trees. We first conducted balance calibration using five intact potted trees, followed by cutting the materials to perform potometric calibration with rootless whole plants. We found that coefficients generated from one calibration approach serve as reliable predictors of reference transpiration when applied to TD measurements obtained from the other approach. Moreover, when applied to a growing season of TD sap flow measurements, balance coefficients yielded transpiration estimates 30 % higher (P < 0.05) than potometric calibrations. The results indicate that the potometric calibration is effective at predicting transpiration at low flow rates; however, as the proportion of high sap flow rates increases, it tends to underestimate transpiration estimates. Future research should focus on enhancing the accuracy of potometric calibration to improve its application in TD measurement studies. This enhancement will facilitate the precise estimation of whole-tree transpiration in the context of climate change, thereby elevating the quality of research in forestry science and promoting the sustainable management of water resources.