Comment on hess-2022-328

Abstract

Tree transpiration considerably contributes to evaporative fluxes to the atmosphere in terrestrial ecosystems. Accurate transpiration quantification promotes the knowledge of water consumption by forests and could favour an adaptive forest management, especially in a global change context. Tree transpiration can be measured by a wide range of methods, and one of the valued ones is sap flow measurements. However, species-specific validations of techniques are required. Hence the objectives of this study were to validate transpiration rate measurements by the heat ratio method (HRM) in juvenile Aleppo pine trees (Pinus halepensis Mill.) by using the probe misalignment correction proposed by Larsen et al. (2020). This study simultaneously recorded the transpiration rate by tree sap flow following the HRM technique (THRM) and tree water losses by load cells (TOBS). These measurements were taken in combination with the environmental variables that control this process such as different vapour pressure deficit (VPD) ranges of air and the soil relative extractable water (REW). The results showed an accurate linear correspondence between TOBS and the transpiration rate measurements both without and with probe misalignment correction, THRM and THRM MIS, respectively, but interestingly underestimations at high transpiration rates were observed. However, underestimations were removed when applying probe misalignment correction. THRM MIS showed a good relation between the VPDxREW interaction. This study supports the notion that HRM offers accurate low values under a wide range of abiotic conditions, and is useful in isohydric species with low transpirations rates like Aleppo pine. To conclude, our results support the validation of both transpiration rate measurements by the THRM and probe misalignment correction in Aleppo pine under different environmental laboratory conditions.