Quantifying canopy conductance in a pine forest during drought from combined sap flow and canopy surface temperature measurements

• Published in: Agricultural and forest meteorology 2022-08, Vol.323, p.108997, Article 108997
• Main Authors: Taborski, Tom, Domec, Jean-Christophe, Chipeaux, Christophe, Devert, Nicolas, Lafont, Sébastien, Wingate, Lisa, Loustau, Denis
• Format: Article
• Language: English
• Subjects: Canopy conductance; Environmental Sciences; Leaf temperature; Leaf-to-air vapour pressure deficit; Penman-Monteith; Sap-flux; Transpiration
• ISSN: 0168-1923; 1873-2240
• DOI: 10.1016/j.agrformet.2022.108997

•Measurement of canopy temperature using near infrared camera enhances the accuracy of canopy conductance determination.•Isothermal assumption leads to substantial bias in the stomatal conductance determination.•The Penman-Monteith equation applied to the canopy layer and combined with a radiative transfer model provides reliable estimates of canopy conductance.•Canopy temperature of a pinus pinaster stand is strongly coupled with the atmosphere.•The use of an infrared camera improve stomatal conductance estimation.•Isothermal assumption leads to bias in the canopy stomatal conductance determination.•At our site, Penman-Monteith inversion with eddy covariance inputs is not reliable.•The divergence between studied methods is higher when the water stress is low. Precise determination of canopy conductance (gs) is needed to quantify the water loss and CO2 exchange from forest canopies and their response to changing environmental conditions. In this study, we combined measurements of the leaf-to-air vapour pressure difference (DL) derived from canopy surface temperature, and tree transpiration to calculate canopy gs in a pine forest at the ICOS site FR-Bil. The period covered was characterized by two consecutive droughts. The inversion of the generalised water transport equation (GT), along with its isothermal simplification (GT’), were used to calculate canopy gs and compared with gs determined from the inversion of the Penman-Monteith equation (PMT). A thermal infrared camera continuously monitored the canopy temperature and allowed to assess the time course of DL with half-hourly resolution. On average a 0.3 °C temperature difference was found between the canopy and surrounding air, with values ranging from −2 °C to +5 °C depending on the time of day, soil moisture and humidity. The three methods used to calculate gs, GT, GT’ and PMT were in agreement under wet soil and low atmospheric demand, but differences up to 40% were found under water stress conditions when the canopy to air temperature differences led to substantial discrepancies between DL and air saturation vapour pressure deficit at 8.2 m in the crown (D8). Under such conditions the GT’ method overestimated gs whereas the PMT method was closer from values estimated with the GT method. The contrasted behaviour of the atmospheric exchanges between the tree canopy and the overall ecosystem limits the use of the above canopy flux measurements alone to quantify the response of surface conductance to e