Precise determination of canopy conductance (g_s) 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 (D_L) derived from canopy surface temperature, and tree transpiration to calculate canopy g_s 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 g_s and compared with g_s 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 D_L 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 g_s, 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 D_L and air saturation vapour pressure deficit at 8.2 m in the crown (D₈). Under such conditions the GT’ method overestimated g_s 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 environmental drivers. Our results also advocate the use of in situ surface temperature measurements to better understand the response of plant canopies to environmental conditions.

需要精确测定冠层电导 ( g _s ) 以量化森林冠层的水分流失和 CO ₂交换以及它们对不断变化的环境条件的响应。在这项研究中，我们结合了从树冠表面温度和树木蒸腾量得出的叶与空气蒸汽压差 ( DL ) 的测量值，以计算 ICOS 站点 FR- Bil松树林中_的树冠gs。所涉时期的特点是连续两次干旱。广义输水方程 (GT) 的反演及其等温简化 (GT') 用于计算冠层g _s并与g _{s进行比较}由 Penman-Monteith 方程 (PMT) 的反演确定。热红外摄像机连续监测冠层温度，并允许以半小时分辨率评估D _{L的时间进程。}平均而言，树冠与周围空气之间的温差为 0.3 °C，数值范围为$-$2 °C 至 +5 °C，具体取决于一天中的时间、土壤湿度和湿度。用于计算g _s、 GT、 GT' 和 PMT 的三种方法在潮湿土壤和低大气需求下是一致的，但在水分胁迫条件下发现差异高达 40% D _L和顶部 8.2 m 处的空气饱和蒸汽压不足 ( D ₈ )。在这种情况下，GT' 方法高估了g _s而 PMT 方法更接近于用 GT 方法估计的值。树冠和整个生态系统之间大气交换的对比行为限制了仅使用上述冠层通量测量来量化表面电导对环境驱动因素的响应。我们的研究结果还提倡使用原位表面温度测量来更好地了解植物冠层对环境条件的响应。