Water use efficiency (WUE), the amount of assimilated carbon per amount of transpired water is central to assess how trees and forests respond to the ongoing climate changes and, particularly, to increasing drought severity. We concurrently evaluated leaf- and ecosystem-scale intrinsic WUE (WUEi) of a Mediterranean evergreen oak (Quercus ilex) forest in southern France using stable carbon isotopic discrimination, leaf gas exchange and eddy covariance fluxes. Nine years of continuous daily eddy fluxes were converted into 7-day composite chronicle of WUEi and then into δ13C input to check, is there a link between eddy and bulk leaf δ13C? The relationship between ecosystem-scale WUEi and water limitation, assessed by the predawn tree potential (ψpd), followed a trapezoidal curve with a plateau value of 158.0 ± 4.0 µmol CO2 mol-1 H2O. WUEi began to decline from -2.4 MPa with gas exchange ceasing at -4.4 MPa. WUEi measured at leaf scale followed a bi-linear relationship with ψpd that peaked at ψpd = -3.15 MPa, reaching 120.5 ± 16.4 µmol CO2 mol-1 H2O. Current year leaf δ13C is linearly related to the averaged δ13C input over the leaf expansion period. In 1-year old leaves, adjustment in bulk δ13C was dependent on the degree of water limitation. Whole-isotopic mass balance at tree or ecosystem scales continue to be a major challenge because large gaps remain in understanding all the processes that influence δ13C of fluxes and C pools.

中文翻译：

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干旱对地中海常绿橡树林内在水分利用效率影响的多尺度分析


水利用效率 （WUE），即每蒸腾水量的同化碳量，是评估树木和森林如何应对持续气候变化，特别是应对日益严重的干旱的核心。我们同时使用稳定的碳同位素鉴别、叶片气体交换和涡度相关通量评估了法国南部地中海常绿橡树 （Quercus ilex） 森林的叶片和生态系统尺度的内禀 WUE （WUEi）。将 9 年连续的日涡流通量转换为 WUEi 的 7 天复合编年史，然后转换为 δ13C 输入以检查涡流和散装叶片 δ13C 之间是否存在联系？通过黎明前树木潜力 （ψpd） 评估生态系统尺度 WUEi 与水限制之间的关系，遵循梯形曲线，平台值为 158.0 ± 4.0 μmol CO2 mol-1 H2O。WUEi 从 -2.4 MPa 开始下降，气体交换在 -4.4 MPa 停止。在叶片尺度上测量的 WUEi 与 ψpd 呈双线性关系，在 ψpd = -3.15 MPa 时达到峰值，达到 120.5 ± 16.4 μmol CO2 mol-1 H2O。当年叶 δ13C 与叶片扩张期间的平均 δ13C 输入呈线性关系。在 1 年生叶中，体积 δ13C 的调整取决于水分限制的程度。树木或生态系统尺度的全同位素质量平衡仍然是一个重大挑战，因为在理解影响通量 δ13C 和 C 池的所有过程方面仍然存在巨大差距。