The carbon isotope ratio (δ¹³C) in tree rings is commonly used to derive estimates of the assimilation‐to‐stomatal conductance rate of trees, that is, intrinsic water‐use efficiency (iWUE). Recent studies have observed increased iWUE in response to rising atmospheric CO₂ concentrations (C _a), in many different species, genera and biomes. However, increasing rates of iWUE vary widely from one study to another, likely because numerous covarying factors are involved. Here, we quantified changes in iWUE of two widely distributed boreal conifers using tree samples from a forest inventory network that were collected across a wide range of growing conditions (assessed using the site index, SI), developmental stages and stand histories. Using tree‐ring isotopes analysis, we assessed the magnitude of increase in iWUE after accounting for the effects of tree size, stand age, nitrogen deposition, climate and SI. We also estimated how growth conditions have modulated tree physiological responses to rising C _a. We found that increases in tree size and stand age greatly influenced iWUE. The effect of C _a on iWUE was strongly reduced after accounting for these two variables. iWUE increased in response to C _a, mostly in trees growing on fertile stands, whereas iWUE remained almost unchanged on poor sites. Our results suggest that past studies could have overestimated the CO₂ effect on iWUE, potentially leading to biased inferences about the future net carbon balance of the boreal forest. We also observed that this CO₂ effect is weakening, which could affect the future capacity of trees to resist and recover from drought episodes.

中文翻译：

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在树木年轮研究中强烈高估了水利用效率对二氧化碳上升的响应。

碳同位素比率（δ ¹³在树的年轮C）通常用于树木的同化对气孔导率，即，固有水分利用效率（IWUE）的派生估算。最近的研究发现，随着大气中CO ₂浓度的升高，iWUE增加（C _a），属和生物群落。但是，从一项研究到另一项研究，iWUE的发生率差异很大，可能是因为涉及许多共变因素。在这里，我们使用来自森林资源清单网络的树木样本量化了两个广泛分布的北方针叶树的iWUE的变化，这些样本是在广泛的生长条件下（使用站点指数SI评估），发育阶段和林分历史记录收集的。使用树木年轮同位素分析，我们在考虑了树木大小，林分年龄，氮沉降，气候和SI的影响后，评估了iWUE的增加幅度。我们还估计了生长条件如何调节树木对C _a上升的生理反应。我们发现树木大小和林分年龄的增加极大地影响了iWUE。考虑到这两个变量后，C _a对iWUE的影响大大降低。iWUE响应于C _a而增加，主要是在肥沃的林木上生长，而iWUE在贫困地区几乎保持不变。我们的研究结果表明，以往的研究可能高估了iWUE的CO ₂效应，可能导致对北方森林的未来净碳平衡的偏见。我们还观察到，这种CO ₂效应正在减弱，这可能会影响树木抵御干旱事件并从干旱中恢复的未来能力。