Laboratory of Tree Ring Research, University of Arizona, Tucson, AZ, USA.
Department of Geospatial Science, Radford University, Radford, VA, USA.
Glob Chang Biol. 2021 Apr;27(8):1560-1571. doi: 10.1111/gcb.15491. Epub 2021 Jan 19.
Increasing water-use efficiency (WUE), the ratio of carbon gain to water loss, is a key mechanism that enhances carbon uptake by terrestrial vegetation under rising atmospheric CO (c ). Existing theory and empirical evidence suggest a proportional WUE increase in response to rising c as plants maintain a relatively constant ratio between the leaf intercellular (c ) and ambient (c ) partial CO pressure (c /c ). This has been hypothesized as the main driver of the strengthening of the terrestrial carbon sink over the recent decades. However, proportionality may not characterize CO effects on WUE on longer time-scales and the role of climate in modulating these effects is uncertain. Here, we evaluate long-term WUE responses to c and climate from 1901 to 2012 CE by reconstructing intrinsic WUE (iWUE, the ratio of photosynthesis to stomatal conductance) using carbon isotopes in tree rings across temperate forests in the northeastern USA. We show that iWUE increased steadily from 1901 to 1975 CE but remained constant thereafter despite continuously rising c . This finding is consistent with a passive physiological response to c and coincides with a shift to significantly wetter conditions across the region. Tree physiology was driven by summer moisture at multi-decadal time-scales and did not maintain a constant c /c in response to rising c indicating that a point was reached where rising CO had a diminishing effect on tree iWUE. Our results challenge the mechanism, magnitude, and persistence of CO 's effect on iWUE with significant implications for projections of terrestrial productivity under a changing climate.
提高水分利用效率（WUE），即碳增益与水分损失的比率，是增强陆地植被在大气 CO 上升（c）条件下碳吸收的关键机制。现有理论和经验证据表明，随着植物维持叶片胞间（c）与环境（c）部分 CO 压力（c/c）之间相对恒定的比例，WUE 会呈比例增加。这被假设为近几十年来陆地碳汇增强的主要驱动因素。然而，比例性可能无法描述 CO 对 WUE 的长期影响，且气候在调节这些影响方面的作用尚不确定。在这里，我们通过使用美国东北部温带森林树木年轮中的碳同位素来重建内在水分利用效率（iWUE，光合作用与气孔导度的比率），评估了 1901 年至 2012 年 CE 年间 c 和气候对 WUE 的长期响应。我们发现，尽管 c 持续上升，但 iWUE 从 1901 年到 1975 年 CE 稳步增加，但此后保持不变。这一发现与对 c 的被动生理响应一致，且与该地区气候明显变湿的趋势相吻合。树木生理学在数十年的时间尺度上受夏季水分的驱动，并且没有维持恒定的 c/c 以响应 c 的上升，这表明上升的 CO 对树木 iWUE 的影响达到了一个临界点。我们的研究结果对 CO 对 iWUE 的影响的机制、幅度和持续性提出了挑战，这对气候变化下陆地生产力的预测具有重要意义。
