Leaf and ecosystem water use effciencies differ in their global-scale patterns and drivers

Water use effciency (WUE) links carbon and water cycling and has been recognized as important in understanding the carbon-water budget of terrestrial ecosystems. However, there are few studies comparing WUE at leaf and ecosystem levels in response to environmental variables on a global scale. Here, we compare global-scale patterns and the drivers of leaf and ecosystem WUEs and quantify the relative influence of biotic and abiotic factors. Using published world-wide δ13C (carbon stable isotope composition) measurements for 6751 C3 plant populations from 174 publications, as well as our own measurements of δ13C for 418 C3 plant populations across drylands in China, and satellite-based datasets of gross primary production and evapotranspiration, we determined global patterns and the drivers of leaf and ecosystem WUEs. Leaf intrinsic WUE (iWUE) and ecosystem WUE displayed almost opposite trends, in response to abiotic factors on a global scale. iWUE was highest in arid regions and lowest in humid regions, whereas ecosystem WUE was lowest in arid regions and highest in humid regions. Phylogeny had a signifcant effect on iWUE. Mean annual temperature (MAT) was the strongest factor in predicting iWUE, whereas the most robust factor in predicting ecosystem WUE was leaf area index (LAI). The data indicate that the two different responses at the leaf and ecosystem levels must be considered when modeling carbon and water balances in response to climate change.水分利用效率（Water Use Efficiency, WUE）连接了碳循环与水循环，在理解陆地生态系统的碳-水收支方面具有重要学术价值。然而，全球范围内对比叶片与生态系统尺度水分利用效率对环境变量响应的研究仍较为匮乏。本研究对比了全球尺度下叶片与生态系统水分利用效率的分布格局及其驱动因子，并量化了生物与非生物因子的相对影响。我们整合了174篇已发表文献中针对全球6751个C3植物种群的碳稳定同位素组成（δ¹³C）实测数据，同时补充了中国干旱区418个C3植物种群的δ¹³C实测数据，此外还使用了基于卫星遥感获取的总初级生产力与蒸散发数据集，以此阐明全球尺度下叶片与生态系统水分利用效率的分布格局与驱动机制。全球尺度下，叶片内在水分利用效率（intrinsic WUE, iWUE）与生态系统水分利用效率对非生物因子的响应趋势几乎完全相反：叶片内在水分利用效率在干旱区域最高、湿润区域最低，而生态系统水分利用效率则在干旱区域最低、湿润区域最高。系统发育对叶片内在水分利用效率具有显著影响。年平均气温（mean annual temperature, MAT）是预测叶片内在水分利用效率的最强驱动因子，而预测生态系统水分利用效率的最稳健因子则为叶面积指数（leaf area index, LAI）。本研究结果表明，在模拟气候变化背景下的碳-水平衡时，必须考虑叶片与生态系统尺度下的两种不同响应特征。