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.

水资源利用效率（WUE）将碳循环与水循环联系起来，并在理解陆地生态系统的碳-水收支方面被认为至关重要。然而，在全球范围内，关于叶片和生态系统水平上WUE对环境变量响应的比较研究为数不多。在本研究中，我们比较了叶片和生态系统WUE的全球尺度模式和驱动因素，并量化了生物和非生物因素的相对影响。通过使用来自174篇出版物中6751个C3植物种群的世界范围内δ13C（碳稳定同位素组成）测量值，以及我们在中国干旱地区对418个C3植物种群进行的δ13C自行测量，以及基于卫星的净初级生产力和蒸散量数据集，我们确定了叶片和生态系统WUE的全球模式和驱动因素。叶片固有WUE（iWUE）和生态系统WUE在全球尺度上对非生物因素的响应趋势几乎相反。iWUE在干旱地区最高，在湿润地区最低，而生态系统WUE在干旱地区最低，在湿润地区最高。系统发育对iWUE有显著影响。平均年温度（MAT）是预测iWUE的最强因素，而预测生态系统WUE的最稳健因素是叶面积指数（LAI）。数据表明，在建模碳和水的平衡以应对气候变化时，必须考虑叶片和生态系统水平上的两种不同响应。