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

Water use efficiency (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 significant 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.