Differences in the patterns and mechanisms of leaf and ecosystem-scale water use efficiencies on the Qinghai-Tibet Plateau

Water use efficiency (WUE) can be employed to characterize the carbon and water cycle of ecosystems. However, little is known about spatial differences in WUE at the leaf (WUELeaf) and ecosystem (WUEEco) scales regarding their responses to environmental variables and elevation (ELE). To understand the underlying drivers of this difference, we obtained the WUELeaf from carbon isotope composition of leaves from previously published reports and our own measurements from the Tibetan Plateau and calculated WUEEco using remote sensing method. The results suggested that the WUE showed different patterns at different scales. Along the elevation gradients, opposite trends of WUE in leaf and ecosystem scales were observed. In the altitude interval of 2000–6000 m a.s.l., WUELeaf increased significantly when the altitude was greater than 3500 m, while WUEEco decreased significantly when the altitude was greater than 4500 m. Specifically, the highest WUELeaf was observed in grasses (10.91 ± 3.34 mmol/mol) in the northwest, while the highest WUEEco existed in forests (1.18 ± 0.57 g C/kg H2O) in the southeast. The most important parameters for the variations of WUELeaf and WUEEco were the climate and vegetation, respectively. The variation of WUE along elevation was not caused by the direct influence of vegetation type, although the WUEs of different vegetations varied. Instead, the interaction between environment and vegetation contributed to the ELE effect on WUE for both scales. These results indicated that the proper scale of WUE for carbon–water coupling research should be selected according to the specific purpose.