Mechanistic differences of leaf and ecosystem-scale water use efficiencies on the Qinghai-Tibet Plateau

Water use efficiency (WUE) is an important indicator of carbon and water cycles in terrestrial ecosystems. However, little is known about differences in water use efficiency at the leaf scale (WUE_Leaf) and ecosystem-scale (WUE_ECO) and response to environmental variables, particularly in plateau ecosystems with gradient effects. We obtained leaf carbon isotope data and calculated leaf-scale water use efficiency on the Qinghai-Tibet Plateau through field surveys and literature collection and calculated ecosystem-scale water use efficiency based on remote sensing data (MODIS). The study analyzed the differences between leaf-scale WUE and ecosystem-scale WUE in terms of vegetation type and spatial distribution and explored the response of water use efficiency to changes in environmental factors at both scales. The results found that the two water use efficiency scales showed different vegetation type trends and spatial distribution. At the leaf scale, WUE_Leaf showed grasses (10.91 mmol/mol) > trees (9.55 mmol/mol) > shrubs (8.34 mmol/mol), and spatially as a whole showed higher in the western high altitudes (Grasses) than in the low eastern altitudes (Trees). In contrast, at the ecosystem scale, WUE_Eco shows trees (1.17 g C/kg H2O) > shrubs (1.05 g C/kg H2O) > grasses (0.53 g C/kg H2O), while at the spatial scale, the eastern low elevation region (Forests) is greater than the western high elevation region (Grasslands). Climate (MAT) and vegetation (EVI) factors are the most important environmental variables affecting the variation of WUE at leaf and ecosystem scales, respectively, on the Tibetan plateau. The effect of altitude on water use efficiency is not caused by the vegetation type, although the WUE varies among vegetation types. Conversely, the effect of elevation is influenced by the interaction between environmental conditions and vegetation. These results suggest that the appropriate water use efficiency scale should be selected for specific purposes in carbon and water cycle studies. When the focus is on the influence of climate on the carbon-water cycle, leaf-scale water use efficiency is more appropriate, while if the effect of vegetation, ecosystem-scale water use efficiency would be more appropriate.