Water use efficiency (WUE) serves as a core function reflecting vegetation-climate interactions and terrestrial carbon-water cycle. However, direct measurements of WUE are largely constrained to indoor experiments, individual plant, and instantaneous physiological reaction, and neglect the effects of long-term climate change and species composition of natural ecosystems.

Here, we built an elevational gradient (40-3800m), which can be viewed as an open-air laboratory for studying plant physiological responses to climate change and species adaptation, and we conducted an intensive field survey (98 sites) and collected datasets (plants, climates, soils) to investigate the geospatial of instinct water use efficiency (iWUE) of grasslands and its potential drivers.

We found an uptrend and downtrend in iWUE for C₃ and C₄ herbs in grasslands with the rising elevation, respectively. Those patterns of iWUE primarily driven by variations in microclimatic factors (e.g,. temperature, vapor pressure deficit), species turnover, and plant stoichiometry rather than species richness.

These results challenge the prevailing climate-centric paradigm in WUE studies, highlighting the critical role of species turnover and nutrient allocation strategies with environmental change. Our results revealed the integrated interactions between species composition and climates could more precisely predict shifts in WUE, thereby providing empirical evidence to strengthen in-depth understanding of carbon-water cycles under future climate change.