Altitude-adaptive water use strategies of grassland are constrained by air dryness and stoichiometry in southwest of China

These contrasting patterns reflect distinct eco-physiological strategies. C₃ plants improve iWUE under the cooler, potentially nutrient-limited in high-elevation conditions through conservative resource-use traits. In contrast, the CO₂-concentrating mechanism of C₄ plants appears constrained at lower temperatures, limiting their iWUE. Our findings demonstrate that iWUE patterns are not simply climate-driven but emerge from pathway-specific interactions between climatic gradients and nutrient availabilities. This study provides a mechanistic framework for forecasting shifts in grassland community structure and carbon-water fluxes under future climate change.