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

Abstract

<h4>Introduction</h4>Understanding the elevational patterns of intrinsic water-use efficiency (iWUE) and their drivers is crucial for predicting plant adaptation and ecosystem responses to climate change. However, how iWUE in different photosynthetic pathways (C₃ vs C₄) varies with elevation, which is interactively shaped by climate and nutrient constraints remains unclear.<h4>Methods</h4>Here, we integrated stable carbon (δ¹³C) and oxygen (δ¹⁸O) isotopes with plant-soil stoichiometry across a grassland elevation transect to interpret these mechanisms.<h4>Results</h4>Our results reveal a fundamental divergence in the response of iWUE to elevation: iWUE increased significantly in C₃ grasses but decreased slightly in C₄ grasses. Using a machine learning approach, we identified vapor pressure deficit (VPD) and leaf stoichiometry (C:P and N:P ratios) as key drivers to shape the altitudinal patterns of iWUE. However, these factors exhibited opposing effects: VPD was negatively correlated with iWUE in C₃ species but positively correlated in C₄ species.<h4>Discussion</h4>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.