Species turnover and climates co-dominate the carbon–water relationship in grasslands along an elevational gradient

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.