Biological factor controls the variations in water use efficiency of an alpine meadow during the growing season in a permafrost region of the Qinghai-Tibet Plateau

The alpine meadow located in permafrost is crucial for ecosystem services of the Qinghai-Tibet Plateau (QTP), which is experiencing precipitation changes in most areas. Water use efficiency (WUE) can quantify the inextricable link between carbon assimilation and water loss in terrestrial ecosystems. However, the temporal variations in WUE and its driving factors across different precipitation years still need to be clarified in alpine meadows on the QTP. Therefore, 4-year carbon and water flux data were used to elucidate the mechanisms behind seasonal and interannual variations in WUE of an alpine meadow in the hinterland of the QTP. Noticeable seasonal variations in WUE were observed during the studied period, with the highest value (1.38±0.38 g C kg−1 H2O) occurring during the mid-growing season (MG, starting around 166 DOY), approximately 2 and 3 times those during the late-growing season (LG, starting around 256 DOY and ending around 282 DOY) and early-growing season (EG, starting around 140 DOY), respectively. Standardized total effects in the structural equation models from NDVI to WUE were highest in all seasons, indicating that NDVI was the primary controlling factor for daily WUE variations. Additionally, energy factors (temperature and solar radiation) also significantly influenced daily WUE variations. The highest mean daily WUE (1.23±0.65 g C kg−1 H2O) was in the mild dry year (2016). However, no significant differences were noted in mean daily WUE in severe dry (2015) and wet (2019) years compared to the normal year (2020) during GS. This can be attributed to the varying sensitivity of carbon assimilation and water loss to biotic and abiotic changes across divergent precipitation years, with WUE exhibiting a greater sensitive to gross primary productivity than to evapotranspiration. These findings suggest that alpine meadows have endured in its harsh environment and have adapted to climatic fluctuations through long-term evolution.