The main source of plant water in alpine meadows dominated by supra-permafrost water under decreasing rain conditions

Climate change has reshaped the eco-hydrological processes of alpine meadows on the Qinghai-Tibetan Plateau, but the response mechanisms of plant water-use strategies to temperature and precipitation changes still lack systematic quantitative analysis. Based on a controlled experiment with warming and precipitation gradients (±30%) in an alpine meadow, this study quantitatively revealed these mechanisms using hydrogen and oxygen stable isotope techniques (δ²H, δ¹⁸O) combined with a Bayesian mixing model (MixSIAR). The results showed that the isotopic composition of plant water exhibited significant seasonal fluctuations, and its enrichment process was synergistically driven by soil warming and moisture deficit. Across all treatments, the 0-20 cm soil water consistently remained the core water source for herbaceous plants (total contribution rate ≥85%). However, the vertical water-use strategies of plants demonstrated high environmental plasticity: precipitation increase induced a significant shallowing of water use (the contribution rate of the 0-10 cm soil layer reached 59%); precipitation decrease drove water use towards deeper layers (the contribution rate of the 10-20 cm soil layer reached 57%), and the contribution rate of supra-permafrost water rose to 5%, acting as an auxiliary water source to alleviate drought stress; under the synergistic effect of warming and increased precipitation, water use between shallow and deep layers tended to be balanced. Based on the aforementioned plasticity mechanisms, this study deduced differentiated ecological restoration recommendations tailored to multiple climate scenarios (precipitation increase, precipitation decrease, synergistic warming and precipitation, and relative stability), providing a theoretical basis and inferential support for the adaptive transition of degraded alpine vegetation from traditional static intervention to "water-dependent dynamic management. Transforming mechanistic understanding into differentiated restoration pathways can provide a scientific basis and practical approaches for sustainable water resource management, degraded vegetation restoration, and regional ecological security maintenance on the Qinghai-Tibet Plateau.