## 5 Conclusions  
This study revealed that the seasonal dynamics of WUE in alpine grasslands were strongly regulated by elevation-sensitive hydrothermal interactions. By integrating partial correlation analysis, SEM, and contribution rate decomposition, we elucidated both direct and mediated pathways through which hydrothermal and radiative factors regulate seasonal WUE dynamics.  
A distinct triphasic WUE pattern emerged along the elevation gradient, with a peak near 3700 m--indicative of a threshold-driven ecosystem response. The analysis reveals that VPD is the primary driver at lower elevations, with a partial correlation coefficient of 0.28 and a direct path coefficient of 0.52, while both VPD and ST jointly control WUE at mid to high elevations, with partial correlation coefficients of 0.39 and 0.38, and direct path coefficients of 0.84 and 0.33, respectively. The influence of both factors intensified with increasing elevation. The SEM results highlight that temperature and precipitation primarily affect WUE indirectly, mediated through VPD and ST. The total effect of temperature on WUE increased from 0.51 to 0.95, while the total effect of precipitation on WUE rose from \(-0.36\) to \(-0.18\). In high-elevation areas, the effect of Temp on WUE through ST was 0.61, and through VPD it was 0.24. Notably, the positive indirect effect of temperature increases with elevation, with the path coefficient shifting from negative to 0.18, suggesting an enhanced temperature sensitivity of alpine vegetation under colder conditions, whereas precipitation consistently suppresses WUE, likely due to its negative impact on radiation availability or stomatal regulation.  
In addition to climatic drivers, the relative contributions of NPP and ET to WUE variation exhibited clear spatial shifts. ET dominated at lower altitudes where evaporative demand was high, with a contribution rate of \(-\)82.50%, while NPP played a more central role at higher elevations, with a contribution rate of 54.71%, where vegetation growth became a limiting factor. This shift underscored the changing balance between carbon acquisition and water loss under varying hydrothermal constraints.  
These findings have significant implications for understanding alpine ecosystem function and resilience under a changing climate. The observed nonlinearity and spatial heterogeneity in WUE response emphasize the need for adaptive, elevation-specific management strategies in fragile mountainous regions. WUE responses to hydrothermal and light factors vary with elevation, reflecting differences between C\({}_{3}\) and C\({}_{4}\) plants. At high altitudes (>3700 m), C\({}_{3}\) plants dominate, and WUE is mainly driven by NPP, so conserving cold-adapted C\({}_{3}\) species is vital. At low altitudes (<3000 m), C\({}_{4}\) plants prevail, with evapotranspiration strongly influencing WUE; thus, improving soil moisture and supporting drought-tolerant C\({}_{4}\) plants is important. Medium elevations (3000-3700 m) see joint control by soil temperature and vapor pressure deficit, requiring diverse plant communities for stability. These patterns support elevation-specific management to enhance ecosystem resilience under climate change.  
Conceptualization, Y.T. and B.Z.; methodology, Y.T. and B.Z.; software, W.Z.; validation, X.X., X.C. and B.Q.; formal analysis, Y.T. and W.Z.; investigation, X.X.; resources, Y.T.; data curation, W.Z.; writing--original draft preparation, W.Z.; writing--review and editing, Y.T. and X.X.; visualization, W.Z.; supervision, B.Z.; project administration, X.C. and B.Q.; funding acquisition, Y.T. All authors have read and agreed to the published version of the manuscript.  
This research was jointly supported by The National Natural Science Foundation of China (No. U21A2021); Beijing and The Water Science and Technology Open Program Funding (2024).  
All data used in this study were obtained from publicly available sources. The processed dataset is available from the authors upon reasonable request.  
The authors declare no conflicts of interest.