## 4 Discussion  
This study showed that the multi-year mean values of NPP and WUE in alpine grassland showed a spatial pattern of gradual increase from northwest to southeast during 2001-2018, with an overall upward trend. This change was mainly driven by the spatial heterogeneity of hydrothermal conditions, which is consistent with the results of existing studies [51]. The variation in WUE along the elevation gradient exhibits a clear phased pattern. The study have shown that WUE reaches its peak at approximately 3700 m, and then gradually decreases with increasing elevation, which is in line with the previous study [52, 53]. This trend reflects the increasing limiting effect of hydrothermal conditions on WUE, especially in high-altitude regions [41]. With the increase in elevation, vegetation growth was more limited by temperature, on the one hand, and with the increase in temperature, NPP had a tendency to increase; but on the other hand, the increase in temperature also led to an increase in ET. The increasing trend of both NPP and ET resulted in an insignificant overall increasing trend of WUE. In this study, we further explored the responses of alpine grassland vegetation WUE to climatic factors such as water, heat, and light. during the growing season under three elevation zones (below 3000 m, 3000-3700 m, and above 3700 m). Partial correlation analysis was utilized to quantify the independent driving effects of climatic factors on NPP, ET, and WUE in alpine grassland across different elevation gradients. At elevations below 3000 m, the dominant role of SM in driving NPP (R around 0.30-0.45) and the significant effects of VPD on ET (R around \(-\)0.50) and WUE (R around 0.28) might be related to the relatively higher proportion of C\({}_{4}\) plants in this region. C\({}_{4}\) plants possess a higher water-use efficiency and a stronger tolerance to heat and drought, enabling them to maintain a high photosynthetic capacity under water-limited and high-VPD conditions. As a result, when VPD increased, ET was suppressed, but carbon fixation remained relatively stable, leading to an improvement in WUE. This also supported the conclusion that vegetation growth in low-elevation areas was more dependent on water availability than on heat conditions [54]. At elevations between 3000 and 3700 m, ST became a key factor influencing NPP (R = 0.23) and WUE (R = 0.39), further emphasizing the importance of heat in the growing season of alpine grassland [55], while the negative dominant effect of VPD on ET (R = \(-\)0.38) still persisted, reflecting the limitation of evapotranspiration by the  
Figure 16: Contribution rates of NPP and ET to WUE under different elevation zones. vegetation under atmospheric drought. Above 3700 m elevation, the dominance of ST and VPD on NPP remained unchanged despite a slight weakening of its influence, while the sensitivity of WUE to both ST and VPD increased, suggesting that soil heat supply and atmospheric moisture stress in high-elevation regions plays a key role in maintaining WUE. This might be due to the stronger response of C\({}_{3}\) vegetation to soil temperature and VPD in high-elevation areas [43; 56]. This response mechanism of spatial heterogeneity was consistent with the water-heat coupling limitation pattern previously found in the Tibetan Plateau ecosystem [57]. The response of WUE to hydrothermal and light factors was mainly related to the response of grassland productivity and vegetation evapotranspiration to hydrothermal factors. According to the results of SEM, there were significant differences in the driving mechanisms of water, heat, and light factors on the coupling process of carbon-water in alpine grassland under different elevation zones. At elevations lower than 3000 m, NPP was mainly driven by ST directly, with a path coefficient of 0.87, while Temp and Prec acted on NPP and ET mainly through indirect effects on the intermediary factors (ST and VPD).