## 4 Discussion
The higher the proportion of C\({}_{4}\) plants in this zone, with their strong heat- and drought-tolerance, could maintain a high photosynthetic efficiency under hot and dry conditions. Thus, temperature and precipitation impact vegetation more by regulating soil temperature and atmospheric dryness rather than directly affecting NPP. As the elevation increased to 3000-3700 m, the direct effect of Temp on NPP and ET increased, reflecting that the thermal factor gradually became the dominant factor, while that of VPD on ET increased, and the inhibitory effect of VPD on ET remained significant. After the elevation exceeded 3700 m, the direct positive effects of Temp (with direct path coefficients of 0.31 and 0.34) and ST (with direct path coefficients of 0.54 and 0.65) on NPP and WUE were further strengthened, reflecting that in the high-elevation freezing and thawing zone, the increase in temperature helped soil thawing and improved water availability, which in turn promoted vegetation growth and water-use efficiency. At this elevation, C\({}_{3}\) plants dominated and were more sensitive to temperature changes, with warming significantly enhancing their photosynthesis and growth; in contrast, the proportion of heat-tolerant C\({}_{4}\) plants was lower, and their response to temperature changes was weaker. This was consistent with existing findings that vegetation activity in alpine grasslands was more sensitive to heat changes [58; 59]. In addition, VPD generally had a significant negative effect on ET at all elevation intervals, suggesting that drought stress had a sustained inhibitory effect on evapotranspiration processes. WUE was determined by the combined effects of ecosystem productivity and evapotranspiration. For different regions, the factors that played a decisive role may vary significantly [60]. The spatial heterogeneity of ET and NPP in WUE changes and the shift characteristics of their dominant factors were revealed by analyzing the contribution of WUE changes at different elevation intervals. The study showed that the negative effect of ET on WUE was dominant at elevations lower than 3000 m, with a contribution rate of \(-\)82.50%. Higher temperatures and more intense evapotranspiration at lower elevations result in high evaporative losses of water, leading to insufficient water supply, which in turn leads to a decrease in WUE [61]. In this region, the proportion of C\({}_{4}\) plants is relatively high. Due to their specialized carbon assimilation pathway, C\({}_{4}\) plants can reduce photorespiration losses more effectively, possess higher water-use efficiency, and exhibit greater drought tolerance, allowing them to maintain a relatively high carbon-fixation efficiency even under moisture limitation [42]. At elevations higher than 3700 m, NPP became the main controlling factor of WUE change, with a contribution rate of 54.71%. With decreasing temperature and harsher climatic conditions, C\({}_{3}\) plants dominate; their carbon assimilation pathway was sensitive to low temperatures, which restricted plant growth and made NPP's influence on WUE more significant [62]. At this time, plant growth was restricted and changes in NPP directly determined the performance of WUE, while ET still played a positive contributing role to some extent, but its influence was more secondary. This phenomenon suggests that the response of plants to climate change at high elevation is more dependent on the capacity of photosynthesis, while the effect of water is relatively weakened [63]. This study systematically investigated the response mechanisms of the WUE of alpine grassland vegetation in Qinghai Province to multiple factors such as water, heat, and light across different elevation zones, revealing the adaptive characteristics of the ecosystem under hydrothermal coupling regulation. The results are generally consistent with existing studies and provide new scientific insights into ecosystem resilience under climate change [51, 56].