## 3 Results
### Response Mechanism Analysis
#### 3.4.1 Results of Partial Correlation Analysis
ET in this elevation zone might be influenced by other factors. In the elevation zones below 3000 m and between 3000 m and 3700 m, the partial correlation coefficient between ST and ET increased with elevation. However, in the elevation zone above 3700 m, the median of the boxplot decreases, which might be due to the fact that at higher elevations, ET is limited by water availability. The partial correlation between VPD and ET was negative across different elevation zones, and the correlation coefficient increased with elevation. In the elevation zones between 3000 m and 3700 m, as well as above 3700 m, the median showed little change. However, in the 3000-3700 m zone, the boxplot distribution was more scattered, suggesting that the response of ET to VPD in this zone was influenced by other factors. The partial correlation between PAR and VPD increased with elevation. The higher the elevation, the more pronounced the vegetation's response to energy, leading to enhanced photosynthesis and transpiration, as shown in Figure 11b. The response of vegetation WUE to various factors differed across elevation zones, showing a different pattern of correlation compared to the response of NPP and ET to the hydrological and thermal factors. The partial correlation between Prec and WUE was negative across different elevation zones, with no significant differences observed. The partial correlation between Temp and WUE showed significant differences across elevation zones. In the \(<\)3000 m and 3000-3700 m zones, the partial correlation coefficients between Temp and WUE were generally negative, with the median close to 0. This might be because the influence of Temp on NPP is slightly smaller than its influence on ET. However, when the elevation exceeds 3700 m, the partial correlation coefficient between Temp and WUE significantly increases. Considering the influence of Temp on both NPP and ET, this suggests that in high-elevation regions, the effect of temperature on vegetation respiration outweighs its impact on transpiration. The partial correlation between SM and WUE was positive in both the \(<\)3000 m and 3000-3700 m elevation zones, with correlation coefficients ranging from 0.1 to 0.2. At elevations greater than 3700 m, the partial correlation between SM and WUE became negative, suggesting that as SM increases, the combined effects of soil evaporation and vegetation transpiration outweigh the photosynthetic activity of vegetation, resulting in a decline in WUE. The response of WUE to ST increased with elevation, indicating that the higher the elevation, the more pronounced the effect of soil temperature on the growth and development of vegetation. The effect of VPD on WUE strengthened with increasing elevation, which could reflect the effect of VPD on NPP and ET in different elevation zones. There was a difference in the response of WUE to PAR under different elevation zones, and the bias correlation coefficient did not change significantly under the zones of elevation of less than 3000 m and 3000-3700 m, and when the elevation was higher than 3700 m, the effect of PAR on WUE was weakened, and this was related to the strengthening of the effect of PAR on ET under this elevation zone, as shown in Figure 11c.