## 3 Results
### Response Mechanism Analysis
#### 3.4.2 Results of Structural Equation Modeling  
As shown in Figure 12a, the changes in Prec and Temp have no direct effect on NPP at elevations less than 3000 m, while SM, ST, VPD, and PAR had direct effects on NPP, which were 0.33, 0.87, \(-\)0.19, and 0.27, respectively. The total effect of Temp on the changes in NPP was 0.77 (\(-\)0.06 + 0.85 \(-\)0.15 + 0.13), which mainly affected NPP through indirect effects on SM (\(-\)0.06), ST (0.85), VPD (\(-\)0.15), and PAR (0.13). The total effect of Prec on the NPP change was 0.20, mainly indirectly through its effect on VPD. In summary, the total effect of ST on NPP change was the largest under this elevation partition. The elevation ranged from 3000 to 3700 m. The results showed that Temp had a direct effect on the changes in NPP, with a total effect coefficient on NPP of 0.89 (0.16 + 0.92 \(\times\) 0.67 + 0.74 \(\times\) 0.09 + 0.38 \(\times\) 0.12) indicating an increased dependence of vegetation on the thermal factor with increasing elevation. The direct effects of SM, ST, and PAR on the NPP change decreased, while VPD had a weak positive direct effect on NPP at this elevation partition. The total effect of precipitation on the NPP change was 0.08, which was a weak effect. At elevations greater than 3700 m, the direct effects of SM, VPD, and PAR on NPP were less than 0.10, indicating that these factors had little effect at high elevations, while the direct effects on Temp and ST were enhanced to 0.31 and 0.54, respectively, with a total effect of Temp on NPP of 0.94, and a total effect of Prec on NPP of \(-\)0.01, as shown in Figure 12c. The model fit indices were as follows: RMSEA values were 0.01, 0.04, and 0.05; CFI values were 1.00, 0.99, and 0.99; and GFI values were 0.99, 0.99, and 0.99, respectively. In the figure, green lines indicate positive path coefficients and red lines indicate negative path coefficients (the same applies to Figures 13 and 14). The results of SEM of ET with Prec, Temp, SM, ST, VPD, and PAR at the three elevation zones of \(<\)3000 m, 3000-3700 m, and \(>\)3700 m are shown in Figure 13. The results showed that at elevations of less than 3000 m, as shown in Figure 13a, Prec and Temp produced a total effect of 0.42 and 0.54 on ET changes, respectively, and precipitation mainly affected ET by indirectly influencing VPD and then ET, and Temp mainly affected ET positively by affecting ST and PAR, and negatively by affecting VPD, with an effect coefficient of \(-\)0.43 (0.79 \(\times\)\(-\)0.55), while higher temperatures and increased VPD led to a decrease in vegetation stomatal conductance and weaker vegetation transpiration. The direct effects of SM, ST, VPD, and PAR on the changes in ET were 0.15, 0.49, \(-\)0.55, and 0.10, respectively. The positive effects of Prec and Temp on ET increased with elevation, with an increase in Prec, adequate water availability, and increased evapotranspiration with higher temperatures. The elevation ranged from 3000 to 3700 m. Temp produced a total effect of 0.51 on ET, with a direct effect of 0.37, and 0.14 from indirect effects on other factors. Prec produced a total effect of 0.41 on ET, with a direct effect of 0.20, and the indirect effect was mainly through the effects of VPD (\(-\)0.67 \(\times\)\(-\)0.20) and SM (0.36 \(\times\) 0.22), as shown in Figure 13b. The results of the SEM of ET and the factors under an elevation greater than 3700 m are shown  
Figure 12: Structural equation modeling of NPP: (**a**) \(<\)3000 m, (**b**) 3000–3700 m, and (**c**) \(>\)3700 m elevation zones; path coefficients that pass the significance test of _p_\(<\) 0.05 are shown. in Figure 13c, showing that the indirect effect of Temp on the total effect of ET included the direct positive effect of SM, which might be related to the increase in elevation, the soil properties in the freeze-thaw region with the increase in temperature, and melting.