## 3 Results
### Control Mechanisms of CUE Changes under Three Scenarios  
The results from the SEM explain the possible influencing paths of CUE changes. Under the scenarios of SSP1-2.6, SSP3-7.0 and SSP5-8.5, MAP, MAT and CO\({}_{2}\) concentrations have little direct impact on CUE, but they have indirect impacts on CUE through GPP and NPP (Figure 4). Under the three scenarios, the direct path coefficient of MAT to GPP is positive and the largest compared with MAP and CO\({}_{2}\) concentration; thus, MAT has the greatest direct impact on GPP compared with MAP and CO\({}_{2}\) concentration. Similarly, MAT has a greater direct impact on NPP than MAP and CO\({}_{2}\) concentration, but it has a negative effect on NPP under the SSP1-2.6 scenario and SSP3-7.0 scenario and a positive effect under the SSP5-8.5 scenario.  
Figure 4: Structural Equation Model (SEM) of CUE and environmental variable factors under three future climate scenarios. SEM images under the SSP1-2.6 scenario are shown in (**a**). SEM images under the SSP3-7.0 scenario are shown in (**b**). SEM images under the SSP5-8.5 scenario are shown in (**c**). The straight line is the linear relationship between potential variables of the structural equation and observed variables, and standardized path coefficients are shown on the relevant path. ** represents the very significant level of the direct path coefficient.  
Under the SSP1-2.6 scenario, the direct path coefficient of MAT to CUE is -0.005, and the indirect path coefficient of MAT to CUE through GPP and NPP is \(-\)0.293. Thus, the total coefficient between MAT and CUE is \(-\)0.298 (Table A2). The direct effects of MAP and CO\({}_{2}\) concentration on CUE are negligible, and their indirect path coefficients are 5.14 \(\times\) 10\({}^{-5}\) and 6.8 \(\times\) 10\({}^{-6}\), respectively. Thus, the comprehensive effect of MAT on CUE is far greater than that of MAP and CO\({}_{2}\) concentration, and MAT has a negative effect on CUE, while MAP and CO\({}_{2}\) concentration have a weak positive effect on CUE.  
Similarly, under the SSP3-7.0 and SSP5-8.5 scenarios, the direct path coefficients of MAT to CUE are \(-\)0.005 and \(-\)0.006, respectively, and the total coefficients are \(-\)0.005 and \(-\)0.004, respectively. Under these two scenarios, the comprehensive path coefficients of MAP to CUE are 1.9 \(\times\) 10\({}^{-4}\) and 3.4 \(\times\) 10\({}^{-5}\), respectively. The comprehensive path coefficients of CO\({}_{2}\) concentration on CUE are \(-\)2 \(\times\) 10\({}^{-5}\) and \(-\)6 \(\times\) 10\({}^{-5}\) (Table A2). Thus, under the SSP3-7.0 and SSP5-8.5 scenarios, the comprehensive impact of MAT on CUE is also much greater than that of MAP and CO\({}_{2}\) concentration. The MAT still has a negative effect on CUE, and MAP has a very weak positive effect on CUE. However, the difference is that the comprehensive effect of CO\({}_{2}\) concentration on CUE changes from a weak positive effect to a weak negative effect after changing from the low radiation forcing scenario to the medium and high radiation scenarios. In Figure 4, R\({}^{2}\) reflects the comprehensive decisive effect of GPP, NPP, MAT, MAP and CO\({}_{2}\) on CUE [50]. It is found that climate variables play an increasing strong role in controlling CUE by adjusting GPP and NPP under the conditions of radiation forcing increasing.