## 4 Discussion
### Control Mechanism of CUE  
An analysis of the mechanisms influencing CUE is, in fact, an analysis of the mechanisms influencing GPP and respiration. In the five models, photosynthesis, respiration, and other physiological processes are simulated, and the leaf area index and biomass are changed, which are reflected in the changes of vegetation types and structures. On the one hand, real changes in GPP can occur if vegetation is removed by natural or human disturbances. The rate of natural disturbances changes with changes in climate, which has indirect effects on GPP. On the other hand, precipitation and temperature will affect plant respiration with different degrees, which leads to the change of NPP and CUE. In this paper, we focus on the direct and indirect impacts of climate factors on CUE. Previous studies showed that climate is an important factor of CUE. Climate change has an indirect impact on the growth of vegetation to a certain extent. Climate change is characterized by key climate indicators, such as temperature, precipitation and CO\({}_{2}\) concentration, and the change in vegetation can be characterized by GPP and NPP, resulting in the change of CUE. [PERSON] et al. used MODIS data to study the CUE of terrestrial ecosystems at the global scale and showed that CUE is dependent on key climate factors, such as temperature and precipitation. The findings in this paper have similarities and differences with their results. Temperature and CUE show a substantial and high negative correlation in three future climate scenarios, and this was consisted for all three climate scenarios. Compared with precipitation and CO\({}_{2}\) concentration, air temperature has the greatest comprehensive impact on CUE, so it is considered that temperature is the leading factor affecting CUE in the future climate change scenario for the Qinghai-Tibet Plateau. [PERSON] et al. [7] showed that an increase in temperature will lead to a decrease in CUE, and in arid and high-temperature areas, vegetation will bear higher respiratory costs, and its NPP will be reduced. [PERSON] et al. [6] found that there is a linear relationship between the average annual temperature and CUE. The increase in temperature will lead to an increase in respiratory enzyme activity and the enhancement of plant respiration [5]. Therefore, NPP will decrease, which will lead to a decrease in CUE, and the absorption of carbon by plants will be weakened. It is shown in the literature that under high temperature, the net photosynthetic rate of leaves tends to decrease due to the increase of mitochondrial respiration and photorespiration, photosynthetic biochemical damage, and stomatal closure [53]. There is a negative correlation between precipitation and CUE in different future climate scenarios; that is, when precipitation increases, CUE shows a downward trend. Some studies showed that an increase in precipitation will lead to a decrease in plant root respiration and a decrease in the plant respiration rate [54], which lowers the carbon consumption of plant respiration, i.e., NPP increases. Simultaneously, the increase in precipitation may increase the stomatal conductance and intercellular CO\({}_{2}\) concentration of leaves and enhance photosynthesis [55]; that is, GPP is also increased. Therefore, the inhibition of precipitation on the respiration rate and the increase in the photosynthetic rate determine its positive or negative effects on CUE. The same results can be verified in the findings of others. [PERSON] et al. [15] found that the global NPP/GPP ratio decreased with increasing precipitation from 2000 to 2003. [PERSON], et al. [56] found that there is a substantial linear negative correlation between CUE and MAP in east Asian grasslands. There are studies show that GPP is generally more sensitive than respiration to extreme drought in a wide range of biological communities. In addition, the sensitivity difference between yield and respiration increased with the increase of drought severity [57].