## 3 Results
### Spatial Correlation Analysis of CUE and Climate Variability  
The climate trends observed in the Three Rivers Basin are consistent with global patterns in the context of global climate change. Evapotranspiration, with a growth rate of 5.0163 mm per year, shows a particularly pronounced increase. In this study, the Pearson correlation coefficient was calculated with the objective of evaluating the relationship between CUE and climate variables. Subsequently, the significance of the correlation was evaluated through the application of the \(T\)-test. The correlation coefficients for CUE with temperature, precipitation, and evapotranspiration within the Three Rivers Source region from 2001 to 2020 ranged from \(-\)0.86 to 0.83, \(-\)0.84 to 0.85, and \(-\)0.93 to 0.89, respectively (Figure 6). Specifically, 1.12%, 2.34%, and 4.93% of the regions showed significance at the 99% confidence level for temperature, precipitation, and evapotranspiration, respectively. In addition, 4.36%, 7.01%, and 9.77% of the regions showed significance at the 95% confidence level for the respective variables. The correlation between CUE and evapotranspiration was particularly noteworthy. It surpassed the correlations with temperature and precipitation in both numerical magnitude and geographic range of significance. Significant positive correlations were clustered around Geermu, Qumalai, and Chengduo. In contrast, significant negative correlations were mainly found in the regions around Ngoring and in the areas around Tongde, Xinghai and Gonghe. Conversely, the relationship between CUE and temperature within the Three Rivers Source region appeared to be relatively weak. An overwhelming 94.52% of areas showed correlations that were neither significantly positive nor negative.  
Figure 5: Spatial distribution of annual mean values of vegetation GPP (**a**), NPP (**b**), autotrophic respiration (**c**), NDVI (**d**), LAI (**e**) and CUE (**f**) in Three Rivers Source region from 2001 to 2020.  
The correlation between the CUE and the climatic variables shows that there are distinct regional patterns within the Three Rivers Source area. In particular, the spatial distribution of CUE showed similarities with temperature and evapotranspiration. Positive correlations were found predominantly in the western Three Rivers Source region, while negative correlations were more common in the eastern region. Conversely, the positive correlation between CUE and precipitation was concentrated in the eastern part of the Three Rivers Source region. The western region showed mainly negative correlations. Western areas, such as Xinghai, Guinan, Tongde, Yushu, Dari, and Henan, are notable areas of reduced CUE within the Three Rivers Source region. In these areas, both significant and non-significant negative correlations were observed between CUE and the climatic variables of temperature, precipitation, and evapotranspiration. All of these variables showed an upward trend. There is a notable anomaly in certain areas of the eastern Three Rivers Source region, such as Tongde, Kuze, and Guinan, where CUE shows a significant negative correlation with precipitation. Interestingly, neighboring regions characterized by similar vegetation types, altitudes, temperatures, and other environmental conditions show a strong and contrasting correlation. This nuanced pattern suggests an increased sensitivity of the steppe CUE to evapotranspiration variations within these specific zones of the eastern Three Rivers Source region.