## 3 Results  
### Spatial Distribution and Change of CUE  
The multiannual average CUE for the whole Three Rivers Source region was calculated to be 0.8536 between 2001 and 2020. The values ranged from 0.70 to 1.00, with a standard deviation of 0.0237, as shown in Figure 1. The limited interannual variability observed in CUE underlines its pronounced annual stability. Furthermore, this stability highlights the substantial carbon sequestration capacity of the Three Rivers Source region.  
Spatially, the CUE shows a distinct gradient. It increases from the south-eastern to the north-western regions of the Three Rivers Source region. Variations in CUE due to different vegetation types in the region are the main reason for such a spatial distribution. Among the 21 county-level administrative divisions in the Three Rivers Source Region, most had an average CUE above 0.8, with Henan Mongolian Autonomous County having a slightly lower value of 0.7963. In particular, CUE values were significantly elevated in the north and north-west of the eastern periphery of the Three Rivers Source region, for example, in Qumalai (0.8660), Maduo (0.8704) and Gonghe (0.8703). In contrast, regions such as Henan and Juzhi had relatively lower CUE values, with a recorded value of 0.8192. The north-western area including Zhiduo, which is the headwaters of the  
Figure 1: Spatial distribution of annual mean CUE of vegetation CUE in the Three Rivers Source region from 2001 to 2020.  
source of water for the Yangtze River), showed a fragmented CUE distribution pattern. This variability can be attributed to several factors specific to the region, including high elevations, abundant lakes, and sparse vegetation cover, as shown in Figure 2.  
As shown in Figure 3, the average annual CUE within the Three Rivers Source region varied between 0.8501 and 0.8578. Such values, which consistently exceed 0.85, mean that the vegetation in the Three Rivers Source region not only has a high CUE but also has a robust capacity for carbon sequestration. Over the two-decade period, particularly high levels of CUE were recorded in 2002 and 2017, while comparatively lower levels were recorded in 2004, 2010, and 2020. The data highlight a relatively stable CUE trajectory over the last 20 years, with a standard deviation of 0.0021 and a range of 0.0077. This stability is further underlined by the peak annual rate of change of 0.0053 (\(p=0.0437\)) per year. However, there was a noticeable fluctuation in the CUE trend. While overall stability was maintained, there was an observable pattern of decline followed by an increase over the 20-year period.  
The rate of CUE variation within the Three Rivers Source region ranges from \(-0.0215\) to 0.0165 per year, culminating in an average decadal shift of 0.0018. Geographically, the  
Figure 3: Annual variation of vegetation CUE in the Three Rivers Source region.  
Figure 2: Annual mean vegetation CUE in Three Rivers Source region from 2001 to 2020 by county.  
CUE trend is decreasing in eastern zones and increasing in western zones. A remarkable 68.84% of the region, particularly in places like Geermu, Zhiduo, Qumalai, Gande, and Jiuzhi, shows an upward trend in CUE. Conversely, the eastern sectors, which include Tongde, Guinan, and Zeku, tend to show declining CUE.  
Overall, the CUE trajectory of the Three Rivers Source region suggests a slow but steady growth pattern. An overwhelming 68.48% of the regions reflected an upward trend in CUE. A staggering 84.33% of regions did not reach the significance level. In addition, regions with extreme increases and decreases in CUE accounted for 3.90% and 1.19%, respectively. There were 8.48% and 2.10% of regions with significant CUE increases and decreases, respectively. Most regions, 56.46% and 27.87%, respectively, showed insignificant increases and decreases in CUE (Figure 4).