## 4 Results  
### ESV and KNDVI of the TRHR Present Significant Spatial Heterogeneity  
The total ESV from 2000 to 2020 tends to increase and then decrease (Figure 3). Among the four major types of service types, regulating value occupies a dominant position and has the most apparent fluctuation; support services are the second most important, with a slower degree of change; and cultural services and supply services have provided a more stable ESV over the past two decades and account for a smaller proportion of the total.  
In general, the ESV of the TRHR demonstrates spatial heterogeneity, with a stable spatial layout of low-value clustering and high-value dispersion (Figure 4). An overall trend of decreasing from southeast to northwest was observed, which is closely related to the characteristic of increasing temperature and humidity from northwest to southeast in the TRHR [77]. As shown in Figure 4\(f\), the ESV in the southeastern TRHR declined as a result of human-caused ecological land degradation. An increasing trend is mainly shown in the middle of the TRHR.  
Figure 4: Spatial distribution (**a**–**e**) and spatio-temporal changes (**f**) in ESV from 2000 to 2020.  
Figure 3: Temporal changes in the four ecosystem services in the TRHR.  
Specifically, the highest-value areas are reflecting the continued clarification of ecosystem boundaries. They are mainly distributed in the central part of the region, Zaling Lake, Eling Lake, and the northwestern part of the Hoh Xili Lake, Xijinwulan Lake, Sun Lake, and other alpine river and lake areas. As multifunctional ecosystems, these rivers and lakes play essential roles in water conservation, soil conservation, carbon sequestration, and oxygen release; so, the watershed areas dominate the ESV in different periods. The areas with high and medium values are scattered in the eastern and southeastern parts of the TRHR, and the trend of low values evolving to medium values in the eastern part gradually increases as time passes. The eastern part is the Yellow River Basin, and the southern part has a more favorable climate, where ecosystems such as alpine meadows and alpine grasslands are widely distributed and can provide stable high-value functions such as carbon sequestration and oxygen release, climate regulation, and environmental purification. Therefore, these zones have higher ESVs relative to the northwest.  
The low-value areas are mainly concentrated in the northwest and part of the central areas of the region, of which the Yangtze River source area occupies a higher proportion, including the core conservation area and the Qinghai Hoh Xil.  
The kNDVI from 2000 to 2020 is shown in Figure 5. There was significant spatio-temporal heterogeneity of the kNDVI in the TRHR. From 2000 to 2020, the kNDVI increased gradually from northwest to southeast (Figure 5a-e) with strong longitudinal zonality. Regarding the spatial distribution features, the high values are primarily concentrated in the southeastern region, where mountain forests and alpine meadows are widespread. In the northwestern part, characterized by glaciers and desert bare land, vegetation cover is low and discontinuous. The linear regression model was used to detect trends of changes in the kNDVI. The pixels with an increasing trend comprised 34.5% of the entire research area, which was much larger than those with a decreasing trend (3%) because of vegetation restoration from barren or sparsely vegetated areas to grassland.  
Figure 5: Spatio-temporal dynamic changes in the kNDVI from 2000 to 2020 in the TRHR.