## 3 Results
### Spatial and Temporal Characteristics  
#### 3.2.1 Spatial and Temporal Characteristics of NPP  
The study analyzed the changes in NPP across the Qinghai Plateau's alpine grasslands from 2001 to 2018 for both the entire year and the growing season (May-September). Figure 4 illustrated the spatial distribution of the mean annual and growing season NPP values. Overall, the NPP in the alpine grasslands increased gradually from the northwest to the southeast, with significant regional variation across the area. The regions with higher NPP values were primarily located in the central and eastern parts of the Yangtze River source, the Lancang River region, and the southeastern part of the Yellow River source. The areas with lower NPP values were mainly found near the Kunlun Mountain range and the western part of the Qilian Mountain range.  
From the annual multi-year average distribution map of NPP in the alpine grassland, it could be seen that NPP generally ranged from 80 to 500 gC/m\({}^{2}\). Of this, 26.06% of the area had NPP values below 80 gC/m\({}^{2}\), and 2.9% of the area had NPP values above 500 gC/m\({}^{2}\). The southeastern region of the entire area generally had NPP values higher than 400 gC/m\({}^{2}\), accounting for 12.17% of the total area, while the northwestern region generally had NPP values lower than 100 gC/m\({}^{2}\), as shown in Figure 4a. The spatial distribution of NPP during the growing season in the study area was consistent with the annual distribution. The regions with high and low NPP values during the growing season align with those observed in the annual distribution. This is because the annual NPP was obtained by accumulating NPP values every 8 days, so it could be concluded that the majority of the annual NPP was contributed by the NPP during the growing season, as shown in Figure 4b.  
This study statistically analyzed the annual NPP of the alpine grasslands in Qinghai Province from 2001 to 2018 and examined the interannual variability. The results are shown in Figure 5a. From the figure, it could be observed that both the annual and growing season NPP showed an upward trend, though with some fluctuations, and the upward trend was consistent for both the whole year and the growing season. From an interannual perspective, the lowest NPP value in the alpine grasslands occurred in 2008 (182 gC/m\({}^{2}\)), while the highest value was recorded in 2018 (247 gC/m\({}^{2}\)). The NPP trend during the growing season was consistent with that of the whole year, with the growing season NPP being approximately 5 gC/m\({}^{2}\) lower than the annual value. Figure 5b shows the multi-year average monthly NPP of alpine grasslands in Qinghai Province from 2001 to 2018. The results indicated that the NPP of alpine grasslands was higher from May to September, which was consistent with the previously determined growing season for alpine grasslands  
Figure 4: Spatial distribution map of mean annual net primary productivity (NPP): (**a**) year, (**b**) growing season.  
(May to September). The NPP value in May was around 10 gC/m\({}^{2}\), gradually increasing thereafter. In July, the NPP reaches its peak value (72 gC/m\({}^{2}\)), and then **began** to decline starting in August, dropping to about 20 gC/m\({}^{2}\) in September. By October, the NPP approached zero. This was because the vegetation had already witnessed during the period from October to April of the following year, resulting in an NPP close to zero.