## 4 Discussion
### Variation of ANPP on Elevation Gradient  
In the comprehensive analysis of altitudinal gradients, we observed a notable upward trend of ANPP within the study area as the elevation increased, gradually stabilizing with minor fluctuations before a significant decline; this finding was aligned with previous academic studies [65; 78]. Additionally, the study area experienced a general decline in the average temperature and precipitation as the altitude rose (Figure 12) [65; 79]. Specifically, in the elevation range of 0 to 400 m, vegetation gradually escaped the influence of human activities in the plains, showing an increasing trend in both abundance and area. This likely serves as a key factor for the significant rise in ANPP within this elevation interval. On the other hand, within the 400 to 1600 m elevation range, the impact of altitude on the average temperature and total precipitation was not significant due to their relatively minor fluctuations. Consequently, the ANPP exhibited variations in this range, but the overall trend remained relatively stable. In the altitude range of 1600 to 2800 m, we observed a trend in ANPP that first decreased and then increased with elevation. Within the elevation range of 1600 to 1900 m, the ANPP exhibited a declining trend, whereas between 1900 to 2800 m it showed an increasing trend. Notably, in the 1600 to 1900 m range, the ANPP primarily exhibited a positive correlation with the average temperature. However, this shifted to a predominantly negative correlation within the 1900 to 2000 m range and continued to demonstrate a negative correlation dominance throughout the 1900 to 2800 m range. Within the altitude range of 1600 to 1900 m, the partial correlation between the temperature and the ANPP, although on the verge of shifting to negative, still presented as positive. In this elevation range, the temperature decreased with increasing altitude (Figure 12), and, concurrently, the ANPP showed a declining trend. This phenomenon could be attributed to the temperature decrease with rising altitude, which is unfavorable for the temperature conditions necessary for photosynthesis, thus limiting the rate of photosynthesis and carbon assimilation processes [78; 80]. Therefore, the ANPP exhibited a downward trend under the influence of decreasing temperatures. Further observation revealed that within the altitude range of 2000 to 2800 m, the partial correlation between temperature and ANPP was negatively correlated. In this elevation range, the temperature continued to decrease with increasing altitude, while the ANPP values showed an upward trend. This trend might be related to the increase in solar radiation within this altitude range. According to previous research, at these altitudes, the positive influence of solar radiation on the ANPP progressively intensifies, exhibiting a significant positive driving effect [65]. Consequently, within this elevation range, the favorable impact of solar radiation on ANPP may surpass the negative effects of decreasing temperature, ultimately leading to an increase in the ANPP, which peaked within this altitude range. In regions above 2800 m, the annual average temperature dropped below 10 \({}^{\circ}\)C and precipitation also fell below 1100 mm, but still remained above 650 mm a\({}^{-1}\) 100m\({}^{-1}\). Both the average temperature and precipitation exhibited a decreasing trend with increasing altitude. The ANPP showed a significant downward trend, consistent with previous research findings [65]. In areas above 5000 m, the ANPP remained below 5 gC m\({}^{-2}\) a\({}^{-1}\). In line with previous studies [51; 81; 82; 83], we observed a negative response of ANPP to precipitation in high altitude areas, potentially due to the fact that in subtropical regions, despite precipitation decreasing with lower altitudes, the values remain between 600 and 800 mm a\({}^{-1}\) 100m\({}^{-1}\) (with a mean value of 1271.27 mm a\({}^{-1}\) 100 m\({}^{-1}\) in low altitude areas), possibly exceeding vegetation growth requirements.