## 4 Discussion  
### Spatial and Temporal Distribution and Changing Trend of ANPP  
Our research results indicate that from 2001 to 2018, the average ANPP across the entire subtropical research area in China exhibited a gradual increasing trend from north to south, a finding that aligns with previously reported trends [58; 59; 60]. The regional average ANPP stood at 677.17 gC m\({}^{-2}\) a\({}^{-1}\), coinciding with earlier research [28]. PNPP, standing for net primary productivity under conditions free from human disturbance, delineates the level of productivity of the most stable and mature vegetation that can be developed under natural conditions. This indicator is an essential tool for assessing the quality status of natural ecosystems, and it effectively distinguishes the impact of human activities on the ecological environment. In this study, the calculation of PNPP was based on the Thornthwaite memorial model. The HNPP, or human-affected net primary productivity, is calculated as the difference between the PNPP and the ANPP, quantifying the influence of human activities on vegetation productivity [52; 53] The computational formulas for PNPP and HNPP are comprehensively detailed in Section 3.2. The findings revealed that the computed PNPP significantly exceeded both the ANPP and the HNPP, with the HNPP also being greater than the ANPP. The descending order of these indices was PNPP, HNPP, and  
Figure 12: The trends of annual average temperature and annual total precipitation on the DEM. ANPP. This concurred with previous research [49; 61; 62] and indicated the efficacy of these indicators in analyzing the study area. The study area transitioned from the Central Plains to the southern part of China. The southern region is characterized by its richness in forest resources and a higher vegetation coverage compared to the central region. The abundance of precipitation, a suitable warm climate, and rich groundwater resources collectively contribute to favorable conditions for vegetation growth [63], resulting in a distribution trend where the vegetation is less dense in the north and denser in the south. Areas with significant increases in the ANPP were detailed in the first section of Section 3 of this document. On a pixel scale, these regions of notable ANPP augmentation were predominantly found in the southeastern part of Yunnan Province, central Henan, the central and northern parts of Anhui, as well as near the mountains and the ecological function conservation areas within these provinces. The increase in the ANPP found within Yunnan Province was consistent with findings from previous studies [64; 65]. There are mainly two reasons for this growth in ANPP in the area. The first reason is attributed to major national ecological restoration projects such as the 'Grain for Green' program, which led to the conversion of non-arboreal vegetation into forests in certain areas [66]. In addition, the retained farmlands, benefiting from favorable hydrothermal conditions, have also contributed to the increase in ANPP [65]. Anhui and Henan provinces are among the regions in China with a substantial distribution of farmlands, hosting a large number of croplands, where research by [PERSON] and [PERSON] et al. observed an increase in the area of croplands for wheat, rice, maize, etc. [67; 68]; thus, the rapid increase in crop yield possibly spurred the increase in the region's ANPP. The rise in mountainous ANPP can be attributed to the combined effects of temperature and precipitation, where, between altitudes of 1600 and 2800 m, photosynthesis in plants was positively correlated with temperature, a relationship that also applies to photoregpiration and dark respiration [69]. Between 2001 and 2018, we observed an increasing trend in the total annual precipitation and the average annual temperature (Section 3.5). Abundant rainfall enriches the soil, providing more nutrients for the vegetation [69]. A naturally warm and humid environment offers improved hydrothermal conditions for mountain vegetation, promoting its growth.