## 4 Discussion
### Spatial and Temporal Distribution and Changing Trend of ANPP
This accounts for the enhancement in the ANPP of the mountain vegetation. National policies can explain the significant increase in ANPP in the ecological function protection areas. Following the issuance of the \"Outline of National Key Ecological Function Protection Area Planning\" by the Chinese Ministry of Environmental Protection, ecological protection areas have become an indispensable part of environmental protection efforts in the country, especially the key ecological function protection areas, which are of great significance and highly valued. The designated protection areas in this study with significant ANPP increases were Southwest Karst Ecological Function Protection Area, Three Gorges Reservoir Area Ecological Function Protection Area, Qinling Mountain Ecological Function Protection Area, Dabie Mountain Ecological Function Protection Area, and Huai River Source Ecological Function Protection Area. These areas are categorized into two types: soil and water conservation ecological function protection areas and water source conservation ecological function protection. According to the \"Outline of National Key Ecological Function Protection Area Planning\", both the soil and water conservation ecological function protection areas and water source conservation ecological function protection areas have established clear standards and guiding principles for vegetation protection. In the soil and water conservation ecological function protection areas, activities that destroy the forest, such as deforestation and slash-and-burn agriculture, as well as agricultural development on steep slopes, are prohibited. These measures aim to slow down vegetation degradation and maintain ecological stability. The area also proposed the need to implement soil and water conservation ecological restoration projects. For the water source conservation ecological function protection areas, the planning outline suggests integration with existing ecological conservation and development projects to enhance the comprehensive management and ecological restoration of forests, grasslands, and wetlands. Overall, these regulationsstrive to strengthen vegetation protection within these ecological function areas by limiting unsustainable land use activities and promoting ecological restoration projects. This way, not only is the ability to retain soil and water resources enhanced, but the net primary productivity of the vegetation within the region is also increased, thus promoting the long-term sustainability of the ecosystem. In the middle and lower reaches of the Yangtze River and the Pearl River Delta urban agglomerations, we observed a significant decline in the ANPP, which is likely related to the high levels of urbanization and economic development in these areas [66; 70]. Urbanization is characterized by the concentration of population and finance, along with an increase in artificial land cover, leading to a substantial reduction in vegetation [71]. In fact, as the middle and lower reaches of the Yangtze River and the Pearl River Delta are core areas of domestic economic development, their rapid economic growth and urban expansion have exerted significant pressure on the local vegetation. Therefore, the accelerating urbanization and economic activities have led to a notable decline in the ANPP in these regions. It is noteworthy that there were differences in the annual average NPP (net primary productivity) among various forest types. In forests, evergreen broadleaf forests exhibited the highest annual average ANPP (728 gC m\({}^{-2}\) a\({}^{-1}\)), followed by mixed forests (646 gC m\({}^{-2}\) a\({}^{-1}\)). The other forest types were deciduous broadleaf forests (632 gC m\({}^{-2}\) a\({}^{-1}\)) and evergreen needleleaf forests (622 gC m\({}^{-2}\) a\({}^{-1}\)), respectively. These findings were consistent with previous research findings [72; 73].