## 4 Discussion
### Effect on Vegetation Net Primary Productivity (NPP)  
On the one hand, the change of land use from one type to another is often accompanied by a large amount of carbon exchange. For example, when forests with high biomass are converted into farmland, grassland or cities with low biomass, large amounts of CO\({}_{2}\) will be released into the atmosphere which may cause a decrease in NPP of vegetation. The expansion of global croplands, pastures, plantations, and urban areas in recent decades generated considerable losses of biodiversity and also potentially undermined the capacity of ecosystems to sustain food production [56]. in the process of industrialization in Beijing, a large amount of land is being changed into construction land, resulting in the increase of impermeable water surface. According to the result of hot spot analysis (Figure 7), when \(p<0.01\), the cold spots are mainly concentrated in the periphery of urban area and water area which may taking negative effects to NPP. On the other hand, in addition to natural factors, policy regulation and economic driving are also important causes of land use change and its spatial and temporal disparities. Typical examples of this are the abandonment of the former republics following the upheaval in Eastern Europe and the collapse of the Post-Soviet Union [57]. With the continuing development of the city, urban planning priorities have gradually changed, the urban greening intensity has increased, and the vegetation condition has steadily improved [58]. It can be seen from Table 5 that the main land types in the hot spot of vegetation NPP are forest and grassland which are mainly distributed in the northwest of Beijing. What is more, the transformation of land use types in hot spot mainly occurs in the transformation from grassland to forest about 21.816 km\({}^{2}\). This is also a partial confirmation of the gradual improvement of vegetation cover in China, which may play a role in relieving the two main problems in Beijing: soil erosion and air pollution. The results of this study (Figures 4 and 5) are similar to those of [PERSON], [PERSON]'s study [59]: from 2001 to 2009, at first, with the expansion of urban areas, ecological land was gradually occupied which led to a decrease in NPP, and finally the NPP increases occurred in highly developed urban zones, which depended to some extent on the growth of green land, local eco-environmental management and urban planning. Therefore, NPP in Beijing showed an obvious upward trend with a series of ecological forestry projects from 2010 to 2020. As Beijing has been highly developed urban zones, the future land use will not change greatly in the next 10 years and the promotion of NPP will exist which is consistent with the predicted results (Figure 6b). In addition, continuous decline areas of the future development of NPP belong to the development area in this study and continuous rise areas account for 84.76%. However, unlike Beijing, land cover and land use changed in some areas have reduced vegetation coverage and net ecosystem production [60; 61]. The reason may be the local inaction to the forest which resulting in the decline of its area under the action of human activities. Therefore, the simulation of future development can provide a new basis for local planning, especially in similar countries and regions in the process of urbanization. In 2019, Nature [62] released research reports pointing out that China alone accounts for 25% of the global net increase in leaf area with only 6.6% of global vegetated area and China is engineering programs to conserve and expand forests with the goal of mitigating land degradation, air pollution and climate change. Although the area of cold spot of NPP changes in Beijing is large (Figure 7), it has more forest cover and the government attaches importance to the protection of ecological environment.