# Spatial and Temporal Variation of GPP and Its Response to Urban Environmental Changes in Beijing
## 4 Discussion  
### Spatiotemporal Distribution of AGPP in Beijing  
In this paper, we observed that the spatial distribution pattern of AGPP in Beijing is generally higher in the northwest and lower in the southeast, which is consistent with the findings of [PERSON] et al. (2018). There may be two factors contributing to this phenomenon: elevation and human activities. On the one hand, the elevation may cause differences in vegetation types, material metabolism, functional structure, and other vegetation itself, and on the other hand, human activities in the mountainous areas in northwestern Beijing and the plain areas in southeastern Beijing are not equally active, and human activities and urban development in the plain areas are relatively more, which may lead to the shrinkage of vegetation area and the degradation of vegetation growth environment. For example, a study by [PERSON] et al. (2018) demonstrated that from 2000 to 2010, 979.74 km\({}^{2}\) of vegetation in Beijing experienced severe degradation, 725.90 km\({}^{2}\) experienced moderate degradation, and 301.65 km\({}^{2}\) was mildly degraded. These degradation phenomena are directly related to the increase of urban development land. The phenomenon of lower AGPP in the area around the urban center is also consistent with the findings of [PERSON] (2018) and [PERSON] (2018), and this result is mostly related to a large number of human construction activities in urban  
Figure 3: GTWR results from 2014 to 2020. (**A**) shows the fitting result of CO, (**B**) shows the fitting result of LST, (**C**) shows the fitting result of O\({}_{3}\), (**D**) shows the fitting result of PRE, and (**E**) shows the fitting result of NO\({}_{2}\).  
development. In addition, this paper also found that most of the AGPP in key ecological protection areas in Beijing showed a significant upward trend, which suggests that the increase of AGPP in Beijing may be related to the implementation of national and urban policies of returning farmland to forest and closing mountains for forestry.  
In terms of temporal variation, this paper found that the AGPP in Beijing exhibited a general upward trend from 2000 to 2020, which is consistent with the findings of [PERSON] (2018) and [PERSON] et al. (2018) Previous studies have shown that the combined effects of climate change, CO\({}_{2}\) fertilization effects, nitrogen deposition, and ecological restoration efforts promote an increase in AGPP, while factors such as drought events and urban development land expansion may cause a decrease in AGPP. In the case of Beijing, major events and ecological restoration work may be the main reasons for the fluctuating rise in AGPP from 2000 to 2020. The years 2004 and 2008 had the greatest ups and downs in AGPP in Beijing and were the two years when ecological work was vigorously promoted to host the Beijing Olympics under the Beijing Urban Master Plan (2004-2020). Under the guidance of the Beijing Urban Master Plan (2004-2020) and the government's attention, the conditions for vegetation growth in Beijing were greatly improved, and AGPP surged.