# Spatial and Temporal Variation of GPP and Its Response to Urban Environmental Changes in Beijing
## 4 Discussion
### Influence of Pollutant Factors on AGPP in Beijing  
In terms of pollutant factors, CO and O\({}_{3}\) generally have a positive effect on AGPP throughout most of Beijing, with O\({}_{3}\) having a less obvious effect. However, in small areas in the northern, southwestern, and southeastern regions, CO and O\({}_{3}\) exhibit negative effects. NO\({}_{2}\) generally exhibits negative impacts on AGPP while showing a positive effect on AGPP in a few areas of the northeast, southwest, and southeast.  
CO can affect vegetation AGPP by affecting the antioxidant and drought resistance of plants [44]. The strong oxidizing properties of O\({}_{3}\) and NO\({}_{2}\) also damage plant structure and have an inhibitory effect on AGPP [16; 17]. At certain concentrations, NO\({}_{2}\) can stimulate plant growth, but at high concentrations, it becomes toxic, adversely affecting plant development [45]. There is a certain gap between the model fitting results and existing research results; the reason could be as follows: O\({}_{3}\) and CO levels can also indicate the intensity of human activities in this study, and vegetation may benefit from positive human interventions, such as more effective vegetation management strategies, which may offset some of the negative impacts of pollutants. This could result in a positive influence of CO and O\({}_{3}\) on AGPP across most areas. Conversely, areas exhibiting negative impacts are likely due to severe local pollution of O\({}_{3}\) and CO, where the detrimental effects of pollution on AGPP surpass the positive effects of human activities. In terms of NO\({}_{2}\), unlike O\({}_{3}\) and CO, the main source of NO\({}_{2}\) in China is industrial emissions [46]. Consequently, the correlation between NO\({}_{2}\) and human activities is relatively low, and the results fitted by GTWR are closer to the natural effects of NO\({}_{2}\). As a result, NO\({}_{2}\) generally has a negative impact on AGPP, with only a small fraction of areas exhibiting a promotional effect on AGPP.  
Therefore, employing more effective vegetation management practices can significantly mitigate the impact of pollutants on AGPP. Focusing on key polluted areas and implementing strategies to reduce pollutant emissions, coupled with appropriate vegetation management, can contribute to the enhancement of AGPP in the Beijing region and facilitate the achievement of sustainable urban development. At present, Beijing has implemented comprehensive initiatives in pollution control, yielding substantial outcomes. As depicted in Figure 7, while O\({}_{3}\) concentrations are anticipated to persist in their upward trajectory due to the rising number of motor vehicles, both CO and NO\({}_{2}\) exhibited a pronounced decreasing trend over the period from 2014 to 2020. This trend underscores the substantial success of Beijing's environmental management strategies.  
Moreover, Beijing has executed a multitude of strategies focused on the realms of urban greening and forest conservation in pursuit of sustainable urban development. For example, by implementing projects to enhance the ecological functions of urban green spaces, improve the quality and efficiency of plain forests, and accurately improve the quality of mountainous forests, the resilience of urban ecosystems has been strengthened. Besides, the Saihanba Mechanical Forest Farm's desertification control has significantly raised forest coverage from 11.4% to 82%, offering critical ecological services to the Beijing Tianjin Hebei region. These measures and achievements reflect Beijing's emphasis and commitment to the construction of ecological civilization.