## 4 Discussion
### Impact of Human Activities on NPP  
In the past 20 years, with the increase in the range and intensity of human activities, anthropogenic disturbance has become a non-negligible driving force affecting NPP in the research region [6; 12]. In the research region, the most significant forms of natural transformation by human activities are changes in land cover. From 2000 to 2019, the NPP of different vegetation land use types in the research region showed an oscillating upward trend (Figure 11). Rapid urban expansion and economic development have caused tremendous changes in the LULC and the ecological environment surrounding the city [6]. From 2000 to 2019, the NPP of different vegetation types showed an oscillating upward trend in the Yangtze River Delta, which was attributed to the implementation of the Grain for Green Project, Yangtze River Protected Forest Project, and Ecological Restoration Project [13; 67]. Therefore, increasing the area of urban green space and rational planning of urban structure could contribute to the increase in NPP. Nevertheless, some studies have shown that the Yangtze River Delta region has lost 0.14 Tg C of NPP per year due to urbanization [12].  
In addition, the impact of urbanization on NPP not only has an impact on land use type changes but also on the rapid growth of urban population. Over the past several decades, China's urbanization rate has increased dramatically from 17.92% to 59.58% [68], and the rapidly increasing urbanization rate may represent a huge population growth. Previous studies have confirmed that the gross primary productivity (GPP) and enhanced vegetation index (EVI) both showed a significant decreasing trend with urban expansion [69]. The dramatic LULC changes caused by rapid urbanization have led to a decline in NPP, which is mainly due to the replacement of cropland and forests by urban land (i.e., construction land) [70]. The NPP was negatively correlated with population in more than 64% of the regions (Figure 9), which indicated that population growth in the region is one of the major factors resulting in the decrease in NPP. Previous studies have shown there is a negative correlation between NPP, economic expansion, and urbanization. As urban population increases, cities have been expanded to surrounding green spaces (including forests, grasslands, croplands, etc.) to accommodate more people, which, in turn, results in the decrease in NPP [71]. [PERSON] et al. calculated the NPP in Hubei province between 2001 and 2012 and studied the spatial and temporal dynamics of the NPP and its relationship with urbanization indicators and found that there was a negative correlation between NPP, population, and GDP. In general, the highest values of negative correlation coefficients were found in urban areas, and the lowest values were found in woodlands. The negative correlation coefficients between population and annual NPP became larger as population density increased [72]. This might explain the negative correlation between population density and NPP in the research region. In recent years, the research region has proposed the \"Yangtze River Delta Regional Integrated Development Plan\", which may help improve cross-basin and cross-regional ecological environmental quality and further enhance regional NPP as the policy is implemented [73]. Therefore, controlling population growth and planning urban development appropriately will contribute to improving the NPP of the study area.