# Spatiotemporal Dynamics of Net Primary Productivity in China's Urban Lands during 1982-2015
## 4 Discussion
The temperature changed little over the years. Whether from the ecological geographical area or the main vegetation types, the temperature in the PRDUA did not reach the optimum temperature of the vegetation [53; 54], and an increase in temperature would still promote the growth of vegetation. The sunshine duration in the PRDUA made a negative contribution to the NPP trends, and this negative contribution had an increasing trend. In the PRDUA, the proportion of forest was higher than that of other urban agglomerations. The decrease of sunshine duration could cause the decrease of solar direct radiation penetrating through the forest canopy and reaching the understory, which may lead to the decrease of light scattering in the canopy and light reabsorption by leaves [59]. Correspondingly, the importance of sunshine duration to NPP gradually increased. [PERSON], et al. [60] also showed that the interannual variation in NPP in southern China was more sensitive to solar radiation than to precipitation. As shown in Figure 11, the proportion of vegetation was more important to the NPP in the PRDUA than in the other two urban agglomerations. Moreover, the proportion of vegetation had the strongest negative contribution to NPP trends in the PRDUA. Even in the 20 km buffer zone, the change rate of the proportion of vegetation was still \(-0.54\%\)/yr (Figure 10), and the rapid reduction in vegetation area threatened the ecological environment. Fortunately, the \"Guangdong-Hong Kong-Macao Greater Bay Area Development Planning Outline\" [61] launched by the Chinese government in 2019 clearly requires the promotion of sustainable development, the creation of ecological protection barriers, the implementation of major projects for the protection and restoration of important ecosystems, the delineation and strict observance of the ecological protection red line, and the strengthening of protection for mountains, hills, and forest ecosystems around the delta. After implementing this ecological protection policy, the rate of reduction in NPP outside the cities in the Pearl River Delta region will be further reduced in the future and, and NPP may increase. There are also some limitations to our research. First, the more detailed NPP changes caused by urban expansion or the change in LULC in China may not be fully captured. This is because the NPP has a spatial resolution of 5 km, but it can still reflect the changes in spatial and temporal scales. We are currently producing NPP products with a spatial resolution of 500 m. We believe that we could better explore this issue in the future. Second, although the NPP products used in the study are produced with low-uncertainty data (such as LAI and FPAR), some uncertainties in their accuracy remain. However, the NPP product was verified through Bigfoot data and compared with the MODIS NPP product, and it was found that the product was reliable. Finally, the temperature, precipitation, sunshine duration, and proportion of vegetation were selected as the variables influencing NPP change. In fact, many other factors influence NPP changes, including human activities, CO\({}_{2}\) concentrations, and nitrogen deposition. Moreover, the NPP products used in the study are calculated based on a LUE model, which does not consider the factors such as the CO\({}_{2}\) concentration and nitrogen deposition. There is a relationship between these factors and other variables, and the effect on NPP is very complex. A simple model cannot well reflect their impact on NPP. Nevertheless, our assessment is still worthwhile because it quantifies the factors that affect urban NPP and clearly shows the differences between different urban agglomerations and buffer zones.