# Spatiotemporal Dynamics of Net Primary Productivity in China's Urban Lands during 1982-2015
## 4 Discussion  
Based on our results, we confirmed that the range of the annual NPP in China is between 2751 and 3343 Tg C, which is consistent with previous studies [37; 38]. In addition, the NPP in China shows a significant increasing trend from 1982 to 2015. After 1997, China's rate of increase in NPP slowed. [PERSON], et al. [39] also demonstrated that the increased rate of NPP after 2000 decreased to 0.008 Pg C/yr, similar to our results. Since the reform and opening-up policy of the late 1970s, China's hydroclimatic conditions and LULC have undergone extensive changes, which have led to dramatic changes in China's terrestrial ecosystem [39; 40; 41]. Rapid urban expansion and economic development have caused tremendous changes to the LULC and ecological environment in and around the cities. In this study, we found that most of the newly added urban lands were converted from croplands. [PERSON], et al. [9] showed that from 1992 to 2015, China's urban expansion put pressure on food security and reduced the average annual food self-sufficiency rate by 2%. In this study, whether within the permanent urban lands or the urban lands in 2015, NPP presented a decreasing trend. Using a piecewise regression analysis, we found that 1997 was the turning point of NPP change. Although the NPP still showed a decreasing trend in the two time periods, the environmental conditions improved, and the rate of decrease decreased significantly. China is a largely agricultural country [42], so both the output and security of grain are related to the country's long-term interests and the foundation of national survival. Since 1978, the Chinese government has been aware of the importance of protecting cropland, and in 1998, the Chinese government formally confirmed the legislation that \"rational use of land and effective protection of cropland is China's basic national policy.\" This also echoes our research results. In addition, a series of ecological and environmental protection policies will also affect changes in NPP, such as ecological redlines, which mainly protect areas with important ecological functions and a fragile ecological environment [43; 44]. [PERSON], et al. [45] indicated that concentrated urban forest planning could alleviate NPP decreases. These policies need to be considered for NPP changes and future predictions. The three urban agglomerations have a strong position in China in terms of both population and economy. However, most of the above studies have explored NPP changes within the city's administrative scope. In this study, the temporal and spatial changes in NPP in the urban lands of the three urban agglomerations were studied separately. Additionally, the NPP changes in the urban land buffer zones (~5 km-20 km) were explored. Furthermore, to investigate the multiple variables affecting urban NPP, we calculated the importance and the contribution of multiple variables to the NPP trends of urban lands and their buffer zones in different urban agglomerations. We expected that by investigating and analyzing the changes in the ecological environment in these three urban agglomerations, we could predict changes in the ecological environment in other developing cities or agglomerations in the future. The Beijing-Tianjin-Hebei region is China's capital economic circle. Our study indicated that in the ETHUA, the NPP showed a decreasing trend, and the decrease in NPP was mainly concentrated in Beijing, Tianjin, and Shijiazhuang. [PERSON], et al. [46] found that in Beijing, the core city of the BTHUA, urban expansion caused a loss of NPP at both the temporal and spatial scales, consistent with our research. In the BTHUA, the proportion of vegetation made a major negative contribution to the NPP trends in urban lands. Its contribution became positive and gradually increased in the ~5-20 km buffer zones. Correspondingly, the importance of the proportion of vegetation also increased. Although the proportion of vegetation in the ~5-20 km buffer zones had a decreasing trend, this trend was slight.