# Spatiotemporal Dynamics of Net Primary Productivity in China's Urban Lands during 1982-2015
## 4 Discussion
[4] found that with urban expansion, the GPP and enhanced vegetation index (EVI) of vegetation within the city both showed a significant decreasing trend. With the expansion of the buffer zones, the reduction rate of NPP in the buffer zone of the YRDUA has gradually become nonsignificant, even though the NPP showed a slight increase in the 20 km buffer zone of urban lands. We found that the importance of meteorological variables increased significantly with the expansion of the buffer zone, and the variable with the fastest increase was temperature. Moreover, the importance of the proportion of vegetation decreased accordingly. The precipitation in the YRDUA showed a decreasing trend, and its contribution to the NPP trend was negative but weaker than that in the BTHUA. This was due to the higher precipitation and the slower decreasing rate of the precipitation than in the BTHUA. Temperature was the variable that made the most positive contribution to the NPP trend. The YRDUA is located in northern subtropical humid area. The optimum temperature of vegetation in this area is ~20-22 \({}^{\circ}\)C [54]. In addition, the optimum temperatures for the main vegetation types in the YRDUA, cropland, and forest, are 24.8 \({}^{\circ}\)C and 23.4 \({}^{\circ}\)C, respectively [55]. The temperature rise promoted the growth of vegetation. The contribution of sunshine duration to the NPP trends changed from negative to positive in the 10 km buffer zone and had an increasing trend. However, in the YRDUA, the variation rate of sunshine duration was relatively minor, almost one-third of that in the BTHUA. The slight variation in sunshine duration also made it less important to NPP. The proportion of vegetation in the YRDUA decreased significantly faster than in the BTHUA. For example, in the 5 km buffer zone, the proportion of vegetation in the YRDUA had a rate of \(-0.81\%\)/yr, while in the BTHUA, the rate was only \(-0.48\%\)/yr (Figure 10). The positive contributions of a series of variables to the NPP trends cannot offset the negative contribution caused by the obvious reduction in vegetation area. Until the 20 km buffer zone, the decline rate of the proportion of vegetation decreased to \(-0.47\%\)/yr, and the other positive contributions were barely higher than the negative contributions. In recent years, the Yangtze River Delta region has also proposed the \"Yangtze River Delta Regional Integration Development Plan,\" which is necessary to improve cross-basin and cross-regional ecological compensation mechanisms [56]. With policy implementation, the future ecological environment will be strictly protected, and the ecological environment around the city will continue to develop healthily. The rate of NPP reduction in the PRDUA was the fastest among the three urban agglomerations. In Foshan and Guangzhou, NPP decreased the most, while NPP increased in southern Shenzhen and northwestern Dongguan. [PERSON], et al. [57] confirmed that tremendous LULC changes caused by rapid urbanization process in the Pearl River Delta region had reduced NPP, mainly because the replacement of cropland and forests with urban lands. A previous study showed that the relationship between precipitation and forest growth had a threshold value [58]. When the average annual cumulative precipitation was between 1000 and 2500 mm, the relationship between mature forest growth and precipitation was not significant [58]. When the average annual cumulative precipitation was higher than 2500 mm, the mature forest growth and precipitation were negatively correlated [58]. The PRDUA is located in the southern subtropical humid area, with abundant precipitation. The accumulated precipitation was significantly higher than the other two urban agglomerations. As shown in Figure 10, precipitation tended to increase in urban lands. In contrast, the precipitation showed a decreasing trend in buffer zones, contrary to the precipitation contribution to the NPP trends. However, this contribution was weak, and its importance to NPP was not high.