# Spatiotemporal Dynamics of Net Primary Productivity in China's Urban Lands during 1982-2015
## 4 Discussion
For the BTHUA, there are many nature reserves around the urban lands. As of 2014, 71 nature reserves have been established in the Beijing-Tianjin-Hebei region, with a total area of 9177.5 km\({}^{2}\)[47]. In addition, the Beijing-Tianjin-Hebei area is located in the \"Three North Shelterbelt Project\" area, which was initiated in 1978 and is the world's largest ecological restoration project [23]. A series of environmental protection measures taken in the Beijing Tianjin Hebei region was also an important factor in reducing theimpact of urban development on the ecosystem. In the BTHUA, the proportion of cropland in the buffer zone was the highest among the three urban agglomerations. To increase the yield of crops, the government has taken a series of field measures, including the promotion of high-quality crop varieties, the optimization of irrigation strategies aimed at reducing water shortages, and rational fertilization. In addition, the government has laid out basic farmland protection measures in Beijing Tianjin Hebei region to restrain urban expansion. In addition, the process of urban expansion and industrialization led to an increase in the CO\({}_{2}\) concentration in the atmosphere. Many studies have suggested that an increase in the CO\({}_{2}\) concentration produces a \"CO\({}_{2}\) fertilization effect\" on vegetation [48; 49; 50]. The experiments on forest plots by [PERSON], et al. [48] showed that when the CO\({}_{2}\) concentration rose from 0.4% to 0.5%, the forest NPP increased by 23%. The increase in the CO\({}_{2}\) concentration promoted the photosynthesis of the vegetation that still existed. All of these contributed to the increase of NPP. Moreover, meteorological factors cannot be ignored. The importance of meteorological variables in urban lands was even higher than that of the proportion of vegetation, and the sunshine duration was the most important meteorological variable, the only one that made a positive contribution to the NPP trends in urban lands. During a given period, the sunshine duration is defined as the sum of that subperiod for which the direct solar irradiance exceeds 120 W/m\({}^{2}\)[51]. Its contribution to the NPP trends was basically positive, and the sunshine duration in the BTHUA was higher than that in the other two urban agglomerations. Although the sunshine duration was decreasing (Figure 10), compared with the solar direct radiation, when the sky has clouds, the photosynthetic rate of the vegetation canopy is higher than when the sky is clear because cloudy days cause light scattering. It is easier for light to enter the canopy to allow more leaves to obtain solar radiation, and the increase of diffuse solar radiation could increase the LUE of crop [52]. In the BTHUA, the temperature increased temporarily promoted the growth of vegetation. The optimum temperature of vegetation refers to the environmental temperature most conducive to vegetation growth and the temperature at which the vegetation's photosynthetic enzyme shows the maximum activity [53]. The BTHUA is located in a warm temperate subhumid area, and the optimum temperature for vegetation in this area is ~18-20 \({}^{\circ}\)C [54]. A previous study which classified the optimal temperature according to vegetation type indicated that the optimal temperature for cropland in China was 24.8 \({}^{\circ}\)C [55]. The precipitation showed decreasing trends (Figure 10) and negatively contributed to the NPP trends. However, a slight change made its importance to NPP less obvious. In the YRDUA, our results showed that the NPP decreased in urban lands and was more significant than that in the BTHUA. The areas where NPP decreased significantly were Shanghai, Suzhou, and Jiaxing, consistent with the conclusions of many other studies. For example, when studying the urban expansion and vegetation dynamics in Shanghai, the central city of the Yangtze River Delta, [PERSON], et al.