# Spatiotemporal Dynamics of Net Primary Productivity in China's Urban Lands during 1982-2015
## 3 Results
### The Contribution of Multiple Variables to the NPP Trends of the Three Urban Agglomerations  
To evaluate the contribution of environmental variables to the NPP trends, we established a fitting relationship between multiple variables and NPP based on the multiple linear regression model, and estimated the contribution of the variable to the NPP trends through the regression coefficients of each variable (Figure 12). In the BTHUA, the proportion of vegetation was the most important factor leading to the downward trend of NPP in urban lands, contributing \(-1.77\) g C/m\({}^{2}\)/yr to the rate of change, followed by the annual cumulative precipitation and the annual average air temperature, with contributions of \(-0.014\) g C/m\({}^{2}\)/yr and \(-0.16\) g C/m\({}^{2}\)/yr, respectively. The annual cumulative sunshine duration promoted the increase of NPP, its contribution was 0.17 g C/m\({}^{2}\)/yr, and obviously it failed to offset the inhibitory effect of other variables on the increase of NPP. In the buffer zones around urban lands, the proportion of vegetation promoted the increase of NPP, and its contribution gradually increased with the expansion of the buffer zones, reaching the highest level in the 20 km buffer zone (2.19 g C/m\({}^{2}\)/yr). Among these three meteorological variables, with the expansion of the buffer zone, the inhibitory effect of the annual accumulated precipitation on the increase of NPP was further enhanced. It gradually became the main factor that made a negative contribution to the NPP trends. In contrast, the annual average air temperature became one of the causes of the increase of NPP, and its contribution gradually increased. The annual cumulative sunshine duration inhibited the increase of NPP in the 5 km buffer zones and contributed \(-0.14\) g C/m\({}^{2}\)/yr to the NPP trends, while it became one of the causes of the increase of NPP in the \(\sim\)10-20 km buffer zones. In the YRDUA, Figure 12b shows that changes in the annual cumulative precipitation and the proportion of vegetation were the reasons for the decrease of NPP, and the vegetation reduction was the main reason for the decrease of NPP. From urban lands to the 20 km buffer zone, the negative contribution of the two to the decrease of NPP weakened. Additionally, the annual average air temperature was the main factor that promoted the increase of NPP and its positive contribution was significantly enhanced with the expansion of the buffer zone, reaching 0.52 g C/m\({}^{2}\)/yr in the 20 km buffer zone. The annual cumulative sunshine duration was the factor that caused the decrease of NPP, but its effect gradually decreased, becoming the factor that caused the increase of NPP in the 10 km buffer zone. With the expansion of the buffer zone, the positive contribution of the annual cumulative sunshine duration to NPP gradually strengthen, but its contribution was slight. In the PRDUA, among all the variables, the annual cumulative sunshine duration and the proportion of vegetation were always the reasons for the decrease of NPP. However, from the urban lands to the 20 km buffer zone, the effect of the proportion of vegetation decreased and its contribution decreased from \(-4.45\) g C/m\({}^{2}\)/yr to \(-1.73\) g C/m\({}^{2}\)/yr. In contrast, the effect of the annual cumulative sunshine duration increased and its contribution increased from \(-0.0065\) g C/m\({}^{2}\)/yr to \(-0.20\) g C/m\({}^{2}\)/yr. In addition, the annual average air temperature was the main factor to promote the increase of NPP. Unfortunately, this promotion and positive contribution also decreased as the buffer zone expanded. The influence of the annual cumulative precipitation to the NPP trends in urban lands and buffer zones in PRDUA was almost negligible. In urban lands, the annual cumulative precipitation made NPP decrease and its negative contribution was only \(-0.006\) g C/m\({}^{2}\)/yr.