# Spatiotemporal Dynamics of Net Primary Productivity in China's Urban Lands during 1982-2015
## 3 Results
### Annual NPP of Urban Lands in China from 1982 to 2015  
In this study, the urban lands extracted from CLUD data for the 1980s and 2015 were used to evaluate long-term NPP variations in China's urban lands. The 1980s urban lands can represent permanent urban lands, and the NPP variations within these areas can reflect the vegetation changes caused by urban development and greening. The 2015 urban lands were utilized to study the trend in NPP caused by urbanization processes.  
As shown in Figure 5a,b, the total NPP and average NPP variations based on the urban lands in the 1980s and 2015 were determined. In Figure 5a, the total NPP of the 1980s and 2015 urban lands both showed a slightly decreasing trend. However, within the 2015 urban lands, the total NPP decrease rate was \(-0.0185\) Tg C/yr, which is higher than that of the 1980s (\(-0.0034\) Tg C/yr). The average NPP has the same trend as the total NPP, and the decrease rates of the average NPP within the 1980s and 2015 urban lands were \(-0.2081\) g C/m\({}^{2}\)/yr and \(-0.3355\) g C/m\({}^{2}\)/yr, respectively.  
According to the trend of the annual total NPP in China (Figure 3) and the 2015 urban lands (Figure 5a), from 1982 to 2015, China's NPP increased by 566.23 Tg C, while the NPP based on the 2015 urban lands decreased by 0.36 Tg C, accounting for 0.063% of China's NPP growth. According to the piecewise regression model, 1997 was extracted as the turning point of the time series. From 1982 to 1997, China's NPP increased by 241.06 Tg C, while the NPP based on the 2015 urban lands decreased by 0.32 Tg C, accounting for 0.13% of China's NPP growth. From 1998 to 2015, China's NPP growth rate slowed, with an increase of 195.98 Tg C, while the NPP based on 2015 urban lands decreased by 1.37 Tg C, accounting for 0.70% of China's NPP growth. From the change in average NPP based on the 1980s urban lands, the rate of NPP change from 1982 to 1997 was \(-2.31\) g C/m\({}^{2}\)/yr, while from 1998 to 2015, the rate became \(-0.21\) g C/m\({}^{2}\)/yr. This result indicated that the decreasing NPP trend for the permanent urban lands weakened, and the ecological environment improved. From the average NPP based on the 2015 urban lands, the NPP change rates in the two periods from 1982 to 1997 and 1998 to 2015 were \(-1.80\) g C/m\({}^{2}\)/yr and \(-1.66\) g C/m\({}^{2}\)/yr, respectively, and the rate of decrease was also slightly reduced.  
Figure 4: (**a**) Slope of NPP obtained from the regression of each single pixel during the period of 1982 to 2015 and (**b**) the relative levels of significance.  
To explore the spatial distribution of land use types converted to urban lands, changes in land cover types were extracted within the 2015 urban lands based on CLUD. As shown in Figure 6, China's urbanization took up a large amount cropland. In 2015, 72.85% of urban lands were converted from cropland, and these urban lands were mainly distributed in the central and eastern regions of China. Additionally, 9.68% and 9.21% of urban lands were converted from forest and grassland, respectively, mainly distributed in the northern and southern regions of China.  
Figure 5: (**a**) Total NPP trend and (**b**) average NPP trend based on urban lands in the 1980s (blue lines, left-hand vertical axis) and 2015 (red lines, right-hand vertical axis).  
Figure 6: Spatial distribution of land use types converted to urban lands. Displayed by land use type with the largest proportion in 3 \(\times\) 3 km pixels.