## 4 Results
### China's Terrestrial GPP  
The spatial distributions of terrestrial GPP in China over the period 2003-2012 were generated from our LPI-POD data assimilation system, and their averages are given in Figure 7a (pixel size 0.5\({}^{\circ}\)\(\times\) 0.5\({}^{\circ}\)). Similar distribution trends were found from 2003 to 2012, with a decreasing gradient from the southeast toward the northwest regions, as suggested in previous studies [63; 64]. The highest GPP estimate (\(>\)2000 gC/m\({}^{2}\)/yr) appeared in the Yunnan province, Southeastern Tibet, south of East and South China, mainly with a distribution of evergreen and mixed forests. Warm temperature, plentiful precipitation, and sufficient radiation in these regions provided an advantageous environment for vegetation growth. The lowest GPP (\(<\)700 gC/m\({}^{2}\)/yr) was concentrated in large portions of Tibet and Northwest China and in central regions of Inner Mongolia province, which is partially covered by ice, bare soil, and rock in a relatively cold climate with scarce precipitation [65].  
The _SD_ was applied to indicate inter-annual variability of the GPP during the period 2003-2012 (Figure 7b). Relatively high _SD_ appeared in most of North China, the northeast area of Northwest China, north of East China, and south of Southwest China, indicating greater volatility in these areas.  
The LPI-POD scheme determined that the average annual GPP in China from 2003 to 2012 was about 6.35 PgC/yr, with estimates ranging from 5.92 PgC/yr in 2003 (Figure 8a) to 6.67 PgC/yr in 2012 (Figure 8b). The GPP in 2003, 2006, and 2007 showed relatively low values, at 5.92 PgC/yr, 6.15 PgC/yr, and 6.18 PgC/yr, respectively. Drought conditions occurred frequently and became a major limitation to plant growth and development, especially the drought stress that occurred during the summer of the three years [66; 67; 68]. In spring 2003, a rainfall shortage caused droughts on a large scale in the northeast. Sustained high temperature and diminished rainfall caused a severe drought in South China during the summer and autumn periods of 2003, and a wide range of regional drought severely affected the photosynthetic activities of plants, thus affecting ecosystem productivity [66]. Compared with the drought years of China, 2012 was a year with less meteorological drought, although the northern regions of Central China and East China experienced moderate or severe droughts during May and June [69].  
Figure 6: (**a–c**) Scatterplot of observed versus estimated monthly gross primary productivity (GPP) at the six sites listed in Table 1.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline  & \multicolumn{3}{c}{**r**} & \multicolumn{3}{c}{**RMSD (gC/m\({}^{2}\)/Month)**} & \multicolumn{3}{c}{**RMSDr (\%)**} \\ \cline{2-10}  & \(\mathbf{GPP_{L}}\) & \(\mathbf{GPP_{LE}}\) & \(\mathbf{GPP_{LP}}\) & \(\mathbf{GPP_{L}}\) & \(\mathbf{GPP_{LE}}\) & \(\mathbf{GPP_{LP}}\) & \(\mathbf{GPP_{L}}\) & \(\mathbf{GPP_{LE}}\) & \(\mathbf{GPP_{LP}}\) \\ \hline EBF & 0.635 & 0.776 & **0.884** & 28.878 & 23.501 & **17.779** & 0.235 & 0.197 & **0.145** \\ ENF & 0.892 & 0.919 & **0.953** & 36.403 & 29.478 & **20.837** & 0.292 & 0.224 & **0.144** \\ MF & 0.894 & 0.921 & **0.943** & 57.504 & 48.142 & **39.781** & 0.587 & 0.462 & **0.335** \\ GRA & 0.821 & 0.862 & **0.889** & 48.036 & 41.478 & **36.527** & 0.917 & 0.817 & **0.723** \\ \hline \end{tabular} Bold numbers represent the optimal results among \(GPP_{L}\), \(GPP_{LE}\) and \(GPP_{LP}\).  
\end{table}
Table 3: Accuracy of model-estimated gross primary productivity (GPP) from three models by withholding plant functional type (PFT).