## 3 Results
### Spatiotemporal Changes in RE in Northern China's Grasslands  
#### 3.2.1 Spatial Patterns of RE in Northern China's Grasslands  
The mean annual RE for northern China's grasslands from 2001 to 2015 was 366.53 g C m\({}^{-2}\) yr\({}^{-1}\), showing clear spatial heterogeneity. However, the spatial patterns for RE over the 15 years did not change significantly (Figure 5a). Regarding the alpine grasslands on the TP, the mean annual RE was 467.44 g C m\({}^{-2}\) yr\({}^{-1}\), exhibiting a clear decreasing gradient from southeast to northwest (Figure 5b). The alpine shrub meadow had the largest RE (573.64 g C m\({}^{-2}\) yr\({}^{-1}\)), followed by the alpine _Kobesia_ meadow (515.28 g C m\({}^{-2}\) yr\({}^{-1}\)), the alpine swamp meadow (380.94 g C m\({}^{-2}\) yr\({}^{-1}\)), and the alpine meadow steppe (129.29 g C m\({}^{-2}\) yr\({}^{-1}\)). Regarding the temperate grasslands in IM, the mean annual RE was 270.43 g C m\({}^{-2}\) yr\({}^{-1}\), exhibiting a clear decreasing gradient from northeast to southwest (Figure 5b). The highest RE value occurred in the meadow steppe (534.28 g C m\({}^{-2}\) yr\({}^{-1}\)) in the northeast, and the lowest value for RE was recorded in the desert steppe in the southwest (88.15 g C m\({}^{-2}\) yr\({}^{-1}\)). The mean annual RE in the typical steppe was 283.15 g C m\({}^{-2}\) yr\({}^{-1}\). Statistics indicated that the RE for the alpine _Kobesia_ meadow and typical steppe accounted for more than 60% of the total RE in northern China's grasslands (Figure 5c).  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**The Growing Season**} \\ \hline  & **Grassland** & \multirow{2}{*}{**SOCD**} & \multirow{2}{*}{**EVI**} & \multirow{2}{*}{**PAR**} & \multirow{2}{*}{**PRE**} & \multirow{2}{*}{**Ta**} & \multirow{2}{*}{**SoilTex**} & \multirow{2}{*}{**VT**} \\  & **Subtypes** & & & & & & & \\ \hline \multirow{4}{*}{TP} & KO & 0.080 & 0.095 & 0.110 & 0.111 & 0.156 & 0.188 & 0.261 \\  & SH & 0.093 & 0.102 & 0.102 & 0.115 & 0.157 & 0.181 & 0.250 \\  & SW & 0.065 & 0.094 & 0.121 & 0.117 & 0.147 & 0.194 & 0.263 \\  & AS & 0.040 & 0.051 & 0.146 & 0.117 & 0.165 & 0.205 & 0.276 \\ \hline \multirow{4}{*}{IM} & MS & 0.050 & 0.128 & 0.096 & 0.169 & 0.117 & 0.182 & 0.258 \\  & TS & 0.038 & 0.083 & 0.125 & 0.184 & 0.120 & 0.188 & 0.263 \\  & DS & 0.034 & 0.042 & 0.152 & 0.155 & 0.110 & 0.200 & 0.306 \\ \hline \multicolumn{6}{c}{**The Non-Growing Season**} \\ \hline  & **Grassland** & \multirow{2}{*}{**SOCD**} & \multirow{2}{*}{**EVI**} & \multirow{2}{*}{**PAR**} & \multirow{2}{*}{**PRE**} & \multirow{2}{*}{**Ta**} & \multirow{2}{*}{**SoilTex**} & \multirow{2}{*}{**VT**} \\  & **subtypes** & & & & & & \\ \hline \multirow{4}{*}{TP} & KO & 0.122 & 0.075 & 0.110 & 0.097 & 0.084 & 0.216 & 0.296 \\  & SH & 0.139 & 0.079 & 0.105 & 0.101 & 0.088 & 0.207 & 0.281 \\  & SW & 0.106 & 0.074 & 0.103 & 0.092 & 0.085 & 0.231 & 0.308 \\  & AS & 0.070 & 0.069 & 0.083 & 0.088 & 0.135 & 0.238 & 0.317 \\ \hline \multirow{4}{*}{IM} & MS & 0.116 & 0.085 & 0.080 & 0.086 & 0.085 & 0.226 & 0.322 \\  & TS & 0.121 & 0.077 & 0.092 & 0.083 & 0.069 & 0.233 & 0.324 \\ \cline{1-1}  & DS & 0.113 & 0.066 & 0.061 & 0.077 & 0.100 & 0.228 & 0.354 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Relative importance statistics for the environmental variables. All the notations in the grassland subtypes column are consistent with those in Figure 4.