## 3 Results
### Model Performance in Simulating NEE under Extreme Climate Conditions
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**(a) Site**} & \multirow{2}{*}{**IGBP**} & \multirow{2}{*}{**Model**} & \multicolumn{3}{c}{**R\({}^{2}\)**} & \multicolumn{2}{c}{**RMSE (g C m\({}^{-2}\) d\({}^{-1}\))**} & \multicolumn{2}{c}{**MAE (g C m\({}^{-2}\) d\({}^{-1}\))**} \\ \cline{5-10}  & & & **Normal** & **Cold** & **Normal** & **Cold** & **Normal** & **Cold** \\ \hline \multirow{2}{*}{AU-Tum} & EBF & RF & 0.23 & 0.20 & 2.75 & 4.32 & 1.94 & 3.15 \\  & CRO & XGB & 0.21 & 0.19 & 2.78 & 4.36 & 2.04 & 3.23 \\  & CRO & RF & 0.77 & 0.67 & 1.76 & 2.21 & 1.14 & 1.41 \\  & & XGB & 0.82 & 0.74 & 1.52 & 1.98 & 0.97 & 1.17 \\  & & RF & 0.91 & 0.83 & 1.15 & 1.49 & 0.83 & 1.00 \\  & DBF & XGB & 0.93 & 0.86 & 1.02 & 1.34 & 0.75 & 0.90 \\  & ENF & RF & 0.89 & 0.88 & 0.63 & 0.51 & 0.43 & 0.34 \\  & & XGB & 0.90 & 0.87 & 0.59 & 0.53 & 0.39 & 0.35 \\  & & RF & 0.59 & 0.50 & 0.88 & 0.95 & 0.58 & 0.63 \\  & & XGB & 0.53 & 0.44 & 0.95 & 1.01 & 0.63 & 0.64 \\ \hline \multirow{2}{*}{**(b) Site**} & \multirow{2}{*}{**IGBP**} & \multirow{2}{*}{**Model**} & \multicolumn{3}{c}{**R\({}^{2}\)**} & \multicolumn{3}{c}{**RMSE (g C m\({}^{-2}\) d\({}^{-1}\))**} & \multicolumn{2}{c}{**MAE (g C m\({}^{-2}\) d\({}^{-1}\))**} \\ \cline{5-10}  & & & **Normal** & **Heat** & **Normal** & **Heat** & **Normal** & **Heat** \\ \hline \multirow{2}{*}{CA-Gro} & MF & RF & 0.77 & 0.75 & 0.70 & 0.68 & 0.46 & 0.42 \\  & & XGB & 0.81 & 0.76 & 0.64 & 0.67 & 0.43 & 0.41 \\  & & RF & 0.62 & 0.50 & 1.14 & 1.06 & 0.77 & 0.70 \\  & & XGB & 0.59 & 0.51 & 1.18 & 1.05 & 0.80 & 0.71 \\  & & RF & 0.87 & 0.75 & 1.12 & 1.45 & 0.74 & 1.00 \\  & & XGB & 0.87 & 0.78 & 1.10 & 1.37 & 0.78 & 0.96 \\  & & RF & 0.81 & 0.71 & 0.67 & 0.99 & 0.46 & 0.63 \\  & & XGB & 0.78 & 0.66 & 0.73 & 1.08 & 0.49 & 0.66 \\ \hline \multirow{2}{*}{**(c) Site**} & \multirow{2}{*}{**IGBP**} & \multirow{2}{*}{**Model**} & \multicolumn{3}{c}{**R\({}^{2}\)**} & \multicolumn{3}{c}{**RMSE (g C m\({}^{-2}\) d\({}^{-1}\))**} & \multicolumn{3}{c}{**MAE (g C m\({}^{-2}\) d\({}^{-1}\))**} \\ \cline{5-10}  & & & **Normal** & **Wetness** & **Normal** & **Wetness** & **Normal** & **Wetness** \\ \hline \multirow{2}{*}{BE-Lon} & CRO & RF & 0.85 & 0.58 & 1.66 & 2.71 & 1.00 & 1.70 \\  & & XGB & 0.84 & 0.64 & 1.75 & 2.52 & 1.15 & 1.63 \\  & & RF & 0.78 & 0.80 & 0.69 & 0.60 & 0.47 & 0.41 \\  & & XGB & 0.83 & 0.82 & 0.60 & 0.56 & 0.41 & 0.36 \\  & & RF & 0.88 & 0.91 & 0.67 & 0.51 & 0.45 & 0.35 \\  & & XGB & 0.90 & 0.92 & 0.61 & 0.48 & 0.42 & 0.32 \\  & & RF & 0.83 & 0.81 & 1.28 & 1.36 & 0.83 & 0.90 \\  & & XGB & 0.86 & 0.83 & 1.18 & 1.28 & 0.82 & 0.86 \\ \hline \multirow{2}{*}{**(d) Site**} & \multirow{2}{*}{**IGBP**} & \multirow{2}{*}{**Model**} & \multicolumn{3}{c}{**R\({}^{2}\)**} & \multicolumn{3}{c}{**RMSE (g C m\({}^{-2}\) d\({}^{-1}\))**} & \multicolumn{3}{c}{**MAE (g C m\({}^{-2}\) d\({}^{-1}\))**} \\ \cline{5-10}  & & & **Normal** & **Dryness** & **Normal** & **Dryness** & **Normal** & **Dryness** \\ \hline \multirow{2}{*}{DE-Hai} & \multirow{2}{*}{DBF} & RF & 0.81 & 0.86 & 1.65 & 1.65 & 1.14 & 1.14 \\  & & XGB & 0.85 & 0.90 & 1.45 & 1.36 & 0.99 & 0.92 \\ \multirow{2}{*}{US-Ton} & \multirow{2}{*}{SAV} & RF & 0.72 & 0.59 & 0.86 & 0.72 & 0.57 & 0.49 \\  & & XGB & 0.73 & 0.43 & 0.85 & 0.85 & 0.59 & 0.56 \\ \multirow{2}{*}{US-Wkg} & \multirow{2}{*}{GRA} & RF & 0.55 & 0.30 & 0.48 & 0.25 & 0.28 & 0.13 \\  & & XGB & 0.54 & 0.14 & 0.48 & 0.27 & 0.30 & 0.15 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Comparative analysis of NEE predicted by the RF and XGBoost in extreme climate conditions, (a–d) denote extreme cold, extreme heat, extreme wetness, and extreme dryness, respectively. Normal refers to the evaluation of the non-extreme samples. The screening method for each extreme site and year is described in Section 2.3.2. (NTS) when simulating the NEE of different biomes at site-level. The prediction results of two models were similar according to BAS (Figure 4a-d) or BFS (Figure 4e-h) method.