## 3 Results
### Modeling of Hourly Carbon Fluxes
The scatter plots of ANFIS modeled and measured hourly NEE flux for both nighttime and daytime datasets at the three sites are presented in Figure 7d-f. As shown in the figures, considerable difference between nighttime and daytime datasets can be found in the magnitude and variation of hourly NEE for an entire testing year, which are largely reduced during nighttime primarily due to the absence of photosynthesis. For the daytime datasets among the three sites, the fit line of the ANFIS model at CA-Oas site is closest to the ideal straight line of slope one, while this model have the highest value of \(R^{2}\) at CA-Obs site. However, for the nighttime datasets among the three sites, the best estimates occur at CA-Gro site in terms of both the slope of the fit line and \(R^{2}\) value. \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Site**} & \multirow{2}{*}{**Model**} & \multicolumn{4}{c}{**GPP**} & \multicolumn{4}{c}{**R**} & \multicolumn{4}{c}{**NEE**} \\ \cline{3-13}  & & \(\mathbf{R^{2}}\) & **NSE** & **Bias** & **RMSE** & \(\mathbf{R^{2}}\) & **NSE** & **Bias** & **RMSE** & \(\mathbf{R^{2}}\) & **NSE** & **Bias** & **RMSE** \\ \hline \multirow{4}{*}{CA-Obs} & ANN & 0.932 & 0.910 & \(-\)0.015 & 0.056 & 0.843 & 0.832 & 0.001 & 0.036 & 0.850 & 0.821 & 0.013 & 0.060 \\  & GRNN & 0.924 & 0.905 & \(-\)0.009 & 0.057 & 0.827 & 0.577 \begin{table}
\begin{tabular}{c c c c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Model**} & \multirow{2}{*}{**Dataset**} & \multicolumn{4}{c}{**CA-Obs**} & \multicolumn{4}{c}{**CA-Oas**} & \multicolumn{4}{c}{**CA-Gro**} \\ \cline{3-13}  & & \(R^{2}\) & **NSE** & **Bias** & **RMSE** & \(R^{2}\) & **NSE** & **Bias** & **RMSE** & \(R^{2}\) & **NSE** & **Bias** & **RMSE** \\ \hline \multirow{2}{*}{ANN} & Daytime & 0.847 & 0.809 & 0.023 & 0.076 & 0.822 & 0.820 & \(-\)0.010 & 0.121 & 0.765 & 0.761 & 0.012 & 0.111 \\  & Nighttime & 0.566 & 0.533 & 0.004 & 0.041 & 0.590 & 0.580 & \(-\)0.005 & 0.052 & 0.799 & 0.674 & 0.015 & 0.041 \\ \hline \multirow{2}{*}{GRNN} & Daytime & 0.824 & 0.784 & 0.024 & 0.081 & 0.814 & 0.814 & 0.003 & 0.123 & 0.750 & 0.677 & 0.061 & 0.129 \\  & Nighttime & 0.400 & 0.385 & 0.006 & 0.047 & 0.601 & 0.589 & \(-\)0.002 & 0.051 & 0.803 & 0.251 & 0.052 & 0.062 \\ \hline \multirow{2}{*}{ANFIS} & Daytime & 0.841 & 0.799 & 0.025 & 0.078 & 0.814 & 0.814 & \(-\)0.005 & 0.123 & 0.753 & 0.751 & 0.008 & 0.113 \\  & Nighttime & 0.544 & 0.511 & 0.003 & 0.042 & 0.419 & 0.414 & \(-\)0.003 & 0.061 & 0.684 & 0.568 & 0.012 & 0.047 \\ \hline \multirow{2}{*}{SVM} & Daytime & 0.844 & 0.807 & 0.022 & 0.077 & 0.822 & 0.820 & \(-\)0.007 & 0.121 & 0.757 & 0.753 & 0.012 & 0.113 \\  & Nighttime & 0.496 & 0.476 & 0.002 & 0.043 & 0.582 & 0.569 & \(-\)0.008 & 0.053 & 0.784 & 0.645 & 0.015 & 0.043 \\ \hline \multirow{2}{*}{MLR} & Daytime & 0.483 & 0.456 & 0.021 & 0.129 & 0.453 & 0.443 & \(-\)0.014 & 0.213 & 0.457 & 0.451 & 0.017 & 0.169 \\  & Nighttime & 0.073 & 0.061 & \(-\)0.003 & 0.058 & 0.121 & 0.176 & \(-\)0.010 & 0.107 & 0.012 & 0.105 & 0.010 & 0.075 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Comparisons of different models with four input variables for predicting hourly net ecosystem exchange (NEE, g C m\({}^{-2}\) hour\({}^{-1}\)) in the testing period for daytime and nighttime datasets in the three boreal forest stands. Figure 7: Eddy covariance measured and ANFIS predicted hourly net ecosystem exchange (NEE) in the testing period. (**a**–**c**) Daily course of hourly NEE for 7 days during the mid-growing season at the three sites; and (**d**–**f**) Scatter plot of hourly NEE both in the daytime and nighttime datasets at each site.