## 3 Results
### Data-Model Comparisons and Model Adjustment
#### 3.1.2 Ecosystem CO\({}_{2}\) Fluxes
Simulated NEP (the difference between GPP and TER) combines the differences of the other two values (Figure 4c). Averaged daily PSIM NEP showed the largest differences from EC NEP values during May due to a poor match with TER, but a better match with TER during winter months resulted in PSIM producing  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline
**Year** & **GPP (g C m\({}^{-2}\) year\({}^{-1}\))** & **TER (g C m\({}^{-2}\) year\({}^{-1}\))** & **NEP (g C m\({}^{-2}\) year\({}^{-1}\))** & **NEP (g C m\({}^{-2}\) year\({}^{-1}\))** \\  & **EC** & **PnET** & **PSIM** & **EC** & **PnET** & **PSIM** & **EC** & **PnET** & **PSIM** \\ \hline
1999 & 1983 & 2440 & 2204 & 1625 & 1805 & 1638 & 357 & 634 & 566 \\
2000 & 2346 & 2127 & 2046 & 1940 & 1641 & 1588 & 406 & 487 & 458 \\
2001 & 2227 & 2089 & 2037 &  
averaged annual total NEP values close to those measured. These net values are arguably the most important as they determine whether the ecosystem is a net sink or source of CO\({}_{2}\) for the year. Differences between measured and simulated daily GPP, TER and NEP (i.e., residuals) were calculated using the PnET and PSIM modules for 1999-2007 (Figure S1). For each component (GPP, TER and NEP), residuals were mostly between \(\pm 5\) g C m\({}^{-2}\) d\({}^{-1}\) for both PnET and PSIM, but the summer residuals were larger in many years, with a maximum GPP residual of \(-13\) g C m\({}^{-2}\) d\({}^{-1}\) in 2006. Residual TER values showed the clearest distinction between PnET and PSIM, mainly due to the absence of autotrophic soil respiration from roots during winter in PnET simulations. The years 2004 and 2006 show the largest summer residuals (negative) for TER from both modules. \begin{table}
\begin{tabular}{c c c c c c c} \hline \multicolumn{8}{c}{**(a)**} \\ \hline \multirow{3}{*}{**Statistic**} & \multicolumn{2}{c}{**Annual GPP**} & \multirow{3}{*}{**Annual TER 1999–2007**} & \multicolumn{3}{c}{**Annual NEP**} \\  & **1999–2007** & & & **1999–2007** & \\  & **PnET** & **PSIM** & **PnET** & **PSIM** & **PnET** & **PSIM** \\ \hline RMSE (\%) & \(14.8\) [FOOTNOTE:]Footnote : No significant total error, * No  
The PnET and PSIM module parameters were adjusted to optimise the fit of the simulations to measurements of annual EC data for GPP, TER and NEP for the period 1999-2007. For the evaluation period, simulations using these parameters were less meaningful because disturbances such as infestations of defoliating moth caterpillars in 2009 and 2010, which are not considered in the simulations, reduced the GPP, TER and, to a lesser extent, NEP, especially in 2010 [28]. It also seems that EC measurements during 1999 to 2010 also showed a significant long-term decrease in annual GPP (\(-46.1\) g C m\({}^{-2}\) year\({}^{-1}\), \(p<0.01\)) and annual TER (\(-44.7\) g C m\({}^{-2}\) year\({}^{-1}\), \(p<0.001\)), with no resultant trend for NEP [29]. These decreasing trends in annual GPP and TER are also represented in the simulations (Figure 3), although this might be partly due to the thinning event in 2007. Simulated annual values show larger declines when using PnET (from linear regression: \(-86.4\) g C m\({}^{-2}\) year\({}^{-1}\) for GPP, \(R^{2}=0.83\), and \(-48.4\) g C m\({}^{-2}\) year\({}^{-1}\) for TER, \(R^{2}=0.87\)) than when using PSIM (from linear regression: \(-26.0\) g C m\({}^{-2}\) year\({}^{-1}\), \(R^{2}=0.52\), and \(-21.7\) g C m\({}^{-2}\) year\({}^{-1}\), \(R^{2}=0.55\), respectively) (see also Section 3.3). Figure 3: Monthly CO\({}_{2}\) fluxes at the Straits Inclosure simulated by PSIM and PnET and measured by eddy covariance (EC): (**a**) GPP, (**b**) TER, (**c**) NEP.