## 4 Discussion
### Seasonal Dynamics in Sasa Community GPP and Annual Sasa GPP  
The seasonal trends differed between the evergreen coniferous forest and the other forests (Figure 7), partly consistent with our hypothesis. The 10-day cumulative _Sasa_ community GPP in the evergreen coniferous forest did not fluctuate greatly, even though the 10-day cumulative PPFD decreased from spring to autumn. This can be explained by the higher light use efficiency in summer and autumn than in spring in the evergreen coniferous forest (Tables 1 and 2). The 10-day cumulative _Sasa_ community GPP was the lowest in summer in both deciduous broadleaved forest and mixed forest because of the low light intensity in both forests and the high T\({}_{A}\) in the gap area in the deciduous broadleaved \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Sasa Community GPP per Leaf** & **Sasa Community GPP** \\ \hline  & **g C m\({}^{-2}\) yr\({}^{-1}\)** & **g C m\({}^{-2}\) yr\({}^{-1}\)** \\ \hline Evergreen coniferous forest & 83.2 & 190.8 \\ \hline Mixed forest & 80.7 & 176.8 \\ \hline Canopy area in Deciduous broadleaved forest & 93.2 & 178.6 \\ \hline Gap area in Deciduous broadleaved forest & 130.8 & 276.7 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Annual _Sasa_ community GPP per leaf area and _Sasa_ community GPP in the four sites. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **Annual Sasa GPP** & **Tree GPP** & **Contribution** \\ \hline  & t C ha\({}^{-1}\) yr\({}^{-1}\) & t C ha\({}^{-1}\) yr\({}^{-1}\) & \% \\ \hline Evergreen coniferous forest & 1.9 & \(11.2\pm 1.3\) & \(13.1\pm 1.3\) \\ \hline Mixed forest & 1.8 & \(8.4\pm 4.0\) & \(19.1\pm 6.9\) \\ \hline Deciduous broadleaved forest & 2.1 & \(16.8\pm 1.7\) & \(11.2\pm 0.9\) \\ \hline Canopy area & 1.8 & - & - \\ \hline Gap area & 2.8 & - & - \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Annual _Sasa_ GPP and tree GPP at the forest ecosystem scale in the four sites and contribution of the _Sasa_ GPP to the forest ecosystem GPP. forest. Moreover, since the solar elevation was lower in autumn, much more sun light was blocked by the branches and trunks of trees. This explains why the 10-day cumulative _Sasa_ community GPP was higher in spring than in autumn. The results that PPFD mostly influenced the _Sasa_ community GPP in all sites, but different factors additionally influenced the _Sasa_ community GPP at each site (Table 3) agree with our second hypothesis. Although _Sasa_ in the gap area in the deciduous broadleaved forest received a relatively large amount of light, still less light reached the forest floor than the forest canopy [34; 35]. This explains why PPFD was still the most important factor that controlled the _Sasa_ community GPP in the gap area in the deciduous broadleaved forest. Plant photosynthetic rate decreases with increasing T\({}_{\rm A}\) above a certain level [36]; that of _Sasa_ decreases above a T\({}_{\rm A}\) of 20-25 \({}^{\circ}\)C [37]. Direct light in gap areas can sharply increase T\({}_{\rm A}\) there [38]. A T\({}_{\rm A}\) of \(>\)30 \({}^{\circ}\)C during CO\({}_{2}\) measurement within the chamber was recorded in the gap area in the deciduous broadleaved forest, and the _Sasa_ community GPP was lower when T\({}_{\rm A}\) was higher at the same PPFD. These facts could explain why the _Sasa_ community GPP in the deciduous broadleaved forest gap area was reduced by T\({}_{\rm A}\) (Table 3d). High RH restricts the transpiration of plant leaves and the CO\({}_{2}\) absorption from the leaves. Since the deciduous broadleaved forest was located in Kayanodaira where fog tends to form easily and to dissolve hardly because of the landform, RH became one of the controlling factors of the _Sasa_ community GPP in the canopy area. The leaf area strongly influences the GPP [5]. Although it affected the _Sasa_ community GPP in the evergreen coniferous forest and mixed forest, it did not do so in the deciduous broadleaved forest (Table 3) on account of the small fluctuations of the _Sasa_ community leaf area there (Figure 4a).