## 4 Results and Discussion
### Indices Selection and Seasonal Variation of All Indices
#### 4.2.2 Seasonal Variation of All Indices  
The variations of C fluxes from 2008 to 2009 are illustrated in Figure 6. The C fluxes are presented as a significant single-peaked variation characteristic in one year. C fluxes can increase to the peak values late in the growing season (summer) and decrease at the end of the growing season each year (Figure 6). The data presented in Figure 7 illustrates the temporal changes of the 19 indices from 2008 to 2009 and contrasts their C flux change trends.  
In order to investigate the seasonal variation of different indices, all indices were divided into different index types (Table 2) and subsets (Figure 7a-f). For different index types, seasonal change trends were variable. The indices in the same subset had a similar seasonal change trend, e.g., C fluxes (single-peaked characteristic annually), especially in the narrowband subsets of Figure 7c-f. NDVI, EVI,  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multicolumn{1}{c}{**Index type**} & \multicolumn{1}{c}{**Name**} & \multicolumn{1}{c}{**Expression form**} & \multicolumn{1}{c}{**Reference**} \\  & & Blue reflectance & Average (R\({}_{450}\)-R\({}_{500}\)) & - \\  & single band & Green reflectance & Average (R\({}_{500}\)-R\({}_{500}\)) & - \\  & & Red reflectance & Average (R\({}_{500}\)-R\({}_{500}\)) & - \\  & & NIR reflectance & Average (R\({}_{500}\)-R\({}_{500}\)) & - \\ \cline{2-3}
**Broadband** & & NDVI & & \\
**index** & & EVI & \(2.5\times((\text{R}_{\text{NIR}}-\text{R}_{\text{abs}})/(\text{R}_{\text{NIR}}+ \text{R}_{\text{abs}}))\) & [59, 60] \\  & ratio index & & \(((\text{R}_{\text{NIR}}-\text{R}_{\text{abs}})/(\text{R}_{\text{NIR}}+ \text{R}_{\text{abs}}))/((\text{R}_{\text{NIR}}-\text{R}_{\text{abs}})/( \text{R}_{\text{NIR}}+\text{R}_{\text{abs}})\) \\  & & Canopy Chlorophyll Content Index & \(R_{\text{Radi}})\) & [61] \\  & & & & \\  & & Chlorophyll/ vegetation index & \(\text{R}_{\text{NIR}}\times\text{R}_{\text{R}_{\text{R}_{\text{R}_{\text{R}_{ \text{R}_{\text{R}_{\text{R}_{\text{R}_{\text{R}_{\text{R}_{\text{R}_{ }_{\text{R}_{\text{R}_{\text{R}}_{\text{R}_{\text{R}}_{\text{R}}_{\text{R} }_{\text{R}_{\text{R}}_{\text{R}}_{\text{R}}_{\text{R}}_{\text{R}}_{\text{Rand some narrowband chlorophyll and nitrogen content indices had a single-peaked seasonal variation each year. All the red-edge indices and the LUE model indices had consistent temporal change trends with GPP and Re. The single band reflectance index did not capture the seasonal variation of C fluxes.  
Figure 6: Temporal changes of C fluxes from 2008 to 2009. The red curve in the left plot illustrates the moving average line within 10 values. (**a–c**) show the annual variation of NEE, GPP and Re respectively from 2008 to 2009.  
Figure 7: Seasonal variation (2008–2009) of 19 indices in five index categories. (**a**) includes four single-band indices; (**b**) includes four broadband ratio indices; (**c**,**d**) include five chlorophyll and nitrogen content indices; (**e**) includes four red-edge indices; (**f**) includes two light use efficiency (LUE) model indices.  
Abbreviations: CCCI (Canopy Chlorophyll Content Index); CVI (Chlorophyll vegetation index); LCI (Leaf Chlorophyll Index); CI (Chlorophyll Index RedEdge 710).  
_4.3. Bio-Indicator Selection and Regression Model Building of C Fluxes_