## 3 Results
### Annual Analysis between MODIS Products and Flux Tower GPP  
#### 3.2.1 NDVI and EVI _vs._ Annual Mean GPP  
When all sites were analyzed at a 32-day resolution, annual mean NDVI and EVI showed comparable correlations with annual GPP (Figure 4). When the GPP of tropical and non-tropical sites were compared separately to EVI, however, the correlation was much lower and non-significant (Table 4 and Figure 4(b)). In contrast, NDVI showed moderate correlation both for all sites and non-tropical sites, but the linear relationship between NDVI and annual GPP collapsed at NDVI greater than 0.75.  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline  & & **Composite** & **All** & **Deciduous** & **Evergreen** & **Non-Tropical** & **Tropical** \\  & & **Period** & **(n = 21)** & **(n = 5)** & **(n = 16)** & **(n = 16)** & **(n = 5)** \\ \hline \multirow{4}{*}{**MOD13**} & **NDVI** & **16-day** & 0.44 (2.08) & 0.64 (2.36) & 0.38 (1.98) & 0.51 (2.14) & 0.13 (1.78) \\  & & **32-day** & 0.47 (1.94) & 0.65 (2.31) & 0.41 (1.80) & 0.54 (2.00) & 0.16 (1.68) \\ \cline{2-8}  & **EVI** & **16-day** & 0.55 (1.70) & 0.78 (1.73) & 0.47 (1.69) & 0.64 (1.70) & 0.11 (1.67) \\  & & **32-day** & 0.54 (1.67) & 0.77 (1.80) & 0.46 (1.62) & 0.62 (1.72) & 0.20 (1.42) \\ \hline \multirow{4}{*}{**MOD15**} & **LAI** & **8-day** & 0.38 (2.21) & 0.60 (2.56) & 0.30 (2.08) & 0.44 (2.31) & 0.13 (1.82) \\  & **16-day** & 0.43 (1.86) & 0.69 (2.05) & 0.34 (2.02) & 0.50 (2.09) & 0.13 (1.75) \\ \cline{1-1}  & & **32-day** & 0.51 (1.70) & 0.69 (2.09) & 0.38 (1.77) & 0.56 (1.89) & 0.10 (1.72) \\ \cline{1-1} \cline{2-8}  & **8-day** & **0.31 (2.42)** & 0.54 (2.81) & 0.23 (2.28) & 0.35 (2.57) & 0.14 (1.81) \\ \cline{1-1}  & FPAR & **16-day** & 0.36 (2.26) & 0.62 (2.45) & 0.27 (2.20) & 0.41 (2.37) & 0.12 (1.77) \\ \cline{1-1}  & & **32-day** & 0.40 (1.90) & 0.62 (2.38) & 0.33 (1.91) & 0.47 (2.19) & 0.12 (1.63) \\ \hline
**MOD17** & **GPP** & **8-day** & 0.68 (1.31) & 0.72 (1.95) & 0.67 (1.11) & 0.82 (1.17) & 0.12 (1.91) \\ \hline \hline \end{tabular}
\end{table}
Table 3: Average \(R^{2}\) between each MODIS product and flux tower GPP for each composite period. The values in parenthesis are root mean square error (RMSE; gC/m\({}^{2}\)/day) calculated by reduced major axis regression analysis. Average \(R^{2}\) and RMSE are the average of all the \(R^{2}\) and RMSE calculated from each year of each site, respectively. n is number of sites used for the calculations.  
Similarly to the short-term analysis, compositing to 32-day did not significantly change the results (Table 4).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multicolumn{2}{c}{} & \multicolumn{2}{c}{**Composite Period**} & **All Sites** & **Non-Tropical Sites** & **Tropical Sites** \\ \hline \multirow{4}{*}{**MOD13**} & **NDVI** & 16-day & 0.50** & 0.48** & 0.30 \\  & 32-day & 0.57** & 0.42** & 0.32 \\ \cline{2-6}  & **EVI** & 16-day & 0.56** & 0.01 & 0.01 \\  & 32-day & 0.54** & 0.01 & 0.11 \\ \hline \multirow{4}{*}{**MOD15**} & **LAI** & 8-day & 0.78** & 0.43** & 0.70* \\  & 16-day & 0.83** & 0.57** & 0.68* \\  & 32-day & 0.88** & 0.68** & 0.66* \\ \cline{2-6}  & **FPAR** & 8-day & 0.56** & 0.46** & 0.64* \\  & 16-day & 0.58** & 0.59** & 0.55* \\  & 32-day & 0.62** & 0.63** & 0.57* \\ \hline
**MOD17** & **GPP** & 8-day & 0.81** & 0.59** & 0.01 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Summary of \(R^{2}\) between each MODIS product and flux tower annual GPP. The stars indicate _p_-value \(<\) 0.05 (*) and _p_-value \(<\) 0.001 (**).  
Figure 3: Scatterplot of 32-day composites of GPP _vs._ (**a**) Normalized Difference Vegetation Index (NDVI), (**b**) Enhanced Vegetation Index (EVI), (**c**) Leaf Area Index (LAI), and (**d**) Fraction of Photosynthetically Active Radiation (FPAR) using data from all the sites.