## 3 Results
### Light Dependence Curves, Carbon Exchange and Vegetation Indices  
Equation (6) describes the diurnal NEE in response to PAR, and this relationship for the Caixuana site is shown in Figure 6. It can be seen that the net CO\({}_{2}\) absorption balance concomitantly increases with PAR. In addition, the CO\({}_{2}\) flux decreases linearly with low solar radiation (PAR < 500 \(\mu\)mol m\({}^{-2}\) s\({}^{-1}\)) in both the dry season (Figure 6a) and the rainy season (Figure 6b). The light saturation point varies according to the time of day, but it is generally above 1000 \(\mu\)mol m\({}^{-2}\) s\({}^{-1}\). The assimilation rate is higher in the afternoon for PAR values, which are up to 1000 \(\mu\)mol m\({}^{-2}\) s\({}^{-1}\) in the dry season and up to around 700 \(\mu\)mol m\({}^{-2}\) s\({}^{-1}\) in the rainy season. There were no significant differences in the CO\({}_{2}\) exchanges detected between the dry and rainy seasons (Figure 7). Both seasons showed a peak CO\({}_{2}\) flux of around \(-20\)\(\mu\)mol CO\({}_{2}\) m\({}^{-2}\) s\({}^{-1}\). The maximum gross productivity of the site occurred in February (Figure 8a), coinciding with the maximum absorption (most negative NEE) and minimum respiration of the ecosystem. The site behaves like a CO\({}_{2}\) sink (negative NEE) for much of the year (from January to May and from September to December). On the other hand, during the dry season (from June to August) the site remained neutral. The seasonal dynamics of vegetation indices (EVI and NDVI) illustrate the pattern of land surface phenology in terms of vegetative canopy structure and vegetation cover development (Figure 8b), which clearly shows that vegetation seasonality is highly responsive to the environmental conditions of the location (Figure 2). Figure 5: Monthly (**a**) and hourly (**b**) averages of the energy balance fluxes: radiation balance (Rn), sensible heat flux (H) and latent heat flux (LE), all in W m\({}^{-2}\). The hatched area indicates the 95% confidence interval. Figure 6: NEE response to PPFD in different seasons ((**a**) = dry; (**b**) = wet) and day periods. At the site under study, NDVI and EVI are highest in August (NDVI > 0.8; EVI > 0.5), following the course of the site's greater radiative supply, decreasing in intensity with decreases in solar radiation and air temperature, showing levels of NDVI < 0.8 and EVI < 0.5, indicating the onset of senescence. The linear correlations between NDVI and EVI with measured GPP (Figure 9) are irrelevant because they showed values close to 0. Figure 9 shows the correlation between the GPP measured at the micrometeorological tower and the vegetation indices (NDVI and EVI) during the study years, with an insignificant correlation between GPP and EVI (R\({}^{2}\) = 0.01) as well as with NDVI (R\({}^{2}\) = 0.0046). As for the tower GPP X MODIS GPP data (Figure 10), the correlation between the data is insignificant (Figure 10), and the greatest similarity between the measurements occurs in the months of July and August (Figure 10), both due to the proximity of the medians and the magnitude of the monthly variability. In the other months, there is a greater underestimation of MODIS GPP and a seasonal signal that does not accompany the measured data. The best association of the data occurs in the months with the highest LAI (Figure 11), which shows values of order > 6.0 m\({}^{-2}\) m\({}^{-2}\) and which are also the months with the lowest variability of the index, which shows a seasonal pattern that is highly related to the radiative availability of the site. Figure 8: Monthly variation of NEE, GPP and RECO fluxes (**a**) and vegetation indices (**b**) at Caxiuana. The hatched area indicates the 95% confidence interval. Figure 7: Average diurnal cycle of NEE at Caxiuana, representing the CO\({}_{2}\) flux above the canopy, for the dry and rainy seasons. The hatched area indicates the 95% confidence interval. Figure 11: Monthly variation of LAI in the Caxiauaná site. The hatched area indicates the 95% confidence interval.