# Progress in Remote Sensing of Photosynthetic Activity over the Amazon Basin
## 4 Case Study of Seasonal Variation in Light-Use Efficiency and SIF across the Amazon Basin
### Results
#### 4.2.2 Comparison of PRI with GPP at Eddy-Flux Tower Sites  
The relationships between PRI and \(\varepsilon\) derived from eddy covariance measurements for the forested sites are presented in Figure 10. Each data point represents a monthly average of PRI and \(\varepsilon\) values. Strong, logarithmic relationships are exhibited for BAN, CAX and RJA with coefficients of determination (\(r^{2}\)) of 0.75 (\(p<0.01\), \(n=12\)), 0.51 (\(p<0.01\), \(n=12\)) and 0.78 (\(p<0.01\), \(n=12\)), respectively. The PRI values were highly variable for the evergreen rain forest K67 (\(r^{2}=0.37\), \(p<0.01\), \(n=12\)) and generally higher during most seasons than observed at the BAN and RJA sites where rainfall is seasonally more variable (Figure 10). Strong logarithmic relationships between eddy-flux-derived \(\varepsilon\) and PRI were also reported in [9; 14; 16] where the relationship between PRI and (\(\varepsilon\)) disappears (or saturates) for high values of \(\varepsilon\). This trend is confirmed by the results shown in Figure 10.  
Figure 11 shows the monthly averages of \(\text{PRI}_{\text{diff}}\), light-use efficiency (\(\varepsilon\)), LAI and GPP for each forested tower site. Overall, BAN and RJA sites showed the lower values compared to K67 and CAX for all months, with a pronounced decline starting in June and continuing until September. Low values of \(\text{PRI}_{\text{diff}}\) are common during this season of low rainfall in southern Brazil (Figure 4 and Appendix A.2). BAN and RJA sites also showed the lower values of light use efficiency and GPP calculated from eddy-flux measurements. However, LAI showed an inverse seasonal variation with higher values concentrating during dry season's months with BAN and RJA showing higher values. Light-use efficiency for the CAX site showed a peak in March (Figure 11). Although it might seem an outlier, the low values of LAI at the same time agree. In general, seasonal trends in LAI are opposite those of PRI, Light Use Efficiency (LUE) and GPP. Similar trends were observed by [PERSON] and collaborators [57] across the Amazon basin. This trend is in agreement with the seasonality of solar radiation (e.g., Figure 4), which might lead to a pattern of leaf flush during this radiation-rich season and leaf abscission during the wettest months due to cloud coverage. LAI estimates for February, March and April were the lowest for CAX compared to other sites, which agrees with the higher values  
Figure 9: Monthly mean dry season \(\delta\)PRI and NDVI for the period 2000 to 2012 expressed in colors denoting \(\geq\)1 standard deviations from all monthly means over the collection. \(\delta\)SIF means (2007 to 2012) for the dry (JJA) and wet (DJF) seasons are similarly depicted (monthly averages available in the Supplementary Material). of LUE recorded for those months. This is because LAI is used to calculate f\({}_{\rm PAR}\) (Equation (4)), which is inversely proportional to LUE based on [PERSON]'s law. Figure 12 indicates a strong logarithmic relation between PRI\({}_{\rm dif}\) and \(\varepsilon\) derived from each site for the months of the dry season (r\({}^{2}\) = 0.70 (\(p<0.01\))). Each data point represents a monthly average of PRI and \(\varepsilon\) for the months of June, July, August and September. Figure 11: Satellite-derived monthly averages of PRI\({}_{\rm dif}\) and LAI compared with calculated values of measurements of light-use efficiency (\(\varepsilon\)) and GPP derived from tower-based measurements at each site. A seasonal decline in PRI\({}_{\rm dif}\) and GPP characterizes all sites during the dry season, particularly those in the south (BAN and RJA).