# 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
Figure 10: Relationship between PRI derived from MAIAC observations (3 \(\times\) 3 km) and \(\varepsilon\) derived from eddy covariance measurements for (**a**) Santárém forest (K67) (PRI = 0.0029 ln(\(\varepsilon\) \(-\) 0.0008), Reserva Jará (RJA) (PRI = 0.0179 ln(\(\varepsilon\) \(+\) 0.0587), Caixuaná National Forest (CAX) (PRI = 0.0036 ln(\(\varepsilon\) \(-\) 0.0004) and Bananal Island (BAN) (PRI = 0.0154 ln(\(\varepsilon\) \(+\) 0.0486) and (**b**) All sites together (right). The way that the values of \(\mathrm{PRI}_{\mathrm{diff}}\) and \(\varepsilon\) are dispersed along the trend lines for each site in Figures 10b and 12 seems reasonable. Forests in the south are likely to experience more stress than forests lying further to the north. Figures 10-12 show distinct separation of sites from one another: BAN and RJA register a larger seasonal range in \(\mathrm{PRI}_{\mathrm{diff}}\) values with consistently low values of \(\varepsilon\). The northern area, typically with frequent cloud cover, experiences less downregulation in photosynthesis (Figure 13) and, as a result, produces much higher values of \(\varepsilon\) than the forested area further to the south. The correlations between \(\mathrm{PRI}_{\mathrm{diff}}\) and \(\varepsilon\) are notably better where seasonal stress occurs than where it does not. We surmise that the relationship between \(\mathrm{PRI}_{\mathrm{diff}}\) and \(\varepsilon\) is considerably stronger when downregulation occurs (Figure 12), because at high values, the relationship saturates (Figure 10b). The relationships between GPP and \(\mathrm{PRI}_{\mathrm{diff}}\) for the dry season (June through September) are defined by Equation (5) and for all months by Equation (6):  
\[\mathrm{GPP}_{\mathrm{dry}}=9588.1(\mathrm{PRI}_{\mathrm{diff}})^{2}+549.88( \mathrm{PRI}_{\mathrm{diff}})+12.53\ (r^{2}=0.72,p<0.001,\,n=16), \tag{5}\]  
\[\mathrm{GPP}_{\mathrm{all\ months}}=-3091.1(\mathrm{PRI}_{\mathrm{diff}})^{2}+ 114.93(\mathrm{PRI}_{\mathrm{diff}})+9.07\ (r^{2}=0.51,p<0.001,\,n=48), \tag{6}\]  
Figure 12: Relationship (\(\mathrm{PRI}=0.011\mathrm{ln}(\varepsilon)+0.0277\)) between \(\mathrm{PRI}_{\mathrm{diff}}\) derived from MAIAC analyses and \(\varepsilon\) derived from eddy covariance measurements for sites at Bananal Island (BAN), Caxiuaná National Forest (CAX), Reserva Jará (RJA) and Santarem forest (K67) for the months of June through September. Figure 13: (**a**) Averaged August precipitation in the Amazon basin from 2000 to 2012 acquired from the Tropical Rainfall Measuring Mission (TRMM); (**b**) Average \(\mathrm{PRI}_{\mathrm{diff}}\) values for the month of August from 2000 to 2012. The lowest values of \(\mathrm{PRI}_{\mathrm{diff}}\) during August and other dry-season months occur in the southern region. During the wet season with extensive cloud cover, GPP is light-limited, and the relationship between \(\mathrm{PRI_{diff}}\) and \(\varepsilon\) weakens because photosynthesis is not downregulated in either sunlit or shaded portions of the canopy (\(\mathrm{PRI_{f}}=\mathrm{PRI_{b}}\)). Figure 14 extends estimates of GPP across the Amazon basin during the dry season based on Equation (5).