## 4 Discussion
### SIF\({}_{757}\) Response during the El Nino 2015-2016 Extreme Drought Event  
Although it was proven with plant-level measurements of active fluorescence that, under controlled environments, the carbon-assimilation of plant species responds differently to drought [69; 70; 71], only few studies have explored the changes in SIF\({}_{757}\) during the drought in the Amazonia (e.g., [7]). We showed that the MBF in the Af KG-climate zone had the highest net SIF\({}_{757}\) decreases compared to the Am and Aw KG-climate zones during the CARA period (Figure 4A). This contrasts with the lowest VPD (higher humidity) in the Af compared to Am and Aw (Figure S3) and demonstrates that the Af environment is more humid, but its vegetation's SIF\({}_{757}\) is more sensitive to variations in the water balance. The estimated drought-driven SIF\({}_{757}\) changes across the ERs (Figure 4B) may be indicators of the changes in GPP dynamics between different drought categories of the same ER. These may also be indicators of mortality [71] of hydraulically vulnerable species [26] that would lead to variations in GPP. Such GPP variations have been reported to have enormous impacts on the structure and function of the Amazonian forest ecosystems [72; 73], as well as on their provisioning of ecosystem services [74; 75]. However, it is difficult to tell when a plant that contributes with chlorophyll fluorescence radiance to a SIF retrieval is dead [76] and, as a consequence, its ecological niche has been occupied by better adapted species [24; 25; 26], or when physiological responses have taken place. This is because an OCO-2 SIF retrieval captures a unit of land with a diverse plant community under constant interaction. If physiological responses are taking place at the plant level, one can expect a reduced photosynthesis [63]. However, at the community level, we observed positive and negative ER-SIF\({}_{757}\) variations. We hypothesize that the variations could also be related to drought-driven ecological dynamics with potential for compelling the ERs' community demography to express a higher or lower \(\Phi_{F}\) and \(\Omega_{C}\) (e.g., the proliferation of tropical plants that increase tree mortality [27], a decrease in pollinators resulting in decreased natality of specific plant species [77], or a decrease in the fauna's floristic resources, thereby affecting the fauna composition [78; 79] and leading to changes in \(\Phi_{F}\) by limiting the plants' N availability [16] from manure and urine stock flows). Nevertheless, similar to previous results [7], we found a widespread SIF\({}_{757}\) reduction during the El Nino 2015-2016 phenomena in which the positive and negative ER-SIF\({}_{757}\) variations suggest that both physiological responses and mortality may be taking place with a higher frequency in ERs with higher SIF\({}_{757}\) variations. Our analysis also provides insights into the magnitude of VPD changes, their correlation with temperature across the Amazon region during the El Nino 2015-2016 extreme drought, and the related SIF response. Notably, the ERs experienced different VPD under drought, which may be related to whether they are able to decouple photosynthesis from transpiration during heat extremes [80]. The decrease in SIF in combination with increases in fPAR in drought areas has been linked to a higher heat dissipation of plants through non-photochemical quenching (NPQ; [81; 82]). fPAR reflects the structure of vegetation, whereas SIF is influenced by structural (APAR and \(\Omega_{C}\)) and physiological (LUE and \(\Phi_{F}\)) components. However, to physiologically consider the allocation of APAR in SIF, NPQ, or photochemistry, PAR measurements are required to calculate APAR. Together with \(\Omega_{C}\), these measurements would allow one to remove the structural components of SIF and provide a proxy for NPQ. Nevertheless, we found large differences in temperature and VPD between drought and non-drought areas of the same ER (Figure 4B).