## 1 Introduction
The findings on the genotype dependency of vegetation fluorescence quantum yield in the band of 692 nm [15] suggest that floristic composition may define the \(\Phi_{F}\) of the forest. \(\Phi_{F}\) differences are particularly pronounced between species with different CO\({}_{2}\) fixation paths (C3, C4, and CAM paths; [15]), while the shading-sensibility suggests that the depth of the forest canopy, the forest structure, and the variations in leaf display (physical shade precursors) are more related to \(\Omega_{C}\) (Equation (1); [9]). In plants, stress response strategies, such as the adjustment of their stomatal morphology and changes in their photosynthetic functions, affect the photosynthetic light use efficiency (LUE, [17]), which in turn may affect the SIF's relationship with the GPP [9]. However, with satellite-based SIF measurements, it is not possible to measure LUE [18] or differentiate its effects on the SIF from the effect of \(\Phi_{F}\) and \(\Omega_{C}\); only the total SIF radiance over the canopy can be studied [9; 19]. SIF radiance's relationship with GPP, which is more mechanically detailed, involves processes including energy partitioning between photosystems I and II, the fluorescence properties of these two photosystems, the canopy structure, the CO2 fixation paths, photorespiration, linear and cyclic electron transports, stoichiometry, and fluorescence radiative transfer modeling in canopies [20]. Biochemical and biophysical responses to drought at the plant level through enzymatic activity leads to changes in leaf biochemistry [21] and reductions in stomatal and mesophyll conductance [22], which result in a lower CO2 uptake. A reduced CO2 uptake limits photosynthetic activity and results in lower SIF values. Drought can also cause mortality via carbon starvation, hydraulic failure, and phloem transport failure [23], which alter the ecological balance by shifting the composition of vegetation communities [24; 25; 26; 27]. In 2009, the first global satellite SIF measurements were obtained from the GOSAT mission instruments [28]; since then, SIF has been employed to study drought's impacts on vegetation [7; 29] and to assess the diversity in leaf phenology [13; 30] across Amazonia. GOSAT-SIF data reveal a productivity reduction of 15% based on the GPP in Amazonia during the 2010 extreme drought event [29]. A new retrieval product based on GOME-2's SIF showed reductions of up to 10%-15% in the eastern part and up to 2%-5% in the western part throughout the El Nino 2015-2016 extreme drought [7]. Further, during the 2015-2016 El Nino, the SIF was below its climatological range, and its reduction was particularly strong in regions with high temperatures and low water availability [7]. The recent NASA Orbiting Carbon Observatory-2 (OCO-2) mission (launched in September, 2014) [31; 32; 33; 34; 35; 36; 37] offers a higher-resolution alternative (footprint of 2.25 \(\times\) 1.3 km) than the previous coarser SIF measurements [38] by GOSAT (footprint of 10 km diameter) [28; 39] and GOME-2 (40 \(\times\) 40 km) [40]. Along with a high spatial resolution, OCO-2's SIF provides the opportunity to study the vegetation responses of specific vegetation types to different climate conditions, thus providing completely new perspectives for SIF analysis [20; 41]. High-resolution SIF products, such as OCO-2, have the potential to resolve climatic and vegetation type-specific responses to water and heat stress [20], such as at the ecoregional scale. Ecoregions (ERs) can be defined as units of land containing a distinct assemblage of species and natural communities of all taxa (beyond plants taxa) with boundaries that approximate the original extent of the natural communities prior to major land-use changes [42].