## 4 Discussion
### SIF Response during Drought Stress  
Water stress adversely impacts photosynthesis, either physiologically or structurally [83]. Physiological alterations include change in leaf biochemistry via reductions in enzymatic activity [84] or a reduction in mesophyll/chlorophyll, and stomatal conductance [85], resulting in lower CO\({}_{2}\) uptake by the plants and thus reduced photosynthesis (i.e., lower SIF values). The structural changes include foliar changes like leaf wilting or rolling (changes in leaf inclination), thus reducing effective leaf area index (LAI), which in turn causes a reduction in fPAR and energy available for photosynthesis and fluorescence emission [69; 86]. Furthermore, plants adopt different mechanisms to minimize water and heat stress, such as chloroplast avoidance movement, during which plants move their chloroplast from the cell surface to side walls of cell [87] so as to minimize APAR [88] and thus SIF. MODIS fPAR decreases due to these protection mechanism, and physiological and structural changes were visible for agricultural areas during the summer drought (Figure S4 and Figure 6). However, for the forest areas we found that MODIS fPAR for drought areas during midsummer (DOY 185-215) was not lower than fPAR in the previous 3 years (Figure 6 and Figure S4). This might imply that a reduction of SIF is likely caused by a reduction in SIF\({}_{\text{yield}}\). SIF variation due to changes in SIF\({}_{\text{yield}}\) can be because of the canopy's physiological (\(\Phi_{\text{F}}\) term of Equation (1)) or structural change (\(\Omega_{\text{C}}\) term of Equation (1)), or both. The PAR absorbed by leaves is partitioned into photochemistry, heat dissipation (NPQ), and fluorescence emission (SIF). Although fPAR (or APAR) did not decrease, the proportion of fPAR (or APAR) that is used in photochemistry (to drive electron transport for carbon assimilation) decreases, resulting in a surplus of photosynthetic energy. Now, two other pathways compete to de-excite the absorbed light that is not used in photochemistry (i.e., the surplus photosynthetic energy), the energy emitted as SIF or energy dissipated as heat through NPQ [69]. The competition between these two pathways depends on environmental stress [89], and during water stress conditions (drought), the reduction in SIF is linked to higher NPQ [66; 90]. This increase in NPQ may result in a weak coupling between SIF/SIF\({}_{\text{yield}}\) and fPAR under water stress conditions (see Figure S8), which seems to be clearly visible for coniferous and mixed forests during midsummer (DOY 185-215; Figure S4). It is also possible that the MODIS fPAR could be erroneous [32] and not be the best estimate of fPAR, especially in case of high vegetation density (as in Europe). The backup algorithm of MODIS fPAR relies on its relationship with NDVI [49], which becomes nonlinear with a sharp drop of NDVI sensitivity to fPAR for moderate to high vegetation density for fPAR values beyond 0.7 (Figure S6). Nevertheless, further research to differentiate the canopy escape term (\(\Omega_{\text{C}}\) term of Equation (1)) from SIF (as demonstrated by [91; 92]) to study changes in fluorescence yield (\(\Phi_{\text{F}}\)) (and SIF\({}_{\text{yield}}\)) during extreme large scale drought event is recommended.  
Our results highlight one more reason why OCO-2 SIF can be useful to study drought impact on vegetation as different vegetation types have different responses to drought, which was not efficiently captured for each vegetation types before because of the coarser resolution of preceding satellite-based SIF data/products. We would like to stress that earlier studies used SIF at coarser spatial resolutions (from GOME-2 or GOSAT) and mostly were not able to differentiate SIF responses for different vegetation types from a heterogeneous land-use area during drought conditions. Nevertheless, regarding the drought response of homogeneous vegetation types, our results are in line with previous SIF-based drought impact studies [31; 32].