## 4 Discussion
### Drought Impact on SIF
We observed that for the forest ecosystems in the drought area, fPAR- and NDVI-anomalies were positive during the summer season (DOY \(<\) 215) (Figure S4b and Figure 6), i.e., when the SIF-anomaly was reportedly negative. While [PERSON] et al. [6] used a reference period spanning 2000-2018, where several normal and even moist summers were observed, the analyses presented here rely on the 2015-2017 period due to the comparably shorter mission length of OCO-2. However, in 2015 another severe drought  
Figure 6: Anomalies (in % difference from “Mean”) of SIF, the Normalized Difference Vegetation Index (NDVI), and fPAR for drought area across different vegetation types as observed from the Orbiting Carbon Observatory-2 (OCO-2) SIF, MODIS NDVI product (MOD/MYD13Q1) version 6, and MODIS fPAR product (MYD15A2H) version 6 in 2018. Positive values indicate a surplus of SIF, NDVI, and fPAR, negative values indicate lower values in comparison to the mean in 2015-2017. The left and right side of blue vertical dashed line represents spring and summer season, respectively. hit central and southern Europe, which might have resulted in forest legacy effects [76], i.e., low productivity and die-back in affected areas in 2016 [77]. Consequently, regarding forests, the fPAR and NDVI values from 2018 do only marginally differ to the period mean (2015-2017) since forests could have been suffering at least in 2015 and 2016, i.e., in 2 of the 3 years included in the reference mean. It is, therefore, not surprising that the extremeness of 2018 in the 4 year NDVI analysis of forests (Figure S4 and Figure 6) did not capture the forest decline as clearly as in [PERSON] et al. [6] (with a 19-year reference period) where the majority of reference years represent years with sufficient water supply. In contrast to NDVI over the four years period, SIF was considerably lower in 2018 compared to the already low mean of the previous three years (2015-2017), which may indicate its higher sensitivity to both physiological and structural drought response from plants [32]. This finding highlights the predominance of SIF over NDVI even if short reference periods are available, for example, SIF available from OCO-2, the upcoming OCO-3, and the TROPOMI in comparison to ESA's SENTINEL-2A [78] (for NDVI). Vegetation indices (VIs), such as the NDVI, are sensitive to canopy structure and pigment concentration with no direct link to photosynthesis [79]. This might lead to a reduced sensitivity of these VIs to capture drought impact when the vegetation, such as high density forests, still remains green (no changes in chlorophyll content) but reduces its photosynthesis initially during heavily stressed drought conditions [80] in early summer during the European hotter drought. Similar conditions might also not have resulted in significant differences of NDVI during the early summer of 2018 from the mean (Figure S4b). Alternatively, higher sensitivity of SIF (with direct links to photosynthesis) to drought impact on high density European forest was evident from our study (Figures 3 and 6). The intense heatwave in Central Europe during July and August resulted in a significant decrease in SIF across all the four vegetation types, which was particularly pronounced for agricultural areas. The heatwave occurred at the end of the already dry summer, which was preceded by a dry spring. As a consequence of persisting drought, many crops desiccated, thus not emitting any fluorescence by the end of July when temperatures were extremely high in Central Europe. Water stress due to high-temperature anomalies during a drought event can further increase heat stress due to an extended period of stomatal closure and subsequent reduction in evaporative cooling [81], thus causing positive feedback resulting in even warmer conditions [82].