## 4 Discussion  
### Drought Impact on SIF  
Our study showed a clear impact of the European hotter drought of 2018 on overall and intra-seasonal SIF variation for different vegetation types as captured by the OCO-2 SIF. Early spring of 2018 was characterized by lower than average temperatures and VPD (i.e., non-stressed stage) followed by above-average temperatures and VPD in the later part of spring (April-May; Figure 3b). Such cooler and wetter conditions in early spring can result in poor crop development and lower productivity [65] and thus lower SIF values ([66]; Figure 3b). Moreover, the initially cooler conditions in spring might have delayed the leaf flushing in deciduous trees, resulting in lower SIF values of broadleaved forests (Figure 3b). This pattern was less evident for the coniferous and mixed forests, probably due to the presence of evergreen trees. In addition, the wet conditions during early spring most likely built up enough soil moisture for the later spring season, thus moderating the sudden warming in later spring season [67; 68]. This combination of ample moisture supply with a rise in temperature resulted in an increase in SIF to above-average values across all vegetation types (Figures 3 and 6). The variation in SIF depends on the variation of both APAR (and fPAR) and SIF\({}_{\text{yield}}\) (Equation (1), [69]). Both SIF and fPAR were increasing during the spring season for both drought and non-drought areas (Figures 3 and 4, Figures S4a and S5a). Moreover, for agriculture areas affected by drought, the peaks of fPAR and SIF were on almost the same day of the year (DOY \(\sim\) 145; Figure 3a and Figure S4a). However, this clear synchronicity was not found for forests. Nevertheless, we conclude that the satellite-based SIF variation was closely coupled with fPAR variation under non-stress conditions (spring for drought areas and  
Figure 5: Boxplots illustrating SIF variation across the four vegetation type during the heatwave (15 July to 15 August) in central Europe (i.e., Germany, Denmark, Belgium, and Netherlands) along with temperature differences. Dots inside the boxplots represent the corresponding mean values. for non-drought areas in spring and summer). Previous studies [13; 31; 32] have also highlighted the synchronicity of satellite-based SIF with fPAR. The warming during late spring continued to the summer season, with the later summer season featuring intense heat. We found decreasing SIF in all investigated vegetation types in the summer of 2018, which was most pronounced in the drought areas. The agricultural areas expressed an immediate decrease in SIF from early summer onwards most likely because of severe soil moisture deficits caused by early water depletion in late spring in course of dry weather conditions [70; 71]. Ongoing drought conditions during the summer season showed continuous decrease in SIF for agricultural areas and with some delay in forests (Figure 3). Similar findings were reported by an NDVI-based study where agricultural areas, and with some delay, forests in central Europe, faced severe decline in NDVI during the summer season [6]. Unlike agricultural crops, forests feature a buffered microclimate [72; 73; 74], and given their deeper rooting system in comparison to crops, they have access to deeper soil water and thus can maintain constant photosynthesis rates for extended periods during drought conditions [75]. Our spring-summer seasonal variation of SIF in 2018 is in close agreement with the one of MODIS NDVI ([6]) for agricultural areas. However, for the forest ecosystems, we found a less extreme SIF decline compared to the NDVI-decline in [PERSON] et al. [6]. To further evaluate this difference we analyzed MODIS NDVI over 4 years (2015-2018), and our results indicate that SIF showed higher sensitivity compared to NDVI over the 4-year period (Figure S4b and Figure 6).