## 5 Conclusions  
This study employed the satellite-based SIF measurements acquired from the recent OCO-2 satellite to study the impact of the 2018 European hotter drought on terrestrial ecosystems. Evidently, OCO-2 SIF displayed the spatial and temporal dynamics of the 2018 European drought across different vegetation types (agricultural area, broadleaved forest, coniferous forest, and mixed forest). Our results show that the agricultural areas were severely affected by the 2018 European hotter drought. While the SIF of forests showed a less strong reduction, legacy effects may become visible in the next years and thus, we recommend closely monitoring their behavior. In general, the impact of the heatwave in Central Europe during the July-August period showed an overall 31% decrease in SIF compared to the reference mean. The SIF variation was closely explained by fPAR variation during non-stressed conditions. However, during water-stress conditions (drought), the SIF variation was attributed to variation in SIFyield. Despite a few technical limitations of the OCO-2 SIF measurement such as the 16-day repeat cycle, it provides an excellent potential to study large-scale vegetation fluorescence variation at high spatial resolution and can improve our understanding of changes in ecosystem productivity during extreme events. Furthermore, owing to its direct origin from chlorophyll, OCO-2 SIF can serve as a complementary dataset to MODIS's vegetation indices (NDVI/EVI). We suggest a more detailed comparison of vegetation indices and OCO-2 SIF to establish the latter's sensitivity to vegetation's structural and physiological changes. Further application of OCO-2 SIF may include process-based SCOPE modeling to understand the variation of complex fluorescence yield due to different environmental stress, as well as its combination with the XCO\({}_{2}\) data (main product of OCO-2 satellite) to study regional carbon budgets. The following are available online at [[http://www.mdpi.com/2072-4292/12/19/3249/s1](http://www.mdpi.com/2072-4292/12/19/3249/s1)]([http://www.mdpi.com/2072-4292/12/19/3249/s1](http://www.mdpi.com/2072-4292/12/19/3249/s1)). Table S1: Relative vegetation class area in percentage for drought and non-drought area. Table S2: Percentage of Nadir OCO-2 measurements mode for the Mean years (2015-2017) and 2018, from total OCO-2 measurements (Nadir + Glint) mode used in the study. Table S3: Comparison of anomalies (2018 - Mean*) in GSIF and OCO-2 SIF measurement (used in this study) for three different areas. Figure S1: Illustration of the spatial distribution and orbit track of OCO-2 SIF soundings of spring and summer season for Europe in 2018. Figure S2: Land cover map of Europe showing agriculture, broadleaf forest, coniferous forest and mixed forest based on Corine Land Cover (CLC) 2018, version 20b2. Figure S3: Representation of climatic water balance (CWB) by the SPI-3 values during spring (May month) and summer (August month). Figure S4: Variation of (a) fPAR and (b) NDVI for drought area across different vegetation types as observed from MODIS fPAR product (MYD15 SA2H) version 6 and MODIS NDVI product (MOD/MYD13Q1) version 6. Figure S5: Variation of (a) fPAR and (b) NDVI for non-drought area across different vegetation types as observed from MODIS fPAR product (MYD15 SA2H) version 6 and MODIS NDVI product (MOD/MYD13Q1) version 6. Figure S6: Relationship between fPAR and NDVI for different vegetation types. Figure S7: Two peaks in the broadleaved forest during summer resulting from a different timing in leaf flushing in the warm summer (Central Europe, blue box) and cool summer (Northern Europe, green box) climate zones of the drought area. Figure S8: Relationship between SIFyield and fPAR across different vegetation types. **Author Contributions:** A.S.: Conceptualization, methodology, software, writing--original draft, formal analysis, visualization, funding acquisition. J.C.: Conceptualization, methodology, writing--review and editing, resources, supervision, funding acquisition, project administration.