## 2 Materials and Methods
### Description of Datasets  
#### 2.2.1 Sif Data  
Light energy absorbed by the leaf chlorophyll molecules has three different pathways: photochemistry, nonphotochemical quenching (NPQ, i.e., heat dissipation), and a small fraction re-emitted as SIF [18]. Theoretically, SIF can be expressed as shown in Equation (1) [22]:  
\[SIF=APAR\times\Phi_{F}\times\Omega_{C}=PAR\times fPAR\times SIF_{yield} \tag{1}\]  
where \(APAR\) is the absorbed photosynthetically active radiation in Watts/m\({}^{2}\), which is the product of photosynthetically active radiation (\(PAR\)) and the fraction of photosynthetically active radiation absorbed (\(fPAR\)) by the vegetation/canopy. \(SIF_{yield}\) is the emitted \(SIF\) per photon absorbed [17] and is also defined as the effective fluorescence yield of the canopy. \(SIF_{yield}\) is the product of fluorescence yield in the wavelength band of measurement (\(\Phi_{F}\)) and an escape probability term (\(\Omega_{C}\)). \(SIF_{yield}\) is expected to vary with the plant's photosynthetic light use efficiency [38; 39] and canopy structural characteristics like leaf area index (LAI). However, with a satellite-based SIF measurement, it is challenging to separate the canopy physiological (\(\Phi_{F}\)) and structural components (\(\Omega_{C}\)) from \(SIF_{yield}\)[32; 40], and this is still an active area of research. For this study, the SIF product from NASA's OCO-2 satellite was used since other satellite-based SIF retrievals were of coarser spatial resolution (GOSAT and GOME-2). OCO-2 SIF is made available by the CO\({}_{2}\) Data Portal managed by the Jet Propulsion Laboratory of the California Institute of Technology, United States ([[https://co2.jpl.nasa.gov/](https://co2.jpl.nasa.gov/)]([https://co2.jpl.nasa.gov/](https://co2.jpl.nasa.gov/))). The OCO-2 spectrometer measures spectra in the O\({}_{2}\)-A band (757-775 nm, full width at half maximum = 0.042 nm), with far-red SIF, retrieved at 757 and 771 nm based on the infilling of Fraunhofer lines [29; 41]. The SIF retrieved at 771 nm is typically about 1.5 times smaller than obtained at 757 nm [41]; therefore, we utilized the SIF retrieved at 757 nm in our study. The major advantage of OCO-2 SIF includes a roughly 100-fold increase in data acquisition frequency (\(\sim\)1000 Hz), which is a factor of 1000-fold increase in data acquisition frequency (\(\sim\)1000 Hz). Figure 1: Drought severity map of Europe in (**a**) spring (SPEI-3 of May) and (**b**) summer (SPEI-3 of August) of 2018. The map is based on the Standardized Precipitation Evapotranspiration Index (SPEI) Global Drought Monitor ([[http://spei.cisc.es/index.html](http://spei.cisc.es/index.html)]([http://spei.cisc.es/index.html](http://spei.cisc.es/index.html))). (**c**) Drought area and non-drought area used in the study. Drought area has 2018 spring and summer SPEI values less than \(-\)1. Non-drought area has 2018 spring and summer SPEI values greater than \(-\)1.  
over GOSAT and finer spatial resolution (\(1.3\times 2.25\) km\({}^{2}\)). This enables OCO-2 to acquire more than \(10^{5}\) clear-sky soundings on land per day, thus providing the opportunity to perform in-depth SIF based analysis, such as regional ecophysiological change detection [42; 43]. However, OCO-2 SIF soundings do not have full spatial coverage (Figure S1). In this study, the SIF retrievals from the Level-2 OCO-2 Lite product, data version SIF v8 (version 8) were used from their first availability (6 September 2014) to 1 October 2018 for the whole European continent. The OCO-2 Lite product is bias-corrected and contains only good quality SIF soundings [44]. The OCO-2 takes measurements in three different observation modes, namely nadir, glint viewing, and a special target mode with different viewing zenith angle (VZA), typically alternatively with a repeat cycle of 16 days. Although SIF values are affected by VZA [45], grouping all the SIF soundings over a large area results in no significant SIF difference in the mean SIF between nadir and the combined modes [13; 16] (see Supplementary Table S1.