# Adjustments to SIF Aid the Interpretation of Drought Responses at the Caatinga of Northeast Brazil
## 3 Results
### SIF and Climate: Seasonality and Trends  
The mean seasonal components of the sampled time-series (2007 to 2017) show differences on vegetation responses between Caatinga ecoregions but these differences seem proportional to each ecoregion's particular climate's seasonality (Figure 4). The variation on timing and intensity of the rainy season shows that most northwestern areas have their dry season from July to December, while at the eastern areas they go from October to March. A clear association between SIF, temperature and precipitation is also visible (Figure 4) but, it is interesting to notice that SIF\({}_{FR}\) and SIF\({}_{R}\) respond differently to the seasonal variation in precipitation and temperature. Precipitation and soil moisture  
Figure 3: Relative cover percentage by ecoregion from ESA GlobCover 2009 product: The acronyms within parenthesis indicate cover density within that class (“CO” for Closed to Open or cover density above 15%, “C” for Closed or cover density above 40% and, “O” for Open or cover density below 15%), Flooding regime (“RF” for Regularly Flooded) and phenology (“EvS4” for Evergreen and Semi-deciduous mix and, “De” for Deciduous). For Mosaic classes, the first class named is dominant occupying 50–70% proportion and the second class named occupies a relative proportion between 20–50% of the land thus classified. “For/Shr” stands for the mixed class Forest and Shrubland, denoting vegetation with height below 5 m. Letters on the horizontal axis are in reference to Figure 1 and the name of each ecoregion is written to the right of its respective bar. presented almost synchronous behaviour with a lag between them that varies among the different areas (results not shown) and, therefore, we did not include soil moisture in our graphs to avoid visual clutter although it was used in subsequent analysis. Results of our correlation tests and time series decomposition analysis demonstrate the synchronicity between fluorescence measured by the GOME-2 instrument and the structural and biochemical phenological processes captured by MODIS MAIAC EVI and NDVI (Appendix A Table A4 and Appendix A Figure A1). Far-red SIF was more correlated to the tested SVIs in the period than red SIF at all ecoregions of the Caatinga (\(\text{SIF}_{FR}\) to SVIs mean \(\tau=0.57\) and \(\text{SIF}_{R}\) to SVIs mean \(\tau=0.42\)). Monthly values of NDVI and EVI were similarly correlated to SIF from both red and far-red wavelengths (Appendix A Table A4). Observed correlations between SIFs and SVIs were generally high but varied among ecoregions and results from Campo Maior, Minor Southern Sertaneja Depression and Chapada Diamantina were considerably lower than those from other ecoregions (Appendix A Table A4 regions a, h and i). The trend components show the long-term variability of climate and its effects on \(\text{SIF}_{FR}\) and \(\text{SIF}_{R}\). The majority of Caatinga ecoregions presented a considerable trend shift in the year 2012 when precipitation decreased and temperature increased at most areas (Figure 5). SIF at both wavelength peaks show a correspondent decrease that is proportional to the climate variables change, particularly in \(\text{SIF}_{FR}\). Red SIF is only available until December 2013 but, it seems to show a relatively weaker response to climate variation than \(\text{SIF}_{FR}\) throughout the sampled period (Figure 5, gray dot-dashed lines). Furthermore, most sites show a period of decreased \(\text{SIF}_{FR}\) emission that continues after 2012, with the exception of Campo Maior--the northwestern area, bordering the savannic Cerrado (Figure 1a). Eastern regions like the Minor Southern Sertaneja Depression (Figures 1h and 5h), Borborema Plateau (Figures 1d and 5d) and Raso da Catarina (Figures 1g and 5g) show another noticeable drop in \(\text{SIF}_{FR}\) output on late 2016. SIF trend responses at all areas is visually proportional to concurrent environmental variation in temperature (simultaneously) and precipitation (with apparent lags).