# Adjustments to SIF Aid the Interpretation of Drought Responses at the Caatinga of Northeast Brazil
## 3 Results
### SIF and Climate: The 2012 Drought  
The results of our trend analysis, showing a major drought at the year of 2012 (Figure 5), can be better understood in a spatial analysis subsetting our data into three periods: a more favorable period from 2007 to 2011, a drought period in the year of 2012 and an intermittent-droughts period from 2013 to 2017. The map of average precipitation shows a clear difference among these three periods and give a spatial perspective of the 2012 drought in the studied area (Figure 7). Spatial averages maps of  
Figure 6: Linear Mixed Models’ (LMM) Fixed Effect Slopes. All variables were rescalled to improve model quality and, therefore, slopes are in relative units. Panel “_a_” represents SIF\({}_{FR}\) LMM slopes, and panel “_b_” represents SIF\({}_{FR}\)-Yield LMM slopes. Ecoregions are represented by letters, corresponding to Figure 1: (a) Campo Maior; (b) Northern Setaneja Depression; (c) Aarripe-Ibiapaba Complex; (d) Borborema Plateau; (e) Major Southern Setaneja Depression; (f) Sao Francisco Dunes; (g) Raso da Catarina; (h) Minor Southern Setaneja Depression; (i) Chapada Diamantina. Significance levels are marked in bars according to the following legend: *** for \(p<0.001\), ** for \(p<0.01\), * for \(p<0.05\) and * for \(p<0.1\). \(n=131\) for each ecoregion.  
Precipitation maps show that the initial period from 2007 to 2011 had more favourable conditions (Figure 7a), while averages for the year of 2012 show a widespread drought in the region (Figure 7b). The subsequent interval from 2013 until 2017 comprised a mix of dry and wet years that was less-favourable than what we observed from 2007 to 2011, if better than 2012 conditions. While the 2012 drought is shown to affect the majority of the Caatinga, dry conditions persisted from 2013 to 2017 particularly at the region's central area (Figure 7c).  
The changes in studied variables observed in the year of 2012, in relation to the period from 2007 to 2011, were large and \(\text{SIF}_{FR}\) dropped by over 40% throughout most sites (Figure 11) while NDVI  
Figure 10: Map of period averages of the correlation between Sun-Induced Fluorescence (\(\text{SIF}_{FR}\)) and NDVI, for the three periods: from 2007/02 to 2011/12 (**a**), 2012 (**b**) and, from 2013/01 to 2017/12 (**c**).  
Figure 9: Map of period averages of NDVI for the three periods: from 2007/02 to 2011/12 (**a**), 2012 (**b**) and, from 2013/01 to 2017/12 (**c**).  
and EVI averages dropped by over 22%. Even \(\text{SIF}_{R}\) and \(\text{SIF}_{FR}\)Yield decreased by over 25% while Temperature increased by nearly 10% and Precipitation Rate and Soil Moisture decreased by 54% and 20% respectively in 2012, in relation to the previous year's average. The ratio between red and far-red SIF increased by over 16% in the year of 2012, with large variation between the different ecoregions of the Caatinga (Figure 11 \(\text{SIF}_{R/FR}\)).  
The particular response of each ecoregion's vegetation to the sampled environmental influences can be better observed by comparing the relative change in NDVI (photosynthesis-related structure and biochemistry), \(\text{SIF}_{FR}\)-Yield (photosynthesis) and their combined response as sampled through \(\text{SIF}_{FR}\). Results from 2012 show a marked drop in variables' means, with vegetation at the Sao Francisco Dunes ecoregion displaying the greatest change in that year among all ecoregions (Figure 12a).  
The milder drought period of 2013 to 2017 (as characterized by Figures 5 and 7c) seems to have had a small impact on the Campo Maior ecoregion. Results show that the Northern Sertaneja Depression and the Borborema Plateau were the most affected ecoregions from 2013 to 2017 in relation to what can be observed from 2007 to 2011 (Figure 12b).