# Adjustments to SIF Aid the Interpretation of Drought Responses at the Caatinga of Northeast Brazil
## 1 Introduction
Secondly, the geometry between light-source, leaf and sensor is stable when using hand-held or bench-mounted fluorometers [8] but, it varies greatly when using satellites or aircrafts for estimating SIF [12]. This means that, for the adequate interpretation of remotely sensed fluorescence, the Sun's angle in relation to the measured plants and sensor should be taken into consideration (e.g., hot spot, latitude, date and time of day) as well as the atmospheric conditions at the time of measurement (e.g., aerosol and clouds), the chemical composition and the structure of the measured plant community (e.g., photosynthetic pigments, leaf clumping, leaf albedo) [3; 10; 12]. Furthermore, the fluorescence emission has two peaks: one around the 685 nm wavelength, corresponding to the red region of the visible spectra and called red SIF (SIF\({}_{R}\)) in this study; and another peak around 735 nm, corresponding to the far-red region of the visible spectra, called far-red SIF (SIF\({}_{FR}\)). While SIF\({}_{R}\) is at a wavelength that is easily re-absorbed by leaves, SIF\({}_{FR}\) photons are generally scattered by emitting vegetation according to chemical composition, thickness and shape of leaves, canopy structure, leaf angles and leaf-area density [13; 14; 15]. Previous studies on red and far-red SIF have found differences on the relationships between these two fluorescence emissions and climate, that depended greatly on vegetation structure and composition [16; 17; 18; 14]. The authors of these studies suggested that more investigation of SIF responses at both wavelengths, and from heterogeneous vegetation, was necessary to improve our understanding on the relationships between SIF, Gross Primary Productivity (GPP), phenology and vegetation responses to environmental stress. Spectral vegetation indices (SVIs) such as NDVI, the Normalized Difference Vegetation Index [19] and EVI, the Enhanced Vegetation Index [20,21] are capable of providing information about abiotic stress effects by observation of leaf pigment composition and structural changes to vegetation and, consequently, about the fraction of Absorbed Photosynthetically Active Radiation (/APAR) [22]. However, chlorophyll fluorescence is more sensitive to environmental changes and can show the presence of stress before it has caused alterations to the plants that would be detectable by other remotely sensed vegetation indices [23,24]. Nevertheless, recent works have shown that over 70% of SIF variability can be attributed to phenology-related structural (e.g., leaf shedding, leaf aging) and biochemical (e.g., chlorophyll synthesis and degradation) changes that can be sampled through reflectance and SVIs [11,25]. This is understandable considering the above-mentioned influences of leaf and plant community structure and biochemistry on the radiative transfer of SIF but, the overall effect of such influences is uncertain, particularly when interpreting results from heterogeneous vegetation [15,16,25]. Therefore, we have chosen to test photosynthetic responses of vegetation using SIF data and also through adjustments made to SIF based on work investigating the effects of vegetation structure and biochemistry on chlorophyll fluorescence [14,15,26] as well as on previous observations concerning SIF data characteristics and radiative transfer [27, 28, 29]. Droughs are a major source of stress for vegetation and agriculture alike and can have devastating consequences for ecosystems and for society. While the potential of SIF for monitoring the onset of drought and its effects on plant populations has already been tested by previous works [30, 31, 32, 33], some including in their methods comparisons of SIF with proxies of structure and biochemistry like /APAR [30] and SVIs [31,32], none has attempted the combination of these data, as we propose, or has included in their analysis measures of SIF\({}_{R}\) and the ratio between red and far-red SIF (SIF\({}_{/FR}\)).