Abstract
Sun-Induced chlorophyll Fluorescence (SIF) relates directly to photosynthesis yield and stress but there are still uncertainties in its interpretation. Most of these uncertainties concern the influences of the emitting vegetation’s structure (e.g., leaf angles, leaf clumping) and biochemistry (e.g., chlorophyll content, other pigments) on the radiative transfer of fluorescent photons. The Caatinga is a large region in northeast Brazil of semiarid climate and heterogeneous vegetation, where such biochemical and structural characteristics can vary greatly even within a single hectare. With this study we aimed to characterize eleven years of SIF seasonal variation from Caatinga vegetation (2007 to 2017) and to study its responses to a major drought in 2012. Orbital SIF data from the instrument GOME-2 was used along with MODIS MAIAC EVI and NDVI. Environmental data included precipitation rate (TRMM), surface temperature (MODIS) and soil moisture (ESA CCI). To support the interpretation of SIF responses we used red and far-red SIF adjusted by the Sun’s zenith angle (SIF-SZA) and by daily Photosynthetically Active Radiation (dSIF). Furthermore, we also adjusted SIF through two contrasting formulations using NDVI data as proxy for structure and biochemistry, based on previous leaf-level and landscape level studies: SIF-Yield and SIF-Prod. Data was tested with time-series decomposition, rank correlation, spatial correlation and Linear Mixed Models (LMM). Results show that GOME-2 SIF and adjusted SIF formulations responded consistently to the observed environmental variation and showed a marked decrease in SIF emissions in response to a 2012 drought that was generally larger than the corresponding NDVI and EVI decreases. Drought sensitivity of SIF, as inferred from LMM slopes, was correlated to land cover at different regions of the Caatinga. This is the first study to show correlation between landscape-level SIF and an emergent property of ecosystems (i.e., resilience), showcasing the value of remotely sensed fluorescence for ecological studies.
Highlights
Monitoring and measuring vegetation responses to climate has long been an area of interest for biologists and ecologists and it merits attention considering the ongoing global climate change and the closely related interplay between carbon flux, climate and the biosphere [1]
While the potential of Sun-Induced chlorophyll Fluorescence (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 fraction of Absorbed Photosynthetically Active Radiation (f 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 SIFR and the ratio between red and far-red SIF (SIFR/FR)
The variables SIFFR-Prod, dSIFFR, SIFFR-s zenith angle (SZA) and standard SIFFR presented a median drop of more than 40% during the year of 2012 in relation to the period from 2007 to 2011 (Figure 11). While these variables include the phenological influences of biochemistry and structure on SIF radiative transfer, SIFFR-Yield has had most of these influences removed but it dropped by a median value of over 25% in 2012
Summary
Monitoring and measuring vegetation responses to climate has long been an area of interest for biologists and ecologists and it merits attention considering the ongoing global climate change and the closely related interplay between carbon flux, climate and the biosphere [1]. Chlorophyll fluorescence is an electromagnetic emission that results from the conversion of Sun light energy into electrochemical energy by the plant’s chlorophyll molecules. Plant biochemistry imposes negative feedbacks on the energy conversion process that acts at different temporal scales to allow it to adapt to the changing environmental conditions of the daily and seasonal cycles of plant life. In this manner, energy conversion rate for photosynthesis is kept proportional to the viable photosynthetic rate within the given environmental conditions and plant biochemical limitations [6]. Chlorophyll fluorescence emission is closely related to photosynthetic yield and productivity [2,7,8]
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