Development of a simple parametric approach to correct fluorescence signal for reabsorption and its implications for GPP estimates: a case study for alfalfa crop

Solar-induced chlorophyll Fluorescence (F) retrievals at canopy level remain challenging due to canopy reabsorption and scattering that affect the F’ spectra at the top of canopy (TOC). This study presents F_TOC retrievals over an alfalfa crop in Italy, based on field spectroscopy data collected from May to July 2018 A novel, simple parametric correction for reabsorption, based on radiative transfer model, was applied to obtain a corrected spectrum (F_RC). We also investigated the relationships of F_TOC and F_RC spectra with Gross Primary Production (GPP) estimated from CO₂ fluxes measured through Eddy Covariance. Fluorescence at selected wavelengths was analyzed and combined to estimate GPP using linear, stepwise, and Random Forest approaches. The Fluorescence yield (Fy) and Light Use Efficiency (LUE) relationship was also investigated. The correlation between GPP and F_TOC peaked on the red-edge and far-red spectral regions, furthermore, combined wavelengths improved GPP estimates around 10%. Meanwhile, GPP – F_RC correlation was constant across spectra. Fy explained 90% of LUE variability. Overall, our results indicate that F_TOC was efficiently correct for reabsorption and that using full F_TOC spectrum enhances GPP estimation in alfalfa and can be seen as a precursor for a better characterization of the link F-photosynthesis for other crops..