A Spectral Fitting Algorithm to Retrieve the Fluorescence Spectrum from Canopy Radiance

Sergio Cogliati,Ilaria Cesana,Matthias Drusch,Uwe Rascher,Lorenzo Genesio,Marco Celesti,Dirk Schuettemeyer,Tommaso Julitta,Roberto Colombo,Pedro Jurado,Franco Miglietta

doi:10.3390/rs11161840

Abstract

Retrieval of Sun-Induced Chlorophyll Fluorescence (F) spectrum is one of the challenging perspectives for further advancing F studies towards a better characterization of vegetation structure and functioning. In this study, a simplified Spectral Fitting retrieval algorithm suitable for retrieving the F spectrum with a limited number of parameters is proposed (two parameters for F). The novel algorithm is developed and tested on a set of radiative transfer simulations obtained by coupling SCOPE and MODTRAN5 codes, considering different chlorophyll content, leaf area index and noise levels to produce a large variability in fluorescence and reflectance spectra. The retrieval accuracy is quantified based on several metrics derived from the F spectrum (i.e., red and far-red peaks, O2 bands and spectrally-integrated values). Further, the algorithm is employed to process experimental field spectroscopy measurements collected over different crops during a long-lasting field campaign. The reliability of the retrieval algorithm on experimental measurements is evaluated by cross-comparison with F values computed by an independent retrieval method (i.e., SFM at O2 bands). For the first time, the evolution of the F spectrum along the entire growing season for a forage crop is analyzed and three diverse F spectra are identified at different growing stages. The results show that red F is larger for young canopy; while red and far-red F have similar intensity in an intermediate stage; finally, far-red F is significantly larger for the rest of the season.

Highlights

Sun-Induced Chlorophyll Fluorescence (F) is becoming a relevant tool for observing vegetation with remote sensing techniques
Few cases are instead characterized by a slight spectral mismatch in the valley in between the two F emission peaks, corresponding to the spectral region among 690–730 nm. This issue can be observed for cases n° 1–14 characterized by leaf area index (LAI) values of 1–2. This behavior can be caused by a slight overcorrection introduced in the F spectrum by the retrieval algorithm, probably caused from a larger contribution of soil onto the canopy reflectance signature
This study aimed at introducing a simplified version of the full F spectrum retrieval algorithm

Summary

Introduction

Sun-Induced Chlorophyll Fluorescence (F) is becoming a relevant tool for observing vegetation with remote sensing techniques. The F emission of plants (650–800 nm) is characterized by a complex spectrum with two distinct peaks in the red (685 nm) and far-red (740 nm) wavelengths. These peaks originate from two different photosystems and they are subjected to extremely diverse absorption and scattering processes along their path through the leaf tissues and the canopy. Red F is strongly reabsorbed by chlorophyll pigments while far-red F is mainly scattered before escaping the canopy and being remotely sensed. Studies based on measurements collected by field spectrometers [12,13,14,15,16] and airborne high-resolution imaging spectroscopy [17,18,19,20,21,22] more frequently considered both the red and far-red F

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