Evaluating the Performance of the SCOPE Model in Simulating Canopy Solar-Induced Chlorophyll Fluorescence

Jiaochan Hu,Xinjie Liu,Liangyun Liu,Linlin Guan

doi:10.3390/rs10020250

Jiaochan Hu, Xinjie Liu + Show 2 more

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The SCOPE (soil canopy observation of photochemistry and energy fluxes) model has been widely used to interpret solar-induced chlorophyll fluorescence (SIF) and investigate the SIF-photosynthesis links at different temporal and spatial scales in recent years. In the SCOPE model, the fluorescence quantum efficiency in dark-adapted conditions (FQE) for Photosystem II (fqe2) and Photosystem I (fqe1) were two key parameters of SIF emission, which have always been parameterized as fixed values derived from laboratory measurements. To date, only a few studies have focused on evaluating the SCOPE model for SIF interpretation, and the variation of FQE values in the field remains controversial. In this study, the accuracy of the SCOPE model to simulate the canopy SIF was investigated using diurnal experiments on winter wheat. First, ten diurnal experiments were conducted on winter wheat, and the canopy SIF emissions and the SCOPE model’s input parameters were directly measured or indirectly retrieved from the spectral radiances, gross primary productivity (GPP) data, and meteorological records. Second, the SCOPE-simulated SIF emissions with fixed FQE values were evaluated using the observed canopy SIF data. The results show that the SCOPE model can reliably interpret the diurnal cycles of SIF variation and provide acceptable results of SIF simulations at the O2-B (SIFB) and O2-A (SIFA) bands with RRMSEs of 24.35% and 23.67%, respectively. However, the SCOPE-simulated SIFB and SIFA still contained large systematical deviations at some growth stages of wheat, and the seasonal cycles of the ratio between SIFB and SIFA (SIFA/SIFB) cannot be credibly reproduced. Finally, the SCOPE-simulated SIF emissions with variable FQE values were evaluated using the observed canopy SIF data. The simulating accuracy of SIFB and SIFA can be improved greatly using variable FQE values, and the SCOPE simulations track well with the seasonal SIFA/SIFB values with an RRMSE of 20.63%. The results indicated a clear seasonal pattern of FQE values for unbiased SIF simulation: from the erecting to the flowering stage of wheat, the ratio of fqe1 to fqe2 (fqe1/fqe2) gradually increased from 0.05–0.1 to 0.3–0.5, while the fqe2 value decreased from 0.013 to 0.007. Our quantitative results of the model assessment and the FQE adjustment support the use of the SCOPE model as a powerful tool for interpreting the SIF emissions and can serve as a significant reference for future applications of the SCOPE model.

Highlights

Solar-induced chlorophyll fluorescence (SIF) refers to the emission of red and far-red light from chlorophyll during the absorption of photosynthetically active radiation under natural sunlight.The SIF spectrum is a continuous broadband spectrum that covers the approximately spectral range of 650–850 nm
The simulated canopy reflectance and gross primary productivity (GPP) separately contain information on two aspects: one is the leaf and canopy characteristic represented by radiative transfer module (RTMo) module, and the other one is the plant physiological and photosynthesis state represented by the biochemical module
The results show that the SCOPE model can provide acceptable accuracy for the SIF at the O2-B band (SIFB) and SIF at O2-A band (SIFA) simulations with fixed fluorescence quantum efficiency in dark-adapted conditions (FQE): Table 4 concludes the quantitative assessment of the errors and deviations in SIFB and SIFA simulations with fixed FQE

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Introduction

Solar-induced chlorophyll fluorescence (SIF) refers to the emission of red and far-red light from chlorophyll during the absorption of photosynthetically active radiation under natural sunlight.The SIF spectrum is a continuous broadband spectrum that covers the approximately spectral range of 650–850 nm. The intensity and shape of the SIF spectrum can reflect the amount of energy absorbed by PSII and PSI [4,5]. Several studies have determined the physics-physiology mechanism connecting function of the photosynthetic apparatus with chlorophyll fluorescence from active florescence induction measurement and demonstrated that the fluorescence signal can be a reliable and observable indicator of the plant’s photosynthetic status [4,5,6]. Since Plascyk introduced the Fraunhofer Line Discrimination (FLD) method [12] to extract SIF signals from the observed vegetation-reflected radiance, various studies have demonstrated the possibility of measuring SIF at Fraunhofer lines or atmospheric absorption bands (e.g., an O2-B band at approximately 687 nm and an O2-A band at approximately 760 nm) on the ground, from airborne platforms, and from satellites (for review, see [13]). SIF’s application for global monitoring of plant photosynthesis has become a hot research area [9,14,19]

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