Abstract
The escape probability of Solar-induced chlorophyll fluorescence (SIF) can be remotely estimated using reflectance measurements based on spectral invariants theory. This can then be used to correct the effects of canopy structure on canopy-leaving SIF. However, the feasibility of these estimation methods is untested in heterogeneous vegetation such as the discontinuous forest canopy layer under evaluation here. In this study, the Discrete Anisotropic Radiative Transfer (DART) model is used to simulate canopy-leaving SIF, canopy total emitted SIF, canopy interceptance, and the fraction of absorbed photosynthetically active radiation (fAPAR) in order to evaluate the estimation methods of SIF escape probability in discontinuous forest canopies. Our simulation results show that the normalized difference vegetation index (NDVI) can be used to partly eliminate the effects of background reflectance on the estimation of SIF escape probability in most cases, but fails to produce accurate estimations if the background is partly or totally covered by vegetation. We also found that SIF escape probabilities estimated at a high solar zenith angle have better estimation accuracy than those estimated at a lower solar zenith angle. Our results show that additional errors will be introduced to the estimation of SIF escape probability with the use of satellite products, especially when the product of leaf area index (LAI) and clumping index (CI) was underestimated. In other results, fAPAR has comparable estimation accuracy of SIF escape probability when compared to canopy interceptance. Additionally, fAPAR for the entire canopy has better estimation accuracy of SIF escape probability than fPAR for leaf only in sparse forest canopies. These results help us to better understand the current estimation results of SIF escape probability based on spectral invariants theory, and to improve its estimation accuracy in discontinuous forest canopies.
Highlights
Solar-induced chlorophyll fluorescence (SIF) is emitted from within the photosynthetic apparatus of all higher plants and is a good indicator of carbon assimilation and plant physiological status [1,2].SIF has been used to characterize the spatiotemporal dynamics of gross primary productivity (GPP), and strong correlations have been found between satellite-derived SIF and landscape-scale GPP across different biomes [3,4,5,6,7,8].Many research studies have reported a strong linear relationship between TOC (Top of Canopy) SIF and ecosystem-scale GPP over different biomes [9]
This study evaluated the feasibility of SIF escape probability estimation methods in discontinuous forest canopies
If the forest background is partly or totally covered by vegetation, normalized difference vegetation index (NDVI) failed to eliminate the effects of background reflectance on the estimation of SIF escape probability, because NDVI failed to totally reduce the contamination of background reflectance on canopy reflectance
Summary
Solar-induced chlorophyll fluorescence (SIF) is emitted from within the photosynthetic apparatus of all higher plants and is a good indicator of carbon assimilation and plant physiological status [1,2]. Canopy radiative transfer models (RTMs) offer an easy and direct way to estimate the total emitted SIF through model inversion This process amounts to the compensation of the observed TOC SIF for the structure-dependent multiple scattering and re-absorption events that occur from the leaves to sensor. Zhang et al [14] proposed to directly derive the canopy interceptance from remotely sensed data at hand (leaf area index (LAI) and clumping index (CI) satellite products) using the method of [25] (i.e., i◦ = 1 − exp(−G(θ)·LAI·CI/COS(θ))) These estimation methods were developed for homogeneous vegetation which tend to the theoretical and have been mainly validated using 1-D radiative transfer models. The performance of estimation methods for escape probability based on SIT was analyzed under different vegetation conditions, such as different leaf optical properties, canopy structure, and background optical properties
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