A practical approach for estimating the escape ratio of near-infrared solar-induced chlorophyll fluorescence

Yelu Zeng,Grayson Badgley,Benjamin Dechant,Youngryel Ryu,Min Chen,J.A Berry

doi:10.1016/j.rse.2019.05.028

Abstract

Solar-induced chlorophyll fluorescence (SIF) has emerged as a leading approach for remote sensing of gross primary productivity (GPP). While SIF has an intrinsic, underlying relationship with canopy light capture and light use efficiency, these physiological relationships are obscured by the fact that satellites observe a small and variable fraction of total emitted canopy SIF. Upon emission, most SIF photons are reabsorbed or scattered within the canopy, preventing their observation remotely. The complexities of the radiative transfer process, which vary across time and space, limit our ability to reliably infer physiological processes from SIF observations. Here, we propose an approach for estimating the fraction of total emitted near-infrared SIF (760 nm) photons that escape the canopy by combining the near-infrared reflectance of vegetation (NIRV) and the fraction of absorbed photosynthetically active radiation (fPAR), two widely available remote sensing products. Our approach relies on the fact that NIRV is resilient against soil background contamination, allowing us to reliably calculate the bidirectional reflectance factor of vegetation, which in turn conveys information about the escape ratio of SIF photons. Our NIRV-based approach explains variations in the escape ratio with an R2 of 0.91 and an RMSE of 1.48% across a series of simulations where canopy structure, soil brightness, and sun-sensor-canopy geometry are varied. The approach is applicable to conditions of low leaf area index and fractional vegetation cover. We show that correcting for the escape ratio of SIF using NIRV provides robust estimates of total emitted SIF, providing for the possibility of studying physiological variations of fluorescence yield at the global scale.

Highlights

The empirical relationship between solar-induced chlorophyll fluorescence (SIF) and gross primary productivity (GPP) is complicated by the fact that we only observe a fraction of emitted SIF photons and that this fraction depends on the direction of observation (Porcar-Castell et al, 2014)
Eq 12 states that the ratio of near-infrared reflectance of vegetation (NIRV) and fraction of absorbed photosynthetically active radiation (fPAR) approximates the canopy escape ratio. To validate this formulation, we began by running two simple simulations in SCOPE: while holding leaf albedo constant and leaf area index (LAI) at 3, we varied canopy leaf angle between erectophile and spherical (Figure 3). fPAR varied by less than 8% across the two simulations, while the NIRV of the two canopies changed by 58.1% (Figure 3A). f esc across the two simulations differed by 54.3%, which is roughly proportional to the differences in the ratio of NIRV to fPAR between the spherical and the erectophile canopies (Figure 3B)
Previous empirical work investigating the relationship of NIRV and fPAR at eddy covariance sites on a per biome basis found similar results: canopies of widely varying architecture, tended to have similar values of fPAR (Badgley et al, 2017)

Summary

Introduction

The empirical relationship between solar-induced chlorophyll fluorescence (SIF) and gross primary productivity (GPP) is complicated by the fact that we only observe a fraction of emitted SIF photons and that this fraction depends on the direction of observation (Porcar-Castell et al, 2014). It is difficult, to distinguish physiological variations in the raw SIF signal from variations in SIF caused by radiative transfer processes. There does not exist an accepted, broadly applicable way to measure the fraction of the total leaf-level SIF emission that makes its way to the top of the canopy and, to the SIF sensor

Methods

Results

Discussion

Conclusion