Systematic Assessment of Retrieval Methods for Canopy Far‐Red Solar‐Induced Chlorophyll Fluorescence Using High‐Frequency Automated Field Spectroscopy

Abstract

Remote sensing of solar‐induced chlorophyll fluorescence (SIF) offers potential to infer photosynthesis across scales and biomes. Many retrieval methods have been developed to estimate top‐of‐canopy SIF using ground‐based spectroscopy. However, inconsistencies among methods may confound interpretation of SIF dynamics, eco‐physiological/environmental drivers, and its relationship with photosynthesis. Using high temporal‐ and spectral resolution ground‐based spectroscopy, we aimed to (1) evaluate performance of SIF retrieval methods under diverse sky conditions using continuous field measurements; (2) assess method sensitivity to fluctuating light, reflectance, and fluorescence emission spectra; and (3) inform users for optimal ground‐based SIF retrieval. Analysis included field measurements from bi‐hemispherical and hemispherical‐conical systems and synthetic upwelling radiance constructed from measured downwelling radiance, simulated reflectance, and simulated fluorescence for benchmarking. Fraunhofer‐based differential optical absorption spectroscopy (DOAS) and singular vector decomposition (SVD) retrievals exhibit convergent SIF‐PAR relationships and diurnal consistency across different sky conditions, while O2A‐based spectral fitting method (SFM), SVD, and modified Fraunhofer line discrimination (3FLD) exhibit divergent SIF‐PAR relationships across sky conditions. Such behavior holds across system configurations, though hemispherical‐conical systems diverge less across sky conditions. O2A retrieval accuracy, influenced by atmospheric distortion, improves with a narrower fitting window and when training SVD with temporally local spectra. This may impact SIF‐photosynthesis relationships interpreted by previous studies using O2A‐based retrievals with standard (759–767.76 nm) fitting windows. Fraunhofer‐based retrievals resist atmospheric impacts but are noisier and more sensitive to assumed SIF spectral shape than O2A‐based retrievals. We recommend SVD or SFM using reduced fitting window (759.5–761.5 nm) for robust far‐red SIF retrievals across sky conditions.

NIRvP: a robust structural proxy for sun-induced chlorophyll fluorescence and photosynthesis across scales

“…which are known to considerably affect SIF magnitude 31 . In the spatial domain, similar regression analyses as for the temporal domain could be conducted but we chose to use the simpler approximation by calculating the ratio of SIF/NIRVP instead as regression intercepts tended to vary relatively little and for the airborne data the regression approach is not applicable.…”

“…As ΦF estimation amplifies SIF retrieval noise 22 , however, very high quality SIF products should be used to avoid the need for aggregation to coarser scales. Second, since SIF and NIRVP share the same structure and radiation components (APAR x fesc) and NIRVP typically has higher signal quality than SIF, evaluating the SIF-NIRVP relationship can be used to assess the quality of SIF retrievals beyond diurnal variations that are strongly driven by PAR 31 . NIRVP might therefore prove helpful in further improving SIF retrieval methods as they continue to be refined 31,60,61 .…”

“…Based on current knowledge, canopy-level SIF variations are expected to be driven predominantly by APAR and fesc rather than ΦF, at least in the absence of strong environmental stress and ecosystems other than evergreen needleleaf forests. At sub-daily time scales, PAR is the dominant driver of SIF given the typically much smaller diurnal variations in the fraction of absorbed PAR (fPAR), fesc and ΦF 22,27,31 . At seasonal time scales, APAR and fesc appear to show stronger variations compared to ΦF 22,23 .…”

On the Covariation of Chlorophyll Fluorescence and Photosynthesis Across Scales

“…This divergence from positive covariation occurs only under very low light, when NPQ is minimal ("PQ-phase," Figure 1d) and is likely not observable from satellite SIF observations made under higher incident light. Further, under low light and low solar angles it is probable that the SIF retrieval uncertainty will be high, rendering the data uninterpretable (Chang et al, 2020; 10.1029/2020GL091098 Grossmann et al, 2018). Remote sensing observations of SIF integrate absorbed light across all sunlit/shaded leaves at varying angles, and it is possible that late morning to afternoon observations do not capture the phase-shift in the ΦP-ΦF relationship (outside of the green circle in Figure 1d).…”

Systematic Orbital Geometry-Dependent Variations in Satellite Solar-Induced Fluorescence (SIF) Retrievals

“…SIF was retrieved with GOME-2 using variants of a principal component analysis (PCA) technique [54][55][56]. Similar PCA approaches have been used in SIF retrievals with other satellite and ground-based instruments (e.g., [35,57,58]). Here, we used the GOME-2B SIF from the version 27 (v27) GOME-2 NASA dataset [54,59] produced with updates as noted below applied to the base unadjusted retrievals provided in that data set.…”