The Response of Spectral Vegetation Indices and Solar‐Induced Fluorescence to Changes in Illumination Intensity and Geometry in the Days Surrounding the 2017 North American Solar Eclipse

Abstract

Remote sensing is a key method for advancing our understanding of global photosynthesis and is thus critical to understanding terrestrial carbon uptake and climate change. Increasingly sophisticated spectral indices including solar-induced florescence (SIF) and the photochemical reflectance index (PRI) are considered good proxies of canopy structure, biochemistry, and physiology. However, the relative influences of illumination intensity and angle on these measures are difficult to unravel, particularly at the scale of whole forest canopies. We exploit the solar dimming during the 2017 North American solar eclipse as well as a clear day before and cloudy day after the day of the eclipse. This novel approach allows us to assess changes in spectral vegetation indices due to illumination intensity independent of changes in illumination angle. Physiologically relevant spectral indices were most affected by dimming, with illumination level explaining 97% of variability in SIF and 99% of variability in PRI during the eclipse. The spectral change in reflectance through the eclipse period revealed changes in PRI were driven by reflectance differences at the 570 nm reference band rather than at the 531 nm signal band associated with xanthophyll pigment interconversions. This study refines our interpretation of vegetation properties from space with implications for our interpretation of signals related to terrestrial photosynthesis derived from sensors spanning a range of illumination conditions and angles.

How Do Sky Conditions Affect the Relationships Between Ground‐Based Solar‐Induced Chlorophyll Fluorescence and Gross Primary Productivity Across Different Plant Types?

“…Therefore, the effects of SZA on CI were not considered in this study. Moreover, the solar‐target‐viewing geometry effects on SIF due to varying SZA were also not considered because the angular effects are minimal under the cloudy conditions (Rogers et al., 2020). $$$\mathrm{C}\mathrm{I}=1-{R}_{\mathrm{d}}/{R}_{\mathrm{g}}$$$ …”

Potential of hotspot solar‐induced chlorophyll fluorescence for better tracking terrestrial photosynthesis

“…Especially, top‐of‐canopy (TOC) SIF emission only accounts for a fraction of total emitted SIF photons from leaves, due to multiple scattering and reabsorption processes within the canopy (Porcar‐Castell et al, 2014). Consequently, TOC observations of SIF is subject to the pronounced bidirectional anisotropic effects (Biriukova et al, 2020; Liu et al, 2016; Rogers et al, 2020; Tol et al, 2009), which is affected by vegetation biochemistry (e.g., chlorophyll content [Cab]), plant canopy structure (e.g., leaf area index [LAI], leaf angle distribution [LAD]; Dechant et al, 2020; Migliavacca et al, 2017), and illumination‐viewing geometries (i.e., sun and sensor angles; Guanter et al, 2012; Liu et al, 2016). The directional effects of SIF are generally quantified explicitly by an escape ratio f esc , which represents the ratio of observed SIF photons escaping from the canopy to all SIF photons emitted from all leaves.…”