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Radiative transfer and viewing geometry considerations for the SIF/GPP relationship
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  • Alexander Norton,
  • Nicholas Parazoo,
  • Andrew Maguire,
  • Katja Grossmann,
  • Troy Magney,
  • Alan Barr,
  • Bruce Johnson,
  • Jochen Stutz
University of California Los Angeles

Corresponding Author:zpierrat@g.ucla.edu

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Alexander Norton
University of Melbourne
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Nicholas Parazoo
Jet Propulsion Laboratory
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Andrew Maguire
NASA Jet Propulsion Laboratory
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Katja Grossmann
University of Heidelberg
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Troy Magney
University of California Davis
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Alan Barr
Global Institute for Water Security, University of Saskatchewan,Environment and Climate Change Canada
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Bruce Johnson
University of Saskatchewan
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Jochen Stutz
University of California Los Angeles,University of Heidelberg
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Solar-Induced chlorophyll Fluorescence (SIF) provides a powerful proxy for determining forest gross primary production (GPP), particularly in evergreen ecosystems where traditional measures of greenness fail. The dynamics of the SIF/GPP relationship, however, are poorly understood under varying viewing directions and light conditions. This is, in large part, due to challenges in measuring SIF at the spatiotemporal scale that is necessary to understand these effects. Therefore, the aim of this work is to utilize high-temporal and spatial resolution SIF measurements to better constrain the response of SIF to ambient canopy illumination and viewing geometry. We use a PhotoSpec instrument and eddy covariance measurements to explore the SIF/GPP relationship under various viewing directions and light conditions during the 2019 and 2020 growing seasons at the Old Black Spruce site in Saskatchewan, Canada. PhotoSpec is a tower-based 2-D scanning spectrometer system capable of taking Fraunhofer-line based SIF retrievals in the red and far-red wavelength ranges with a 0.7 degree field of view at a ~30 second time resolution. Measured SIF and GPP are combined with SCOPE modelling results to provide a mechanistic understanding of the physical and ecophysiological drivers for the SIF/GPP relationship in the Boreal Forest. Our results show that viewing direction and solar zenith/azimuth angles are important for the SIF signal under direct light conditions, but not under diffuse. Furthermore, the SIF/GPP relationship changes under direct and diffuse light conditions at a 30 minute, daily, and monthly resolution. Our ability to use SIF as a proxy for GPP depends on a quantitative understanding of radiative transfer within the canopy and how scanning geometry impacts SIF measurements. These results provide an important insight into these relationships in the Boreal forest, a region where GPP has been traditionally difficult to track using remote sensing.