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Physical and ecophysiological controls on the relationship between solar-induced chlorophyll fluorescence and gross primary productivity across diurnal and seasonal scales in the boreal forest
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  • Zoe Pierrat,
  • Troy Magney,
  • Nicholas Parazoo,
  • Katja Grossmann,
  • Bruce Johnson,
  • Alan Barr,
  • Jacob Bortnik,
  • Alexander Norton,
  • Andrew Maguire,
  • David Bowling,
  • Ulrike Seibt,
  • Christian Frankenberg,
  • Jochen Stutz
Zoe Pierrat
University of California Los Angeles

Corresponding Author:zpierrat@gmail.com

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Troy Magney
University of California Davis
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Nicholas Parazoo
Jet Propulsion Laboratory (JPL)
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Katja Grossmann
Heidelberg University
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Bruce Johnson
University of Saskatchewan
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Alan Barr
University of Saskatchewan
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Jacob Bortnik
University of California Los Angeles
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Alexander Norton
Jet Propulsion Laboratory, California Institute of Technology
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Andrew Maguire
NASA Jet Propulsion Laboratory
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David Bowling
University of Utah
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Ulrike Seibt
University of California Los Angeles
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Christian Frankenberg
NASA Jet Propulsion Laboratory
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Jochen Stutz
University of California Los Angeles
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Abstract

Solar-Induced Chlorophyll Fluorescence (SIF) is a powerful proxy for gross primary productivity (GPP) in Boreal ecosystems. However, SIF and GPP are fundamentally different quantities that describe distinct, but related, physiological processes. Recent work has highlighted non-linearities between SIF and GPP at finer spatial (leaf- to canopy- level) and temporal (half-hourly) scales. Therefore, questions have arisen about when, where, and why SIF is a good proxy for GPP and what the potential sources for divergence between the two are. The goal of this study is to answer two specific questions: 1) At what temporal scale is SIF a good proxy for GPP and 2) What are the predominant physical and ecophysiological drivers of nonlinearity between SIF and GPP in boreal ecosystems? We collected tower-based measurements of SIF (and other common vegetation indices) with PhotoSpec (a custom spectrometer system) and eddy-covariance GPP data at a 30-minute resolution at the Southern Old Black Spruce Site (SOBS) in Saskatchewan, CA. We applied a combination of statistical and machine learning approaches to disentangle the influence of structural/illumination effects and ecophysiological variations on the SIF signal. Our results show that at a high temporal resolution (half-hourly), SIF and GPP are predominantly dependent on photosynthetically active radiation (PAR). Therefore, the non-linear light response of GPP drives non-linearity between SIF and GPP. Additionally, canopy structure and illumination effects become important to the SIF signal at high temporal resolutions. At the seasonal timescale, SIF and GPP exhibit co-varying responses to PAR, even when accounting for changes in canopy structure. We attribute changes in the light responses of SIF and GPP to sustained photoprotection over winter which co-varies with changes in temperature. Finally, we show that the relationship between SIF and GPP has a seasonal dependence caused by small differences between the light use efficiencies of fluorescence and photosynthesis. Accounting for this seasonally variable relationship will improve the use of SIF as a proxy for GPP.