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\begin{document}

\title{Radiative transfer and viewing geometry considerations for remote
sensing as a proxy for carbon uptake in boreal ecosystems}



\author[1]{Zoe Pierrat}%
\author[2]{Alexander Norton}%
\author[1]{Lea Baskin Monk}%
\author[3]{Nicholas Parazoo}%
\author[4]{Andrew Maguire}%
\author[5]{Katja Grossmann}%
\author[6]{Troy Magney}%
\author[7]{Alan Barr}%
\author[7]{Bruce Johnson}%
\author[1]{Jochen Stutz}%
\affil[1]{University of California Los Angeles}%
\affil[2]{Jet Propulsion Laboratory, California Institute of Technology}%
\affil[3]{Jet Propulsion Laboratory (JPL)}%
\affil[4]{NASA Jet Propulsion Laboratory}%
\affil[5]{Heidelberg University}%
\affil[6]{University of California Davis}%
\affil[7]{University of Saskatchewan}%


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  \date{May 26, 2024}


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\begin{abstract}
The boreal forest plays an important role in the global carbon cycle but
has remained a significant source of uncertainty. Remote sensing can
help us better understand the boreal forest's role in the global carbon
cycle. A faint light signal emitted by plant's photosynthetic machinery,
known as solar-induced chlorophyll fluorescence (SIF), is a promising
remotely sensed proxy for carbon uptake, also known as gross primary
productivity (GPP), due to its connection to photosynthesis and its
strong relationship with GPP when observed by satellite. However, SIF
and GPP are fundamentally different quantities that describe distinct,
but related, physiological processes. The relationship between SIF and
GPP is therefore complicated by both physical and ecophysiological
controls. In particular, the dynamics of the SIF/GPP relationship are
poorly understood under varying viewing directions and light conditions.
This is further complicated in evergreen systems where canopy clumping
and the presence of needles create a unique radiative environment. We
use a combination of tower-based SIF and GPP measurements from a boreal
forest field site compared with a coupled biochemical-radiative transfer
model to understand illumination effects on the SIF/GPP relationship. We
find that GPP is amplified under cloudy sky conditions in both
measurements and model results. SIF on the other hand, shows no
significant difference between sunny or cloudy sky conditions in modeled
results, but does show a difference in measurements. We suggest that
these differences may be due to viewing geometry effects that are
important for SIF under sunny sky conditions or the presence of
clumping. Accounting for the differences in the SIF/GPP relationship
therefore is critical for the utility of SIF as a proxy for GPP. In
summation, our results provide insight into how we can use remote
sensing as a tool to understand photosynthesis in the boreal forest.%
\end{abstract}%



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