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
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.