Sustained Non-Photochemical Quenching Shapes the Seasonal Pattern of
Solar-Induced Fluorescence at a High-Elevation Evergreen Forest
Abstract
The Western US accounts for a significant amount of the forested biomass
and carbon uptake within the conterminous United States. Warming and
drying climate trends combined with a legacy of fire suppression have
left Western forests particularly vulnerable to disturbance from
insects, fire and drought mortality. These challenging conditions may
significantly weaken this region’s ability to uptake carbon from the
atmosphere and warrant continued monitoring. Traditional methods of
carbon monitoring are limited by the complex terrain of the Rocky
Mountains that lead to complex atmospheric flows coupled with
heterogeneous climate and soil conditions. Recently, solar induced
fluorescence (SIF) has been found to be a strong indicator of GPP, and
with the increased availability of remotely-sensed SIF, provides an
opportunity to estimate GPP and ecosystem function across the Western
US. Although the SIF-GPP empirical linkage is strong, the mechanistic
understanding between SIF and GPP is lacking, and ultimately depends
upon changes in leaf chemistry that convert absorbed radiation into
photochemistry, heat (via non-photochemical quenching (NPQ)), leaf
damage or SIF. Understanding of the mechanistic detail is necessary to
fully leverage observed SIF to constrain model estimates of GPP and
improve representation of ecosystem processes. Here, we include an
improved fluorescence model within CLM 4.5 to simulate seasonal changes
in SIF at a sub-alpine forest in Colorado. We find that when the model
includes a representation of sustained NPQ the simulated fluorescence is
much closer to the seasonal pattern of SIF observed from the GOME-2
satellite platform and a custom tower mounted spectrometer system. We
also find that average air temperature may be used as a predictor of
sustained NPQ when observations are not available. This relationship to
air temperature is promising because it may allow for efficient spatial
upscaling of SIF simulations, given widespread availability of
temperature data, but not NPQ observations. Further improvements to the
fluorescence model should focus upon distinguishing between the impacts
of NPQ versus the de-activation of photosystems brought on by
high-stress environmental conditions.