Remotely sensed solar-induced chlorophyll fluorescence (SIF) is linked to the functional status of leaves and may provide novel information on the broad scale dynamics of photosynthesis compared with traditional reflectance-based methods. This is especially true in evergreen ecosystems which exhibit seasonally changing light use efficiency while maintaining chlorophyll content all year. Although SIF has the potential to transform our ability to study the terrestrial carbon cycle, a lack of physiological understanding of fluorescence emission under natural conditions remains a barrier to realizing the full potential of SIF. Numerous studies have shown a linear relationship between remotely sensed SIF and carbon uptake across ecosystems, however non-linearities emerge at finer scales, and the underlying mechanistic link between SIF and photosynthesis is complicated by the physiology of plant response to environmental conditions. In this study we combined three years of eddy covariance flux data with high spatio-temporal resolution canopy SIF observations from a tower-mounted PhotoSpec scanning spectrometer system in a subalpine conifer forest in Colorado (Niwot Ridge AmeriFlux core site). We compared the light response of gross primary productivity (GPP) with that of SIF to assess differences between the quantum yields of SIF and photosynthetic carbon assimilation at the canopy scale.
Results/Conclusions:
Our results show that the environmental cues that dictate the onset of SIF differ from those for GPP. SIF exhibited a light response weeks prior to the onset of eddy covariance estimated GPP. This result suggests that photosystems become activated in spring ahead of when the eddy covariance method is able to detect photosynthetic production following winter dormancy. In contrast, the light response of SIF closely matched that of GPP during both the growing season and fall senescence, supporting the use of SIF as a remotely sensed proxy for photosynthetic carbon uptake during much of the year. That SIF and GPP were out of phase in spring represents a challenge for the use of SIF as a phenological predictor of start of growing season in evergreen coniferous forests.