On the Covariation of Chlorophyll Fluorescence and Photosynthesis Across Scales

Recent advances in remote sensing of solar-induced chlorophyll fluorescence (SIF) have garnered wide interest from the biogeoscience and Earth system science communities, due to the observed linearity between SIF and gross primary productivity (GPP) at increasing spatiotemporal scales. Three recent studies, Maguire et al., (2020, ), He et al. (2020, ), and Marrs et al. (2020, ) highlight a nonlinear relationship between fluorescence and photochemical yields and show empirical evidence for the decoupling of SIF, stomata, and the carbon reactions of photosynthesis. Such mechanistic studies help advance our understanding of what SIF is and what it is not. We argue that these findings are not necessarily contradictory to the linear SIF-GPP relationship observed at the satellite scale and provide context for where, when, and why fluorescence and photosynthesis diverge at smaller spatiotemporal scales. Understanding scale dependencies of remote sensing data is crucial for interpreting SIF as a proxy for GPP. Plain Language Summary When exposed to light, plants re-emit a small amount of light from chlorophyll molecules called fluorescence. Remote sensing instruments are now capable of measuring chlorophyll fluorescence (which is emitted between 650-850 nm) from canopies to the globe (solar-induced chlorophyll fluorescence; SIF). A growing number of papers have highlighted an empirical linear relationship between SIF and whole-ecosystem photosynthesis (gross primary productivity; GPP). These advances have excited the broader Earth science research community, but recent studies have pointed out that the linear SIF-GPP relationship at coarse scales does not hold true at smaller spatiotemporal scales. In this commentary, we synthesize three recent studies that provide insight into the relationship between fluorescence and photosynthesis at leaf and canopy scales, under natural and controlled conditions. At fine spatiotemporal scales, fluorescence can be decoupled with photosynthetic carbon uptake, but we argue that satellite measurements are often too coarse in time and space to observe the SIF-photosynthesis decoupling and that the integration of canopy processes explains the observed linearity. As such, SIF plays an important role as an estimate of GPP at spatial and temporal scales relevant for monitoring global terrestrial productivity, benchmarking terrestrial biosphere and earth system models, and managing ecosystems. Key Points Solar-induced fluorescence (SIF) is widely used as a remote estimate of ecosystem gross primary productivity (GPP), but why does it work? Three recent studies point to inherent nonlinearities in the fluorescence-photosynthesis relationship at fine spatiotemporal scales We synthesize mechanisms to suggest that these results are not contradictory to the increasingly linear SIF:GPP relationship across scales