Solar-induced fluorescence (SIF) is being rapidly adopted as a proxy of photosynthesis at unprecedented scales. The connection between SIF and photosynthesis originates at the photosystem level via light absorption and subsequent energy partitioning between photochemistry, regulatory heat dissipation, and chlorophyll fluorescence emission, but the two signals are intricately connected at the leaf and canopy scales too. At the leaf level, diurnal variations in ChlF can relate to changes in photochemical or non-photochemical quenching, antenna and pigment dynamics, or chloroplast movements, whereas seasonal variations can include sustained changes in photochemical and non-photochemical quenching related to the photoinhibition of reaction centres or the adjustment in the protein-pigment structure of the photosystem. At the canopy level, variations in ChlF can reflect changes in total leaf area, canopy structure, species-specific phenology, dynamics of background, or changes in illumination or sun-target-sensor geometry. Clearly, while some of these factors strengthen the link between SIF and photosynthesis, others can compromise it. Mutiscale datasets are therefore critical to assess how the link between fluorescence and photosynthesis propagates across scales. We here present results from a recent multiscale campaign conducted in Southern Finland during the winter-to-summer transition in an evergreen boreal forest. Measurements spanned during five months and covered five species and two different canopy positions, providing a unique record of the multiscale spring recovery of photosynthesis in a boreal forest. Measurements included: Fluorescence lifetime, ChlF spectra, Fast kinetics, PAM fluorometry PSI and PSII activity, leaf absorption, foliar pigments, photosynthetic proteins, photosynthetic gas exchange, canopy SIF and spectral reflectance, ecosystem CO2, water, COS, and VOC fluxes, as well as drone-based measurements of ecosystem SIF from 30 to 500 m altitude.
Results/Conclusions
Temporal dynamics in the leaf-level spectral fluorescence signal differed between species and were found to be strongly modulated by sustained forms of NPQ. In turn, spatial variation in leaf-level fluorescence properties was strongly controlled by the prevailing light environment experienced by the foliage. At the canopy level, temporal dynamics in SIF depended on both leaf-level factors as well as seasonal changes in canopy structure, whereas spatial variation as registered with the drone was related to species composition. The results emphasize the intricate variability of factors that make up the SIF signal of a satellite pixel, where the interpretation of the SIF signal depends on spatial, temporal and biological context. This should not be only regarded as a challenge but also as an opportunity for new SIF-based ecological studies.