The photosynthetic response of spectral chlorophyll fluorescence differs across species and light environments in a boreal forest ecosystem

Chlorophyll fluorescence can serve as a proxy of photosynthesis in boreal forests. When sustained non-photochemical quenching (NPQ_S) relaxes towards summer, leaf chlorophyll fluorescence (ChlF) emission increases along with photosynthesis. Yet, other physical and physiological factors can also leave a measurable imprint on the fluorescence emission spectra, and disrupt this relationship. We measured spectral ChlF in leaves of three dominant evergreen species in the boreal ecosystem (Scots pine, Norway spruce and lingonberry) growing under contrasting light environments and throughout the spring recovery of photosynthesis. We also measured photosynthetic, biochemical and morphological traits. Correlations between traits and ChlF spectral components were analyzed to identify the mechanisms underlying both the spatial variation found between species and light environments, and the temporal variation along the spring recovery of photosynthesis. Spatially, we found evidence of baseline differences in leaf-level ChlF magnitude, which we attribute to species- and light environment-specific changes in leaf morphology. Temporally, ChlF magnitude followed the relaxation of NPQ_S towards summer, but only in upper canopy foliage and lingonberry, suggesting a seasonal compensation effect between sustained photochemical quenching (PQ_S) and NPQ_S, potentially decoupling the seasonal relationship between ChlF and photosynthesis in shaded foliage. Finally, we show subtle changes in the shape of the ChlF spectra that took place independently of chlorophyll concentration dynamics, pointing to the complexity of NPQ_S which could reflect structural rearrangements in the thylakoids and changes in the relative contribution of PSI to emitted ChlF. We conclude that the diversity of species and light environments found within an ecosystem generates a baseline level of variation in leaf spectral ChlF as well as contrasting seasonal photosynthetic acclimation patterns. These sources of variability should be taken into account when developing quantitative models for the interpretation of ChlF data, in particular for applications involving high resolution SIF imaging systems capable of resolving different plant individuals and their parts.