Contributions of the understory and midstory to total canopy solar-induced chlorophyll fluorescence in a ground-based study in conjunction with seasonal gross primary productivity in a cool-temperate deciduous broadleaf forest

Solar-induced chlorophyll fluorescence (SIF) can represent gross primary productivity (GPP) in many types of terrestrial vegetation. In principle, the chlorophyll-a fluorescence signal responds to the amount of light absorption and the fraction of energy distribution in photosystems. Therefore, it is mechanistically linked with CO2 assimilation. Recently, radiative transfer models have traced the processes of emission, interception and reabsorption within a canopy in heterogeneous tree stands. These processes influence the expected relationship between SIF and GPP. We inferred that the vertical profile of SIF could reveal those processes. In addition, the sum of SIF emission from different pathways within the canopy must be equivalent to the total SIF emission from all leaves. The purposes of this study were: (1) to clarify the seasonal and diurnal variations of ground-based SIF observed above the overstory, midstory and understory layers (strata); (2) to examine whether the sum of SIF observed from the three layers could represent total SIF emissions, and (3) to examine whether SIF above the understory could reveal the photosynthetic activity within the vertical profile of a forest. To estimate how much SIF was present after its emission from leaves within the canopy, we conducted spectral radiation observations in three vertical layers: above the understory, midstory and overstory (8, 14 and 18 m from the ground, respectively) to retrieve the red and far-red SIF in a deciduous broadleaf forest ecosystem in Japan from April to November 2020. We found that SIF above the overstory and the sum of SIF from the three layers increased sharply at the leaf-onset of the overstory and was correlated with each season's GPP. The sum of far-red SIF from the three layers was proportional to the total SIF estimated using the escape ratio approach for SIF above the overstory. In this study we demonstrated that SIF above the understory could detect the increased photosynthesis of the understory in spring, compared with the GPP and the vertical profile of the CO2 concentrations. In summary, SIF observed in three forest layers detected the seasonal change of GPP within the canopy. Thus, a multi-layer approach can be used to understand the relationship between SIF and photosynthesis within the canopy.