From SIF to photosynthesis: Achievements, challenges and opportunities

Abstract:

Solar induced chlorophyll fluorescence (SIF) has increasingly been used as a remote sensing proxy for the photosynthesis of terrestrial vegetation. Both experimental data and the Soil Canopy Observation of Photosynthesis and Energy fluxes (SCOPE) model show that the lion share of the SIF-photosynthesis relation is explained by variations in the solar radiation absorbed by chlorophyll. Over a season the intensity of photosynthetic active irradiance (PAR) and the fractional absorption (fPAR) contribute about equally to the variability in SIF and photosynthesis. However, variations in light use efficiency of photochemistry and the emission efficiency of fluorescence in photosystems also play a non-negligible role. Finally, the scattering and re-absorption after emission cause an observation angle dependence of SIF.

We derived the 'Fluorescence Correction Vegetation Index' (FCVI = NIR-RED) from radiative transfer theory: a reflectance index to normalize proximal SIF of dense vegetation for absorption of PAR and re-absorption of SIF after emission, thus resulting in a photosystem emission efficiency that is more comparable to active leaf fluorescence data. We show that SIF/FCVI explains the seasonal cycle of the fluorescence emission efficiency in proximal SIF measurements above dense vegetation. This shows that it is possible to combine SIF and reflectance measurements to obtain information about energy dissipation in photosystems. However, the optical properties of soil and atmosphere pose a challenge for unraveling physiological structural effects on SIF measured from satellites or above sparse vegetation. Coupled soil-vegetation-atmosphere modelling is needed to obtain a complete understanding of SIF in such cases. We summarize these challenges and identify opportunities in the combined use of data of different satellite missions.