Satellite footprint data from OCO-2 and TROPOMI reveal significant spatio-temporal and inter-vegetation type variabilities of solar-induced fluorescence yield in the U.S. Midwest

Abstract

Solar-induced chlorophyll fluorescence (SIF) measured from space has been increasingly used to quantify plant photosynthesis at regional and global scales. Apparent canopy SIF yield (SIF_(yield apparent)), determined by fluorescence yield (Φ_F) and escaping ratio (f^(esc)), together with absorbed photosynthetically active radiation (APAR), is crucial in driving spatio-temporal variability of SIF. While strong linkages between SIF_(yield apparent) and plant physiological responses and canopy structure have been suggested, spatio-temporal variability of SIF_(yield apparent) at regional scale remains largely unclear, which limits our understanding of the spatio-temporal variability of SIF and its relationship with photosynthesis. In this study, we utilized recent SIF data with high spatial resolution from two satellite instruments, OCO-2 and TROPOMI, together with multiple other datasets. We estimated SIF_(yield apparent) across space, time, and different vegetation types in the U.S. Midwest during crop growing season (May to September) from 2015 to 2018. We found that SIF_(yield apparent) of croplands was larger than non-croplands during peak season (July–August). However, SIFyield apparent between corn (C4 crop) and soybean (C3 crop) did not show a significant difference. SIF_(yield apparent) of corn, soybean, forest, and grass/pasture show clear seasonal and spatial patterns. The spatial variability of precipitation during the growing season could explain the overall spatial pattern of SIF_(yield apparent). Further analysis by decomposing SIF_(yield apparent) into Φ_F and f^(esc) using near-infrared reflectance of vegetation (NIRV) suggests that f^(esc) may be the major driver of the observed variability of SIF_(yield apparent)