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

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, SIF_{yield 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 (NIR_V) suggests that f^esc may be the major driver of the observed variability of SIF_{yield apparent.}