Assessing bi-directional effects on the diurnal cycle of measured solar-induced chlorophyll fluorescence in crop canopies

Solar-induced chlorophyll fluorescence (SIF) has shown to be a good proxy of gross primary production (GPP) across multiple spatial and temporal scales. The dependence of top-of-canopy measurements of SIF on the illumination and observation directions has been recognized as an important factor in the relationship between SIF and GPP across multiple spatiotemporal scales. In this study, we investigated angular effects in diurnal observed SIF (SIFobs) measurements taken with a multi-angle SIF measuring system in a wheat-corn rotational field during the growing season of 2018. Our results reveal strong angular dependencies in diurnal measurements of top-of-canopy SIF for both red SIFobs (RSIFobs) and far-red SIFobs (FRSIFobs). We also observe that the correlation between SIFobs (RSIFobs and FRSIFobs) and GPP depends on both the illumination and viewing directions. To mitigate angular effects in SIFobs, an approach based on spectral reflectance measurements was used. The derived canopy total SIF emission (SIFtotal) at both red and far-red bands (RSIFtotal and FRSIFtotal) showed less angular dependencies and stronger relationships to APAR and GPP than RSIFobs and FRSIFobs. These results highlight the important role of angular effects in SIF when interpreting directional SIFobs retrievals from space, which normally include large variations in sun-target-viewing geometries. This study contributes to our understanding of angular effects on SIF-GPP relationships and subsequently help improve the estimation of GPP from SIF data.