Ground-Based Multiangle Solar-Induced Chlorophyll Fluorescence Observation and Angular Normalization for Assessing Crop Productivity

Solar-induced chlorophyll fluorescence (SIF) provides remotely sensible signals for monitoring gross primary production (GPP). Ground-based multiangle observations of both red and far-red SIF above wheat and maize canopies were conducted to examine angular effects on SIF. With these new measurements, we were able for the first time to refine and apply an algorithm developed for angular normalization of both red and far-red SIF measurements. The angular normalization improved the correlation of SIF with GPP derived from eddy covariance measurements at the instantaneous scale (1 min), with increases of the diurnal coefficients of determination (of sunlit SIF with GPP) up to 0.21 for far-red SIF and 0.3 for red SIF based on analysis on 6 sunny days. The improvement was slightly smaller for far-red SIF than for red SIF, attributing to that the observed angular variation of SIF in the red band was greater than that in the far-red band due to weaker multiple scattering in the red band in the canopy. In addition, at the hourly time scale, far-red sunlit SIF shows its advantage to track GPP for heterogonous canopies, while angular normalization of red SIF is effective for homogeneous canopies. In comparison with another angular normalization method based on the escape ratio using datasets over both wheat and maize canopies, the two kinds of method show similar ability to improve the correlation between SIF and GPP, while the results suggest a limitation of SIF in estimating GPP for dense canopies where the fraction of shaded leaves are large.