Forest Woody Structures Decrease the Radiative Transfer Modelled Far-red Solar-induced Fluorescence Radiance More Than Its Canopy Escape Ratio

Abstract:

Space and airborne top of canopy (TOC) solar-induced fluorescence (SIF), a subtle optical signal originating from photosynthetic processes of green plants, is used to estimate the gross primary productivity (GPP) of terrestrial vegetation, including forests. To reduce the GPP estimation inaccuracy, the angularly anisotropic SIF signal must be corrected for its interactions with canopy structures. To investigate the potential impacts of selected forest structural traits and elements, i.e. leaf area index (LAI), leaf angle distribution (LAD), canopy closure (CC) and the presence/absence of wood, we coupled the leaf model Fluspect-Cx with the 3D DART canopy radiative transfer model and simulated propagation of SIF signal (radiative budget) through two structurally complex but architecturally different canopies: an Australian White peppermint (Eucalyptus pulchella) and a European Norway spruce (Picea abies) stands. 3D representations of the stands were constructed based on terrestrial laser scans of individual trees and required optical properties were measured in the field. The radiative budget of far-red SIF (740 nm) was simulated for several virtual scenarios by modifying or removing only a single canopy structural trait at the time. Removal of woody components from clumped erectophile DART canopies of W. peppermint with CC of 80 and 40% and LAI of 2.5 increased the far-red TOC SIF radiance by 24 and 14%, while the hemispherical SIF escape ratio (SIF TOC radiance/total SIF emission) from the TOC increased by 10 and 8% and in nadir direction by 6 and 4%, respectively. Interestingly, the change in CC from 80 to 40% triggered a similar change in far-red SIF hemispherical and nadir escape, i.e. a respective decrease by 8 and 6%. The elimination of woody material in the N. spruce canopy (spherical LAD, LAI = 8.5, CC ~ 80% and single generation of needle leaves) increased the far-red TOC SIF radiance almost twice (90%), whereas the hemispherical and nadir far-red SIF escape ratio increased by 42 and 40%, respectively. The greater change in the far-red TOC SIF radiance over its canopy escape ratio indicates that the effect of wood shadowing is larger than direct optical interactions. Our results demonstrate a potentially significant influence of wood that should be considered when interpreting remote sensing SIF observations.