Multi-angular instrument for tower-based observations of canopy sun-induced chlorophyll fluorescence

Abstract

Gross primary production (GPP) is an important component in the global carbon cycle. Sun-induced chlorophyll fluorescence (SIF) provides remotely sensible signals for monitoring GPP. The angular distribution of SIF is heterogeneous. However, SIF is usually measured from one angle only, which may not represent the canopy-scale value. In this study, a ground-based system of multi-angle observation was used to measure the SIF over the growing season of a paddy rice. This system includes two spectrometers (Ocean Optics HR4000) with the identical conditions (i.e., full width at half maxima, slits, and grating) and a cooling system. This system can monitor SIF signals continuously over plant canopies. The SIF was measured at four viewing zenith angles (VZA = 32°, 42°, 52°, and 62°) with eleven view azimuth angles (VAA = 30°, 60°, 90°, 120°, 150°, 180°, 210°, 240°, 300°, 330°, and 360°, respectively). The ground observation of the SIF varied greatly with the angle of observation relative to the sun. The SIF values in the hotspot direction were higher (from 25% to 40%) than those in the non-hotspot direction, and the lowest SIF values appeared at the dark spot.