Assessing the potential of sun-induced fluorescence and the canopy scattering coefficient to track large-scale vegetation dynamics in Amazon forests

• Published in: Remote sensing of environment 2018-01, Vol.204, p.769-785
• Main Authors: Köhler, Philipp, Guanter, Luis, Kobayashi, Hideki, Walther, Sophia, Yang, Wei
• Format: Article
• Language: English
• Subjects: Amazon forest dynamics; Annual variations; Atmospheric effects; Biodiversity; Canopies; Canopy scattering coefficient; Chlorophyll; Computer simulation; Concept of canopy spectral invariants; Directional area scattering factor; Fluorescence; Forests; Global ozone; Global ozone monitoring experiment; GOME-2; I.R. radiation; Monitoring; OCO-2; Optical properties; Ozone; Ozone monitoring; Parameters; Photosynthesis; Photosynthetically active radiation; Pollution monitoring; Radiation; Radiative transfer; Reflectance; Remote sensing; River basins; Scattering coefficient; Seasonal variations; Sensitivity analysis; Spatial heterogeneity; Spectrometers; Sun; Sun-induced chlorophyll fluorescence; Theft; Three dimensional models; Vegetation
• ISSN: 0034-4257; 1879-0704
• DOI: 10.1016/j.rse.2017.09.025

Two new remote sensing vegetation parameters derived from spaceborne spectrometers and simulated with a three dimensional radiative transfer model have been evaluated in terms of their prospects and drawbacks for the monitoring of dense vegetation canopies: (i) sun-induced chlorophyll fluorescence (SIF), a unique signal emitted by photosynthetically active vegetation and (ii) the canopy scattering coefficient (CSC), a vegetation parameter derived along with the directional area scattering factor (DASF) and expected to be particularly sensitive to leaf optical properties. Here, we present the first global data set of DASF/CSC and examine the potential of CSC and SIF for providing complementary information on the controversially discussed vegetation seasonality in Amazon forests. A comparison between near-infrared SIF derived from the Global Ozone Monitoring Experiment-2 (GOME-2) instrument and the Orbiting Carbon Observatory-2 (OCO-2) (overpass time in the morning and noon, respectively) reveals the response of SIF to instantaneous photosynthetically active radiation (PAR). Large-scale seasonal swings of GOME-2 SIF amount up to 21% (regarding the annual maximum) and peak in October and around February, while OCO-2 SIF peaks in February. However, both time series agree very well if SIF is normalized by overpass time and wavelength. We further examine anistropic reflectance characteristics with the finding that the hot spot effect significantly impacts observed GOME-2 SIF values. On the contrary, our sensitivity analysis suggests that CSC is highly independent of sun-sensor geometry as well as atmospheric effects. The slight annual variability of CSC (3%) shows no clear seasonal cycle, while a relatively high spatial standard deviation points to a high degree of spatial heterogeneity in our study domain within the central Amazon Basin. •Very good agreement between normalized SIF from OCO-2 and GOME-2.•SIF shows a similar but less pronounced hot spot effect compared to NIR reflectance.•Canopy scattering coefficient (CSC) seems to reverse the usual saturation effect.•CSC is highly independent of directional and atmospheric effects.