A New Approach for Measuring Photosynthetic Light-use Efficiency from Space Using Multi-angular Satellite Observations

Satellite remote sensing of Gross Primary Production (GPP) will greatly enhance our understanding of the terrestrial carbon cycle, as it allows globally continuous estimates of plant CO 2 uptake at regular intervals from space [1]. Remotely sensed GPP can be defined as the product of the incident photosynthetically active radiation (the radiation between 400 and 700 nm wavelength, PAR), the fraction of PAR absorbed by the green canopy elements (f PAR) and the light-use efficiency İ (gCMJ-1) with which the absorbed PAR is used to produce biomass [2, 3]. Recent decades have seen considerable progress determining PAR and f PAR from satellite observations [4, 5], remote sensing of İ, however, remains challenging. A possible means to determine İ remotely is the photochemical reflectance index (PRI), a narrow waveband index that measures the photosynthetic activity of leaves using a xanthophyll absorption band at 531 nm. A large number of studies have demonstrated a relationship between İ and PRI over a wide range of species plant functional types[6, 7], the same and other work, however, has also shown that PRI is affected by numerous other factors such as the sun-view geometry, leaf angle distribution, leaf area, canopy structure and pigment pool size, making an upscaling to landscape and global levels difficult [8, 9]. A promising approach to resolve these dependencies is to characterize the anisotropy of the surface reflectance via continuous, multi-angular spectral observations. Using a tower-based, automated, multi-angular spectroradiometer instrument (AMSPEC, Fig. 1), we demonstrated in previous work that stand-level PRI reflectance observed in a Douglas-fir forest can be defined as a function of the sun-observer geometry, the sky condition at the time of measurement and the physiological status of the vegetation canopy observed (i.e. İ) [8, 10]. This physiological component of the reflectance signal can be extracted by stratifying spectra into homogeneous subsets of observations with respect to both sky Fig 1: The AMSPEC radiometer system measures canopy reflectance at vertical zenith angles between 32° and 78°, completing a full rotation in 15 min.