Absorbed photosynthetically active radiation and sun-view geometry effects on remote sensing relationships

Quantifying the amount of photosynthetically active radiation (PAR) absorbed by vegetation is an essential consideration for determining useful vegetative photosynthetic capacity and surface conductance values for regional and global carbon cycle studies. This study was conducted to compare absorbed photosynthetically active radiation at the FIFE-89 Konza prairie sites to that of the KUREX-91 steppe grassland sites and to investigate variations in relationships between absorbed PAR and spectral vegetation indices derived from bidirectional reflectance factors. Incoming, reflected and transmitted PAR were measured from which fractions of reflected, transmitted and absorbed PAR were computed at selected FIFE prairie and KUREX steppe sites. Fractions of direct and diffuse PAR transmitted through canopies were estimated. Fractions of absorbed PAR were much lower at the FIFE sites (ranging from 0.35 to 0.65) than those at KUREX (ranging from 0.75 to 0.95) which can be explained by differences between leaf area index, leaf angle distribution, and direct and diffuse sky conditions. Scattering of PAR may be an important parameter in canopy light penetration particularly in canopies of large LAI, with non-photosynthetically active vegetation components and illuminated at large solar zenith angles. The magnitude of spectral vegetation indices computed and plotted as a function of the fraction absorbed did not differ considerably even though LAI and the fraction of absorbed PAR did. Adjusting for background improved the distinction between spectral vegetation indices at the two experimental sites. Relationships between fraction of absorbed PAR and spectral vegetation indices derived from bidirectional reflectance factors were not consistent over illumination and view angles.