Absorbed Photosynthetically Active Radiation of Steppe Vegetation and Sun-View Geometry Effects on Apar Estimates

Abstract

Instantaneous fractions of absorbed photosynthetically active radiation (APAR) were measured at the Streletskaya Steppe Reserve of the Soviet Union in conjunction with canopy bidirectional-reflected radiation measured at solar zenith angles ranging between 37 and 74" during KUREX-91. APAR values were higher for KUREX-91 than those for FIFE-89 and the amount of APAR of a canopy was a function of solar zenith angle, decreasing as solar zenith angle increased at the Reserve. Differences in absorption are attributed to leaf area index (LAI) and leaf angle distribution and subsequently transmitted radiation interactions. LA1 was considerably higher at the Reserve than those at the FIFE site. In addition, leaf angle distributions of the Reserve approach a uniform distribution while distributions at the FIFE site more closely approximate erectophile distributions. Reflected PAR components at KUREX-91 and FIFE-89 were similar in magnitude and in their response to solar zenith angle (near constant value). However, transmitted PAR increased with increasing solar zenith angle at KUREX-91 and decreased with increasing solar zenith angle at FIFE-89. Transmitted PAR at FIFE-89 was considerably larger than those at KUREX-91. Spectral vegetative indices derived from canopy bidirectional reflected radiation are a function of solar and viewing directions indicating that solar and viewing directions must be considered in using remotely-sensed data to derive surface biophysical parameters.