Biophysical properties affecting vegetative canopy reflectance and absorbed photosynthetically active radiation at the FIFE site

Leaves of the dominant grass species of the First International Satellite Land Surface Climatology Project (ISLSCP) Field Experiment (FIFE) site reflect and transmit radiation in a similar manner to other healthy green leaves. Visible reflectance factors (RFs) and transmittance factors (TFs) were lower for older leaves than younger leaves except during senescence, when RF and TF values were higher. Near‐infrared (NIR) RF values increased and TF values decreased with leaf age, with the reverse occurring as the leaf underwent senescence. Leaf optical properties were not found to be dependent on leaf water potential in the range from −0.5 to −3.0 MPa. Canopy bidirectional reflectance factor (BRF) values generally increased with increasing view zenith angle (θ υ ). Maximum values were in the backscatter direction, whereas BRF values in the visible region were lowest at oblique off‐nadir θ υ in the forward scatter direction and at or near nadir in the NIR region. Solar principal plane BRF values varied most at large solar zenith angles (θ s ). Visible and mid‐infrared canopy BRF values decreased and NIR BRF values increased with leaf area index (LAI). Soil BRF distributions in the solar principal plane varied slightly with θ s and θ υ and varied considerably for wet and dry surfaces. Spectral vegetation indices (SVIs) varied with θ s and θ υ ; values were lowest in the backscatter direction and highest in the forward scatter direction. The fraction of absorbed photosynthetically active radiation (APAR) increased with increasing θ s . APAR had a strong linear relationship to nadir‐derived SVI values but not to oblique off‐nadir‐derived SVI values. The relatively small dependence of off‐nadir SVI values on θ s should allow daily APAR values to be estimated from measurements made at any time of the day.