Multiple view zenith angle observations of reflectance from ponderosa pine stands

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Abstract
Bi-directional reflectance factors (BRF(λ)) from dense and sparse ponderosa pine (Pinus ponderosa) stands, derived from radiance data collected in the solar principal plane by the Advanced Solid-State Array Spectroradiometer (ASAS), were examined as a function of view zenith angle (0rpar;. BRF(λ) was maximized with 0 nearest the solar retrodirection, and minimized near the specular direction throughout the ASAS spectral region. The dense stand had much higher BRF anisotropy (maximum BRF/minimum BRF) in the red region than did the sparse stand (relative differences of 5·3 versus 2·75, respectively), as a function of 0, due to the shadow component in the canopy. Anisotropy in the near-infrared (NIR) was more similar between the two stands (2·5 in the dense stand and 2·25 in the sparse stand); the dense stand exhibited a greater hotspot effect than the sparse stand in this spectral region. Two common vegetation transforms, the NIR/red ratio and the normalized difference vegetation index (NDVI), both showed a θ dependence for the dense stand. Minimum values occurred near the retrodirection and maximum values occurred near the specular direction. Greater relative differences were noted for the NIR/red ratio than for the NDVI. The sparse stand showed no obvious dependence on 0 for either transform, except for slightly elevated values toward the specular direction.