REFLECTANCE ANISOTROPY FOR A SPRUCE HEMLOCK FOREST CANOPY

The bidirectional reflectance distribution characteristics of a spruce -hemlock forest were studied during field campaigns conducted in Augus t-September of two different years using the three-channel PARABOLA ra diometer system and a SE590 spectrometer mounted for sampling above a forest canopy. Canopy bidirectional reflectance data were acquired und er clear, cloudless sky conditions over a large range of solar zenith angles. Pyranometer measurements of the total hemispherical upwelling and downwelling solar radiation above and below the forest canopy were simultaneously acquired. In addition, the PARABOLA instrument was dep loyed at different heights within the canopy, under overcast sky condi tions, to measure the directional downward spectral radiation transmit ted. The highly absorbing forest canopy, with a total plant area index of 3.9, reflected only approximately 2.5% of the red (0.662 mum) hemi spherical irradiance. Reflectances for wavebands in the photosynthetic ally active radiation (0.4-0.7 mum), the near-infrared (NIR, 0.826 mum ), and the shortwave infrared (SWIR, 1.658 mum) were approximately 3%, 26%, and 15%, respectively. The bidirectional spectral reflectance fa ctors in the solar principal plane were highly anisotropic, while thos e in the plane perpendicular to the solar principal plane were much cl oser to Lambertian. The forwardscatter was not much greater than the n adir reflectance, differing from the usual dense vegetative cover case s previously examined, especially for the NIR waveband. The hot spot m aximum reflectance, occurring at the retrosolar view angle, was pronou nced in all three wavebands. This prominent hot spot effect has not be en observed for the NIR in other vegetation canopy types (e.g., grassl ands, agricultural crops, and semiarid and desert scrub). View angle e ffects were more prominent than solar zenith angle effects, except at large solar zenith angles. Hemispherical spectral reflectance factors and albedo increased with increases in solar zenith angle. Substantial variation was observed in the normalized difference vegetation index (NDVI) for different view and solar zenith angles with a nominal value of 0.75.