Reflectance anisotropy for a spruce-hemlock forest canopy

Abstract

The bidirectional reflectance distribution characteristics of a spruce-hemlock forest were studied during field campaigns conducted in August-September of two different years using the three-channel PARABOLA radiometer system and a SE590 spectrometer mounted for sampling above a forest canopy. Canopy bidirectional reflectance data were acquired under 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 deployed at different heights within the canopy, under overcast sky conditions, to measure the directional downward spectral radiation transmitted. The highly absorbing forest canopy, with a total plant area index of 3.9, reflected only ∼ 2.5% of the red (0.662 μm) hemispherical irradiance. Reflectances for wavebands in the photosynthetically active radiation (0.4–0.7 μm), the near-infrared (NIR, 0.826 μm), and the shortwave infrared (SWIR, 1.658 μm) were approximately 3%, 26%, and 15%, respectively. The bidirectional spectral reflectance factors in the solar principal plane were highly anisotropic, while those in the plane perpendicular to the solar principal plane were much closer to Lambertian. The forwardscatter was not much greater than the nadir reflectance, differing from the usual dense vegetative cover cases previously examined, especially for the NIR waveband. The hot spot maximum reflectance, occurring at the retrosolar view angle, was pronounced in all three wavebands. This prominent hot spot effect has not been observed for the NIR in other vegetation canopy types (e.g., grasslands, agricultural crops, and semiarid and desert scrub). View angle effects 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.