抄録
Reports of polarization of light by plant canopies in relation to their agronomic characteristics are scarce. In this study radiance responses of an eight band field spectroradiometer, expressed as reflectance factors, were measured using rotary polarizer and responses were related to the ratio of the legume green dry biomass to the total (legume and grass) green dry biomass (L/T) for five plots of a mixed seeding of pasture grasses (Lolium italicum, Dactylis glomerata, and others) and legumes (Trifolium repens and T. pratense) at Tochigi, Japan. Wavelength bands of the field spectroradiometer were centered at 491, 560, 662, 833, 1100, 1200, 1648 and 2210 urn. Degree of polarization (P) was calculated from the maximum and minimum of reflectance factors that were acquired by manually rotating the polarizer filter wheel from 0° to 160° at 10° intervals. The field of view at the canopy surface was 30 cm in diameter and observations were made looking toward the Sun from a zenith view angle of 45°. Each of the five plots was measured four times on May 9, 1994. During the measurement intervals the angle of incidence (the average of the view and solar zenith angles) were 32.5-33.0, 34.0-35.5, 36.0-38.5, and 39.5-43.0°. Total green dry biomass for the 5 pasture plots varied from 300 to 464 g/m2 and legume green dry biomass ranged from 12 to 220 g/m2. Observed ratios of L/T in the plots were 0.04, 0.05, 0.35, 0.46, and 0.70. Reflectance factors for the 491, 560, 662, 833, 1100, 1200 and 1648 nm bands, P for the 491, 560, 662, and 833 nm bands, and four vegetation indices-NDVI (two band difference/sum ratio for the 833 and 662 nm band reflectance factors), RVI (ratio of 833 and 662 nm reflectance factors), MSI (ratio of 1648 and 833 nm reflectance factors) and DVI (difference between 1100 and 1200 nm reflectance factors)-were statistically tested to detected differences in L/T among the pasture plots. Only NDVI, RVI, and P at 560 and 662 nm were statistically significant, and they were not significantly affected by angle of incidence. Whereas NDVI and RVI saturated at L/T greater than approximately 0.5, P at 560 and 662 nm increased almost linearly as L/T increased. In the linear regression between P and L/T, the coefficient of determination, r2 was higher for 662 nm (0.73) than for 560 nm (0.60). Among the radiometric variables tested in this study, P at 662 nm was the most promising index to predict L/T.
