Comparison of Bi-directional Reflectance Distribution Functions of Black Spruce Forest in Snow and No-snow Seasons in Alaska

The Bidirectional Reflectance Distribution Function (BRDF) of the forest is an important clue for remote sensing to reveal the forest structure such as Leaf Area Index (LAI) and above-ground biomass. The BRDF is required for the robust development of forest radiative transfer model, which is applied to the forest structure analysis based on satellite data. To acquire in-situ BRDF of the forest, we carried out the field survey of BRDFs at a boreal forest in no-snow season (July 2010) and snow season (March 2011) in Alaska, and compared them. A black spruce forest, a typical boreal evergreen forest in Alaska, located in the Poker Flat Research Range of University of Alaska Fairbanks (65 07'24"N, 147 29'15"W, 210 m MSL) was targeted. Since the forest homogeneously extends about 500 m wide and the terrain is relatively even, this forest site is highly suitable for the validation of the remote sensing measurement. The tree stand density was about 4000 tree/ha, and the highest tree was 6.4 m. The forest floor is covered by the green vegetation such as moss and grass in summer, while the vegetation on the floor is completely covered by snow during winter and early spring. The observations of the BRDF taken place around the noon of July 7 and 8, 2010 (no-snow season) and March 16 and 17, 2011 (snow season) from the top of the tower (17 m) constructed in the forest. We measured the reflected irradiance from the forest by the spectroradiometer (MS-720; EKO Instruments) changing the viewing angle from 20 to 70 degrees and -20 to -70 degrees(off-nadir angle; positive and negative angles mean forward and back scatter angles, respectively) with 5 degrees interval in the principal plane. Irradiances in the orthogonal (cross) plane were also measured in the same manner. The global radiation was simultaneously measured by the other spectroradiometer for the calculation of the reflectance. The BRDF in the principal plane in the no-snow season showed a kind of bowl-shape distribution with its minimum and maximum at approximately 30 and -70 degrees in visible and near-infrared bands, respectively, that is, the forward scatter was generally smaller than the back scatter. By contrast, in the snow season, the back scatter was generally smaller than the forward scatter, that is, the reverse of that in the no-snow season. These results will be used for the development of the forest radiative transfer model aimed to evaluate the forest biodiversity and ecosystem functions.