Seasonal variation in the structure and red reflectance of leaves from yellow poplar, Red Oak, and Red Maple

A laboratory goniometer was used to measure the light scattered from leaves as a function of view angle in the principal plane for yellow poplar ( Liriodendron tulipifera L.), red oak ( Quercus rubra L.), and red maple ( Acer rubrum L.). The source was a parallel-polarized helium-neon laser (632.8 nm). Yellow poplar leaves had the highest reflectance of the three species, which may have been due to its shorter palisade cells and more extensive spongy mesophyll. Prior to senescence, there was a significant decrease, but not total extinction, in the reflectance of the beam incident at 60° from nadir on the adaxial (upper) side of the leaves of all three species. This was interpreted as a Brewster angle effect. The reflectance increased as the leaves senesced. The abaxial (lower) side of the leaf did not exhibit the reflectance decrease at the Brewster angle, indicating that multiple reflections were obscuring the effect. The multiple reflections could have been from either the spongy mesophyll or from the abaxial surface (which was rougher than the adaxial). Low-temperature scanning electron microscope (SEM) observations showed differences in the surface wax patterns among the three species but did not indicate a cause of the reflectance changes other than possibly the accumulation and aging of the wax. SEM observations confirmed the greater roughness of the abaxial side of the leaf. This type of information on leaf bidirectional scattering should be useful for linking remote sensing observations with leaf optical properties.