Shoot scattering phase function for Scots pine and its effect on canopy reflectance

Spectral and directional reflectance properties of coniferous forests are known to differ from those of broadleaf forests. Many reasons have been proposed for this, including differences in the optical properties of leaves and shoots, the latter being considered the basic unit in radiative transfer modeling of a coniferous canopy. Unfortunately, very little empirical data is available on the spectrodirectional scattering properties of shoots. Here, we present results of angular measurements (using an ASD FieldSpec 3 spectroradiometer mounted on LAGOS) of ten Scots pine shoots in the spectral range 400--2000 nm. The shoots were found to scatter anisotropically with most of the radiation reflected back into the hemisphere where the radiation source was positioned. To describe the measured directional scattering pattern, we propose a phase function consisting of isotropic and Lambertian scattering components. Next, we used the proposed scattering phase function in a Monte Carlo radiative transfer model. Angular reflectance of a modeled horizontally homogeneous shoot canopy has, due to shoot scattering anisotropy, an enhanced “dark spot” as compared with a canopy composed of isotropic scatterers and a quantitatively similar leaf canopy.