Silvilaser 2019 - Poster Presentations »
Deciduous Broad Leaf Bidirectional Scattering Distribution Function (BSDF): Measurements, Modeling, and Effects on Leaf Area Index (LAI) for Forest Ecological Assessments
Lidar remote sensing has shown high accuracy/precision for quantification of forest biophysical parameters needed for ecological management. Although the significant effect of Bidirectional Scattering Distribution Functions (BSDF) on remote sensing of vegetation is well known, current forest metrics derived from lidar data seldom take leaf BSDF into account. Despite the importance of BSDF effects, leaf directional scattering measurements are almost nonexistent, particularly for transmission. Previous studies have been limited in spectrum, lacked models to capture all angles beyond measurements, and did not adequately incorporate transmission scattering. Furthermore, many current remote sensing simulations, which are vital to our understanding of lidar data, assume leaves with Lambertian reflectance, opaque leaves, or apply purely specular or Lambertian transmission. The accuracy of these assumptions and the effect on simulation results are currently unknown. This study captured deciduous broadleaf BSDFs from the visible through shortwave infrared spectral regions, accurately modeled the BSDF for extension to any illumination angle, viewing zenith or azimuthal angle, and assessed the effect of leaf BSDF on LAI derived from waveform lidar. The goniometer of the Rochester Institute of Technology-2 (GRIT-T) was used to make leaf bi-conical directional reflectance and transmittance measurements. Leaves from three species of large trees, Norway Maple (Acer platanoides), American Sweetgum (Liquidambar styraciflua), and Northern Red Oak (Quercus rubra) were measured. Data then were fit through nonlinear regression to physical-based microfacet BSDF models, resulting in normalized root mean square errors less than 15% for reflectance and 30% for transmission, averaged across all wavelengths. Leaf physical parameters, including the index of refraction and a relative physical roughness, were extracted from the microfacet models delineating the three species. The effect on LAI (due to leaf BSDF) and dependence on wavelength, lidar footprint, view angle, and leaf angle distribution (LAD) all were explored using the Digital Imaging and Remote Sensing Image Generation (DIRSIG) remote sensing radiative transfer simulation model. The largest effects on LAI were observed at visible wavelengths, small footprints, oblique interrogation angles, and small LADs. These effects were attributed to (i) the BSDF becoming almost purely specular in the visible, (ii) small footprints having fewer leaf angles to integrate over, (iii) oblique angles causing diminished backscatter due to forward scattering, and (iv) larger LADs resulting in diffuse lidar signals. Armed with the knowledge from this study, researchers are able to select appropriate sensor configurations to account for or limit BSDF effects in forest lidar data.