Rochdi, N, Fernandes, R, Chelle, M (2006). An assessment of needles clumping within shoots when modeling radiative transfer within homogeneous canopies. REMOTE SENSING OF ENVIRONMENT, 102(2-Jan), 116-134.
The impact of needle clumping within shoots on radiative canopy modeling is investigated using discrete homogenous numerically generated vegetation canopies together with a forward ray-tracing model (PARCINOPY). Three numerical shoot designs including one detailed and two simplified (decahedron and flat) shoots were considered. The detailed shoot used measurements of Scots Pine shoots used in previous ray-tracing studies. The simplified shoots dimensions preserved the hemispherically averaged interaction cross-section of the detailed shoot, while the optical properties of their surfaces were assigned using those of the detailed shoot. At shoot scale, our simulations confirm previous studies that shoot albedo is relatively insensitive to the ratio of needle transmittance to reflectance within observed ranges. However, the peak of the BRDF in the hot-spot direction was enhanced with increasing leaf area index (LAI) as needle reflectance increased relative to needle transmittance at fixed needle albedo. The shape of the decahedron shoot scattering phase function more closely resembled that of the detailed shoot simulation than did the flat shoot. At canopy scale, the shoot simplification induces reduction of the leaf area index of 4 STAR where STAR corresponds to the hemispherically averaged silhouette to needle area ratio. Both simplified shoot parameterizations were similar to the detailed shoot parameterization when considering canopy scale Near Infra Red reflectance, transmittance, albedo and BRDF. However, the BRDF for RED wavelengths is better described by a flat shoot parameterization than the decahedron shoot canopy, especially with increasing LAI. Notwithstanding these agreements between parameterizations, the partitioning between single and multiple scattered photons is not conserved with simplified shoot canopies. This divergence tends to be higher for the flat leaf simulation, especially in the single scattering at large view zenith angles. The results described in this work indicate that the simple flat leaf model can be substituted for the detailed shoot model when modeling the total bidirectional reflectance and the canopy hemispherical reflectance and transmittance within homogeneous canopies. Taking into account this parameterization might improve considerably the reflectance and radiative transfer models, which to date do not consider the effect of clumping within the shoot. Further work to upscale these results for complex canopy architecture such as branches and crowns is required. However, this set of simulations could serve as a useful initial set of cases for testing of radiative transfer models over needleleaf canopies. Crown Copyright (c) 2006 Published by Elsevier Inc. All rights reserved.