Modeling BRF and Radiation Regime of Boreal and Tropical Forests: I. BRF

Monitoring of forest evolution and functioning with remote sensing depends on canopy BRF (bidirectional reflectance factor) sensitivity to biophysical parameters and to canopy PAR (photosynthetically active radiation) regime. Here, we study the canopy BRF of a tropical (Sumatra) and three boreal (Canada) forest sites, with the DART (discrete anisotropic radiative transfer) model. The behavior of PAR regime of these forests is analyzed in a companion article. We assessed the BRF sensitivity to some major experimental parameters (scale of analysis, viewing and illumination directions, sky radiation) and compared it with BRF sensitivity to commonly studied biophysical quantities: Leaf area index (LAI) and leaf optical properties. Simulations showed that BRF directional anisotropy is very large for all forests. For example, maximum relative reflectance difference with view zenith angle less than 25° is around 0.5 in the visible, 0.4 in the short wave infrared, and 0.25 in the near-infrared for tropical forest. We showed that this BRF variability associated with experimental conditions can hamper the remote detection of forest LAI and tree cover change such as deforestation of tropical forest. DART BRFs of the boreal sites were favorably compared with ground (PARABOLA) and airborne (POLDER) measured BRFs. This work stressed 1) the potential of the DART model, 2) the importance of accurate field data for validation approaches, and 3) the very strong influence of canopy architecture on forest BRF; for example, depending on forest sites, a LAI increase may imply that nadir near-infrared reflectance increases or decreases.