Modeling BRF and radiation regime of boreal and tropical forests: I. BRF

Monitoring od 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 (Ca nada) forest sites, with the DART (discrete anisotropic radiative transfer) model. The behavior of PAR regime of these forests is analysed in a compan ion article. We assessed the BRF sensitivity to some major experimental par ameters (scale of analysis, viewing and illumination directions, sky radiat ion) and compared it with BRF sensitivity to commonly studied biophysical q uantities: Leaf area index (LAI) and leaf optical properties. Simulations s how;ed that BRF directional anisotropy is very large for all forests. For e xample, maximum relative reflectance difference with view zenith angle less than 25 degrees is around 0.5 in the visible, 0.4 in the short wave infrar ed and 0.25 in the near-infrared for tropical forest. We showed that this B RF variability associated with experimental conditions can hamper the remot e detection of forest LAI and tree cover change such as deforestation of tr opical forest. DART BRFs of the boreal sites were favourably 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 arch itecture on forest BRF; for example, depending on forest sites, a LAI incre ase may imply that nadir near-infrared reflectance increases or decreases. (C) Elsevier Science Inc., 1999.