A MODELING APPROACH OF PAR ENVIRONMENT IN A TROPICAL RAIN-FOREST IN SUMATRA - APPLICATION TO REMOTE-SENSING

Little is known about photosynthetically active radiation (PAR) within tropical rain forests in relation to their architecture and functioni ng processes. Past studies, mostly empirical, already showed the poten tial of remote sensing for investigating these aspects. However, till now modeling approaches were mostly limited by the absence of physical models able to simulate both remote sensing images and the 3-D within stand radiation in heterogeneous covers. This paper builds upon a new radiative transfer model (Discrete Anisotropic Radiative Transfer (DA RT)) that provides this information. The modeling study was applied to a tropical rain forest site in central Sumatra. Intensive field work led to detailed structural data on a 1 ha plot. Reflectance images and within stand radiation were simulated for different scales of analysi s, viewing and illumination configurations, and leaf biomass. This str essed the great dependence of remote sensing measurements and within s tand absorbed PAR upon experimental conditions. We also investigated t he impact of forest structure on two hypotheses commonly used for simu lating CO, assimilation and for relating this quantity to remote sensi ng: within stand PAR exponential profiles, and constant ratio of light absorption among leaves. Finally, this work stressed the potential of the DART model, alone or with remote sensing, to better understand PA R environment and consequently photosynthetic responses of tropical ra in forest. (C) 1998 Elsevier Science B.V. All rights reserved.