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 functioning processes. Past studies, mostly empirical, already showed the potential 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 (DART)) 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 analysis, viewing and illumination configurations, and leaf biomass. This stressed the great dependence of remote sensing measurements and within stand absorbed PAR upon experimental conditions. We also investigated the impact of forest structure on two hypotheses commonly used for simulating CO2 assimilation and for relating this quantity to remote sensing: 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 PAR environment and consequently photosynthetic responses of tropical rain forest.