Modeling of the radiation regime and photosynthesis of a finite canopy using the DART model. Influence of canopy architecture assumptions and border effects

The scope of this work was to investigate the impact of the border effects and the 3-D architecture of a fallow field on: 1) its bidirectional reflect ance factor (BRF); 2) its PAR (photosynthetically active radiation) regime; and, to a lesser extent, 3) on its carbon assimilation. For this purpose, laboratory BRF measurements were conducted on a sample of a fallow field. M oreover, we modified a 3-D radiative transfer model in order to simulate th e visible and near infrared BRF of finite and heterogeneous media. Several scene representations were used (finite and infinite scenes with/without I- D or 3-D distribution of leaf area index [LAI]). Results showed that border effects and LAI distribution strongly affect the BRF, with variations as l arge as 40% depending on the scene representations and on the spectral doma in. PAR profiles and instantaneous canopy carbon assimilation of an infinit e scene (natural conditions) were also studied with the 3-D model. The resu lts stressed that, in the case of a fallow field, the use of a simple LAI p rofile provides enough information to accurately simulate the effects of th e architecture on the PAR regime and the carbon assimilation of a fallow fi eld.