RATP: a model for simulating the spatial distribution of radiation absorption, transpiration and photosynthesis within canopies: application to an isolated tree crown

The model RATP (radiation absorption, transpiration and photosynthesis) is presented. The model was designed to simulate the spatial distribution of r adiation and leaf-gas exchanges within vegetation canopies as a function of canopy structure, canopy microclimate within the canopy and physical and p hysiological leaf properties. The model uses a three-dimensional (3D) repre sentation of the canopy (i.e. an array of 3D cells, each characterized by a leaf area density). Radiation transfer is computed by a turbid medium anal ogy, transpiration by the leaf energy budget approach, and photosynthesis b y the Farquhar model, each applied for sunlit and shaded leaves at the indi vidual 3D cell-scale. The model typically operates at a 20-30 min time step . The RATP model was applied to an isolated, 20-year-old walnut tree grown in the field. The spatial distribution of wind speed, stomatal response to environmental variables, and light acclimation of leaf photosynthetic prope rties were taken into account, Model outputs were compared with data acquir ed in the field. The model was shown to simulate satisfactorily the intracr own distribution of radiation regime, transpiration and photosynthetic rate s, at shoot or branch scales.