Preliminary measurement and simulation of the spatial distribution of the Morphogenetically Active Radiation (MAR) within an isolated tree canopy

Light quality, i.e. the solar radiation spectrum, is involved in developmen tal processes of plants, including trees. Characterisation of morphogenetic ally active radiation (MAR) within a canopy is necessary in order to take i nto account photoregulation of the architecture in tree simulation models. This study was a first attempt at describing and simulating the spatial dis tribution of light quality within a walnut tree crown (Juglans regia L.) us ing both spectral measurements and a radiation transfer model based on the turbid medium analogy. Both measurements and simulation were qualitatively in agreement. They showed large differences in light quality between shaded and sunlit areas. The range of measured and simulated values was in agreem ent with values reported in the literature. The quantitative comparison bet ween measurements and model outputs showed large discrepancies. The latter were attributed to the rough treatment of scattering in the model, the smal l amount of punctual measurements made in the tree, and the high sensitivit y of input parameters such as the diffuse to incident radiation ratio and c anopy structure description. Nevertheless the model was mostly able to desc ribe the range of MAR values (phytochrome equilibrium Phi(c), blue transmit tance) found in the tree canopy.