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 developmental processes of plants, including trees. Characterisation of morphogenetically active radiation (MAR) within a canopy is necessary in order to take into account photoregulation of the architecture in tree simulation models. This study was a first attempt at describing and simulating the spatial distribution of light quality within a walnut tree crown (Juglans regia L.) using 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 agreement with values reported in the literature. The quantitative comparison between measurements and model outputs showed large discrepancies. The latter were attributed to the rough treatment of scattering in the model, the small amount of punctual measurements made in the tree, and the high sensitivity of input parameters such as the diffuse to incident radiation ratio and canopy structure description. Nevertheless the model was mostly able to describe the range of MAR values (phytochrome equilibrium Phi(c), blue transmittance) found in the tree canopy.