MEASUREMENT AND MODELING OF THE PHOTOSYNTHETICALLY ACTIVE RADIATION TRANSMITTED IN A CANOPY OF MARITIME PINE

Modelling the photosynthesis of a forest requires the evaluation of th e quantity of photosynthetically active radiation (PAR) absorbed by th e crowns and the understorey. In this article a semi-empirical model, based on Beer's law is used to study PAR absorption and its seasonal v ariation. Our purpose was to confirm that the PAR and the solar radiat ion follow the same interception laws for both the direct and diffuse paa, using correct values of needle transmission and reflection coeffi cients. The model developed took into account the direct and the diffu se radiation. The radiation rescattered by the crowns was neglected fo llowing an estimation using the Kubelka-Munk equations, which indicate d that the term was small. The model was calibrated and tested from th e measurements taken in a maritime pine forest during the summer and a utumn of 1995. The comparison between the results of the model and the measurements was satisfactory for the direct radiation as well as for the diffuse radiation. In conclusion, although the measurement waveba nds are different, the penetration of the PAR can be estimated using t he same simple semi-empirical model already established for solar radi ation.