Original article Measurement and modelling of radiation transmission within a stand of maritime pine

Summary — A semi-empirical model of radiation penetration in a maritime pine canopy was developed so that mean solar (and net) radiation absorption by crowns and understorey could be estimated from above-canopy measurements only. Beam radiation Rb was assumed to penetrate the canopy accord-ing to Beer’s law with an extinction coefficient of 0.32; this figure was found using non-linear regression techniques. For diffuse sky radiation, Beer’s law was integrated over the sky vault assuming a SOC (stan-dard overcast sky) luminance model; the upward and downward scattered radiative fluxes were obtained using the Kubelka-Munk equations and measurements of needle transmittance and reflectance. The penetration of net radiation within the canopy was also modelled. The model predicts the measured albedo of the stand very well. The estimation of solar radiation transmitted by the canopy was also satis-factory with the maximum difference between this and the mean output of mobile sensors at ground level being only 18 W m-2. Due to the poor precision of net radiometers, the net radiation model could not be tested critically. However, as the modelled longwave radiation balance under the canopy is always between-10 and-20 Wm-2, the below-canopy net radiation must be very close to the solar radiation balance. model / solar radiation / net radiation / penetration / maritime pine Résumé— Mesure et modélisation de la transmission du rayonnement à l’intérieur d’une par-celle de pins maritimes (Pinus pinaster Ait). Un modèle semi-empirique de pénétration du rayon-nement dans un couvert de pins maritimes a été établi, dans le but d’estimer l’absorption moyenne du rayonnement solaire et du rayonnement net par les houppiers et le sous-bois à partir des seules mesures faites au-dessus du couvert. Le rayonnement direct est supposé le pénétrer selon la loi de Beer, avec un coefficient d’extinction de 0,32; cette valeur a été obtenue par des techniques de régres