Ozone uptake by an evergreen Mediterranean forest (Quercus ilex L.) in Italy - Part II: flux modelling. Upscaling leaf to canopy ozone uptake by a process-based model

Ozone (O-3) induces deleterious effects on plants by its oxidising capacity. Efforts have aimed at defining O-3 critical loads which are based on O-3 penetrating into the plant through the stomata, being an important mechanism of O-3 removal. Recently, papers have proposed the use of process-based models for estimating the O-3 dose at the community level. These models should define the stomatal O-3 flux (F-st) on vegetation, correlating it to O-3-induced injury and hence to improve the data-base available for decision makers. A process-based model has been utilised for the quantification of O-3 flux (F) toward a Holm oak forest and its repartition in F-st and non-stomatal flux (F-nost) during daylight. The model outputs have been compared with eddy covariance measurements. Simulation of O-3 fluxes under the climatic limitations of summer 2003 showed that the mean values of F-st represented only 28.9% of F, which was similar to eddy covariance (31.5%). The use of LAI(effective) to link the leaf F-st to the canopy scale O-3 stomatal flux (F-effective; on ground unit) is shown to be useful not only for the model validation but also for upscaling purposes from leaf to canopy. Forest acts as a LAI = 1, i.e. a "real" big leaf for canopy O-3 uptake. Changes in O-3 fluxes were more closely related to the factors that control O-3 deposition than to the factors controlling O-3 concentration. Simulations showed that Holm oak forest was most effective at taking up O-3 under moderate to high irradiance and high physiological activity, but not in limiting environmental conditions. The model used here does not require a great number of input variables and it is based on simple assumptions with respect to other more recognised models. So, the use of this model can have useful applications for the risk assessment of level II for the forests