Resumen X que a los niveles ambientales del contaminante. Por ello, se recomienda que los análisis de riesgo de O3 para pastos anuales se basen siempre en niveles críticos basados en flujos (POD). El uso de modelos de crecimiento del pasto, ligados a la aproximación multi-específica del DO3SE para el cálculo de la absorción de O3 a escala de dosel, son mas precisos para refinar los análisis de riesgo que los basados en índices de concentración como el AOT40. biomass species composition and gas exchange response parameters were measured, as well as forage quality parameters that are usually less studied. Based on the quantification of the effects, and the modelling exercises for calculating O3 uptake inside the plants through a new multi-species approach of the Deposition of Ozone and Stomatal Exchange (DO3SE) model, new O3 exposure-and doseresponse relationships were derived and threshold limits (critical levels) for this pollutant were proposed for the protection of annual communities. The canopy approach is a novel consideration which brings forward the current individual species approach. Moreover, three years of field data on growth, phenology and species composition from a natural Dehesa pasture were collected and analyzed. ABSTRACTFinally, based on all the information available, a 3-years O3-risk assessment study was performed for a natural Dehesa pasture. The pasture yield at canopy level showed similar response patterns to the treatments for the two growing seasons analysed: O3 caused a yield loss that was partially mitigated by N-fertilization; simultaneously, O3 reduced the fertilization effect of N. Accordingly, O3 decreased the community Gross Primary Production Abstract XV due to both a global reduction of ecosystem CO2 exchange and an increase of ecosystem respiration. The results at the species level showed heterogeneous responses to both factors, O3 and N. Legumes did not react to N but were very sensitive to O3:Trifolium species responded negatively, while Ornithopus responded positively, taking advantage of the greater sensitivity of clovers to O3. Thus, the legumelegume competition plays an important role in the system under polluted environments. The grasses and the herb were more O3-tolerant than the Trifolium but more responsive to N, especially the grasses. The significant interactions between factors followed the general pattern described at canopy scale, a loss of effectiveness of N in O3-polluted atmospheres and an ability of the fertilizer to counterbalance the damage induced by O3; but both effects were dependent on the species, and on the O3 and N levels. The direction of the interaction can reverse if N is too high at the highest O3 levels, as it was found for T. striatum roots. In this situation, the fertilizer increased the O3-induced root reduction. Both factors might affect the competitive relationships among species through their heterogeneous responses to O3 and N, affecting the structure and composition of Mediterranean annual pastures.Ozone and N-inputs also affected the car...