The Mediterranean Basin presents an extraordinary biological richness but very little information is available on the threat that air pollution, and in particular reactive nitrogen (N), can pose to biodiversity and ecosystem functioning. This study represents the first approach to assess the risk of N enrichment effects on Spanish ecosystems. The suitability of EMEP and CHIMERE air quality model systems as tools to identify those areas where effects of atmospheric N deposition could be occurring was tested. For this analysis, wet deposition of NO3(-) and NH4(+) estimated with EMEP and CHIMERE model systems were compared with measured data for the period 2005-2008 obtained from different monitoring networks in Spain. Wet N deposition was acceptably predicted by both models, showing better results for oxidized than for reduced nitrogen, particularly when using CHIMERE. Both models estimated higher wet deposition values in northern and northeastern Spain, and decreasing along a NE-SW axis. Total (wet+dry) nitrogen deposition in 2008 reached maxima values of 19.4 and 23.0 kg N ha(-1) year(-1) using EMEP and CHIMERE models respectively. Total N deposition was used to estimate the exceedance of N empirical critical loads in the Natura 2000 network. Grassland habitats proved to be the most threatened group, particularly in the northern alpine area, pointing out that biodiversity conservation in these protected areas could be endangered by N deposition. Other valuable mountain ecosystems can be also threatened, indicating the need to extend atmospheric deposition monitoring networks to higher altitudes in Spain.

Long‐term effects of ozone (O₃) exposure and drought stress were assessed on two subspecies of Quercus ilex: ssp. ilex and ssp. ballota. Two‐year‐old seedlings were continuously exposed for 26 months in open‐top chambers to three O₃ treatments: charcoal filtered air, non‐filtered air and non‐filtered air supplemented with 40 nl·l⁻¹ O₃. Additionally, two irrigation regimes were adopted: half of the plants were well‐watered and the others received half of the water supplied to control plants. Growth, shoot water potential and gas exchange rates were assessed seasonally, and biomass accumulation was determined at the end of the experiment. Drought stress caused higher reductions of gas exchange, growth and biomass accumulation than O₃ exposure in both subspecies. The combination of O₃ and drought stress caused further decreases of accumulated aboveground biomass but no additive effects were observed on gas exchange rates or root biomass. Thus, drought stress did not protect Q. ilex from O₃ effects on biomass when the response of the whole plant was considered. Q. ilex ssp. ballota was more sensitive to O₃ and ssp. ilex was more affected by drought stress. The different O₃ sensitivity was not only related to pollutant uptake but also to the ability of plants for resource acquisition and allocation. Based on biomass dose–response functions, Q. ilex is more resistant to O₃ than other European evergreen tree species, however, O₃ represents an additional stress factor that might be impairing plant ability to withstand current and future climate change.