Climate change, alteration of atmospheric composition, land abandonment in some areas and land use intensification in others, wildfires and biological invasions threaten forests, shrublands and pastures all over the world. However, the impacts of the combinations between global change factors are not well understood despite its pressing importance. Here we posit that reviewing global change factors combination in an exemplary region can highlight the necessary aspects in order to better understand the challenges we face, warning about the consequences, and showing the challenges ahead of us. The forests, shrublands and pastures of the Mediterranean Basin are an ideal scenario for the study of these combinations due to its spatial and temporal heterogeneity, increasing and diverse human population and the historical legacy of land use transformations. The combination of multiple global change factors in the Basin shows different ecological effects. Some interactions alter the effects of a single factor, as drought enhances or decreases the effects of atmospheric components on plant ecophysiology. Several interactions generate new impacts: drought and land use changes, among others, alter water resources and lead to land degradation, vegetation regeneration decline, and expansion of forest diseases. Finally, different factors can occur alone or simultaneously leading to further increases in the risk of fires and biological invasions. The transitional nature of the Basin between temperate and arid climates involves a risk of irreversible ecosystem change towards more arid states. However, combinations between factors lead to unpredictable ecosystem alteration that goes beyond the particular consequences of drought. Complex global change scenarios should be studied in the Mediterranean and other regions of the world, including interregional studies. Here we show the inherent uncertainty of this complexity, which should be included in any management strategy.The present review is an outcome of the research project MONTESConsolider (CSD2008-00040), funded by the Spanish Ministry of Economy and Competitiveness.Peer Reviewe

Mediterranean Basin ecosystems, their unique biodiversity, and the key services they provide are currently at risk due to air pollution and climate change, yet only a limited number of isolated and geographically-restricted studies have addressed this topic, often with contrasting results. Particularities of air pollution in this region include high O3 levels due to high air temperatures and solar radiation, the stability of air masses, and dominance of dry over wet nitrogen deposition. Moreover, the unique abiotic and biotic factors (e.g., climate, vegetation type, relevance of Saharan dust inputs) modulating the response of Mediterranean ecosystems at various spatiotemporal scales make it difficult to understand, and thus predict, the consequences of human activities that cause air pollution in the Mediterranean Basin. Therefore, there is an urgent need to implement coordinated research and experimental platforms along with wider environmental monitoring networks in the region. In particular, a robust deposition monitoring network in conjunction with modelling estimates is crucial, possibly including a set of common biomonitors (ideally cryptogams, an important component of the Mediterranean vegetation), to help refine pollutant deposition maps. Additionally, increased attention must be paid to functional diversity measures in future air pollution and climate change studies to establish the necessary link between biodiversity and the provision of ecosystem services in Mediterranean ecosystems. Through a coordinated effort, the Mediterranean scientific community can fill the above-mentioned gaps and reach a greater understanding of the mechanisms underlying the combined effects of air pollution and climate change in the Mediterranean Basin.

The study reported was conducted to establish the impacts of photochemical oxidants (primarily ambient ozone, O₃) on the yield of watermelon (Citrullus lanatus) at a site on the east coast of Spain. Fruit yield and quality were monitored in plots established in a commercial watermelon field exposed, in open‐top chambers (OTCs), to non‐filtered air (NFA; near‐ambient levels of ozone) or charcoal‐filtered air (decreased levels of photochemical oxidants including O₃; CFA), or to ambient air (AA), during the 1988 and 1989 growing seasons. Ambient levels of O₃ were found to exceed present UN‐ECE (United Nations Economic Commission for Europe; Convention on Long‐Range Transboundary Air Pollution) critical level guidelines for the protection of crop yield by approx. twofold in 1988 and by approx. fivefold in 1989. Plants exposed to NFA and AA developed visible O₃ injury on the upper surface of sun‐exposed older leaves, and fruit yield (annual marketable fruit weight and number) was found to be depressed in OTCs ventilated with NFA in comparison with those receiving CFA. Consistent with inter‐annual variations in O₃ exposure, greater yield losses were experienced in 1989 (39%) than in 1988 (19%), an effect mediated predominantly by a decline in fruit number rather than average fruit weight. Exposure to ambient levels of ozone also slightly decreased fruit quality (4–8% decline in soluble solids content). Leaf gas exchange measurements made in the field in 1988 revealed effects of O₃ on fruit yield and quality to be associated with a decline in the net CO₂ assimilation rate per unit leaf area under light saturation (A_sat) and stomatal conductance to water vapour (g_s), and enhanced rates of dark respiration. A/c_i curves (where A is the net CO₂ assimilation rate per unit leaf area and c_i is the mole fraction of CO₂ in the leaf intercellular air space) constructed for plants grown in laboratory‐based closed chambers, and exposed to an accumulated O₃ exposure similar to that experienced by plants in the field, suggested that the likely cause of the decline in photosynthetic capacity was (1) a decrease in the amount and/or activity of Rubisco and (2) an impaired capacity for regeneration of ribulose 1,5‐bisphosphate, which was not mediated through changes in the photochemical efficiency of photosystem II (F_v/F_m, where F_v is variable chlorophyll a fluorescence and F_m is maximum chlorophyll a fluorescence). No shift in the relative stomatal limitation to photosynthesis was observed under the influence of O₃, suggesting that the decline in g_s induced by the pollutant in both field and laboratory was the result, and not the cause, of the decrease in A_sat. Ozone exposure also caused a decrease in C isotope discrimination (approx. 0.5‰), a shift that revealed a departure from predicted theory based on supporting leaf gas exchange measurements. The study demonstrates that ambient levels of photochemical oxidants on the Spanish Mediterranean coast are high enough to adversely influence the yield and physiology of an economically important crop grown in the region, and the magnitude of the effects was greater than would be predicted from exposure–yield‐loss relationships for grain and fodder crops in central and northern Europe. Implications for the derivation of UN‐ECE ozone standards are discussed, along with the mechanistic basis of the observed yield decreases.

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.