Background Ozone (O3) is a secondary pollutant produced during the atmospheric photo-oxidation of volatile organic compounds (VOCs), which also acts as a powerful greenhouse gas (GHG) in the troposphere (the lowest layer of the atmosphere). As mean temperature and global radiation rise, so does the level of tropospheric ozone. Recent studies have shown an increasing impact of ozone on ecosystems and forests. When deposited on leaves, ozone impacts both superficially and internally; with necrosis appearing hours after exposure. The absorption of ozone molecules also acts on transpiration and stomatal opening, reducing the rate of photosynthetic activity. A good knowledge of ozone concentrations in the air is not sufficient to obtain a reliable picture of the development of foliar damage. It is considered more appropriate to establish cause-effect relationships, based on the amount of ozone going into the foliage. Objectives The main objective of the FO3REST project was to refine the criteria and establish validated thresholds for forest protection against ozone and climate change. By comparing measured and modelled canopy-level stomatal fluxes, FO3REST aimed to provide knowledge for updating the current Deposition of Ozone and Stomatal Exchange (DO3SE) model for calculating ozone flux and for including plant defence in the model for European beech and Aleppo pine. The project aimed to establish new standards, based on the quantity of ozone absorbed by plants as opposed to external exposure. Results The FO3REST project developed new criteria and thresholds for assessing the impacts of ozone on vegetation. For European policy purposes, monitoring of the detrimental impacts of ozone on forests has been done using an ozone exposure index called AOT40 (Accumulated Ozone exposure over a Threshold of 40 Parts Per Billion), which measures ambient concentrations in the atmosphere. However, ozone effects on vegetation do not only depend on atmospheric concentrations, but also on the ozone uptake by the plants through their stomata. A new index, based on the stomatal ozone flux, called Phytotoxic Ozone Dose above a threshold Y of uptake (PODY), has been under discussion as a new standard. However, until the project, this index had only been studied under controlled ‘laboratory’ conditions. To address this, the project evaluated the performance of different ozone risk metrics, namely POD0, POD1 and AOT40 under field conditions, and assessed the large-scale impacts of ozone on Mediterranean forests. The project team conducted experimental studies on exposure to ozone and visible ozone damage in forest plots in south-east France and northern Italy. They demonstrated that POD0 is the most effective of the metric indices to represent plant injury due to ozone pollution, being better correlated with observed ozone-induced injuries in field plots than the other indices. The presentation of sound field-based results, showing that POD0 was more appropriate to assess the impact of ozone than the currently-used AOT40 index, represents considerable methodological progress for quantifying ozone effects on vegetation at the regional scale. Critical levels of POD0, based on observations of ozone-induced injury, were defined for six tree species typical of the Mediterranean area: Pinus cembra (Swiss pine), Pinus halepensis (Aleppo pine), Pinus sylvestris (Scot’s pine), Pinus pinea (stone pine), Pinus pinaster (maritime pine) and Fagus sylvatica (European beech), taking into account a percentage of needle or leaf area affected with ozone-induced injury of 5 to 15%. The project team ranked species in order of sensitivity to ozone, and identified the most useful indicator species. The project team measured and modelled ground-level ozone in the forest plots. This allowed a validation of the DO3SE model, used to calculate ozone stomatal flux from ozone concentrations and meteorological data, with new parameters utilised for several tree species. These results will contribute to better monitoring and assessment of the impacts of ozone on forests, especially in a context of climate change, because the two factors (ozone pollution and climate change) interact and both affect forest health. The project outcomes are especially relevant to Mediterranean forests, which are particularly vulnerable to the impacts of climate change and ozone pollution. The FO3REST project team disseminated their results via a website, workshops and seminars, reports and scientific papers. In particular, they addressed the International scientific community and policymakers, so as to induce changes in the legislation on forest protection against ozone (e.g. EC Directive 2008/50). More appropriate standards (based on stomatal uptake rather than exposure indices) should contribute to more appropriate forest protection strategies and more effective ozone abatement policies, hence to a better preservation of European forests. The project demonstrated a novel integrated approach to derive scientifically-based thresholds and critical levels for protecting forest vegetation against ozone in a changing climate, which combined several fields of expertise: forest pathology, tree physiology, statistics, modelling, forestry and atmospheric sciences. The beneficiaries ensured that their innovative methods were readily transferable to similar geographic area around Europe. By contributing to the protection of European forests against ozone pollution in a climate change context, the project contributes to preserving the social and economic benefits provided by these forests. The identification of particularly sensitive species will be useful for forest managers to adapt their management, for example, while the project also contributed to raising environmental awareness among school children and local authorities. Further information on the project can be found in the project's layman report and After-LIFE Communication Plan (see "Read more" section).