Background Climate change and air pollution are interlinked, representing a challenge for the management of European forests, with ozone (O3) being the most phytotoxic air pollutant for trees. European standards for the protection of ecosystems, including forests, have been defined through experiments carried out under controlled conditions. Yet, research has highlighted the need to also conduct ‘open-air’ experiments and epidemiological investigations based on observations with high temporal resolution (e.g. one hour intervals). Another shortcoming concerns the monitoring of O3 concentrations, which at remote forest sites is a particularly challenging task due to a lack of electricity, making the use of passive samplers unavoidable. Unfortunately, such monitoring systems suffer from low temporal resolution (e.g. two weeks to one month), making it impossible to correlate O3 data with epidemiological observations. Most European countries carry out forest monitoring, but the different approaches used by the countries make comparisons at the European level problematic, in particular in the case of O3 injury, where obsolete measurement tools (passive samplers) are still used in the European monitoring network (ICP-Forests), which generate errors when calculating recent stomatal uptake based indices. Therefore, there is a need for a harmonised and up-to-date European forest monitoring scheme for O3. Objectives The LIFE MOTTLES project aims to define scientifically-based thresholds and critical levels for the protection of forests from O3 in the changing climate scenario. To do this, an innovative integrated monitoring station system for the continuous measurement of parameters affecting European forest ecosystems sustainability will be set up and tested across three European countries. This will support the elaboration of recommendations and adaptive management strategies for sustainable forest management and stimulate the development of usable legislative standards for protecting forests against O3. The project will address the 2015 EU policy priorities (2020 Biodiversity Strategy and 2013 Forest Strategy), by anticipating the adverse effects of climate change and taking appropriate measures to prevent associated damage. Furthermore, as there is a demand for tools supporting the assessment of impacts of policies both at EU and national level, LIFE MOTTLES will facilitate the development of science-based strategies for policy-makers and managers to enable the protection of forests against O3 and climate change. This will serve as a decision-support tool for national and EU authorities. Expected results: The establishment of an innovative network of monitoring stations across Europe, for the continuous investigation of parameters affecting forest ecosystems sustainability, in particular the O3-induced diseases on vegetation; The creation of open-access data for complete integration of the new network with the European Forest Data Centre and the Forest Information System of the European Commission; Increased knowledge on air pollution effects on forest resilience to support the elaboration of adaptive management strategies; Efficient and effective integrated policies and measures for practical implementation of a sustainable forest management to maintain a healthy forest environment; Scientifically-based thresholds and critical levels to be considered as new legislative standards for protecting forests from O3 pollution and for a long-term monitoring strategy; Usable and comprehensible guidelines with recommendations, standards and measure methodologies for sustainable forest management; An assessment as to whether some EU regions are more exposed and vulnerable to the effects of O3; and Incorporation of the outputs into policy and management processes for forest protection, in particular through the adoption of an integrated monitoring system, and through support for future EU air quality decision-making by providing, for the first time, large-scale epidemiologically-validated and species-specific critical O3 levels.