A new “Mission: Impossible”: A mechanistic understanding of ozone impact and risk assessment on forest ecosystems?

Understanding the role of ozone pollution on forest ecosystems is crucial to any effort to mitigate climate change by stabilizing atmospheric carbon dioxide concentrations. Ozone risk assessment should be founded upon spatio-temporal resolution of the pollutant distribution and the biological processes incited by the phytomedically relevant “effective ozone dose”. This because the real impact of ozone and plant responsiveness is not based solely on ozone uptake, the final effect being also due to plant sensitivity per unit of ozone intake. These phenomena of defense (stress tolerance), in turn, are linked to the capacity of the antioxidant, detoxification (and repair) mechanisms. All of these processes are multifactorial, plant-related and influenced by environmental factors. At the moment, the current metric descriptor for selecting air quality indices inferring ozone risks in forest trees is still the AOT40 concept, based on an accumulated external ozone exposure above a fixed threshold, rather than an internal uptake rate. The shortcomings of exposure concept may be overtaken by adopting flux-related critical levels, but models of ozone flux require detailed micrometeorological information and are applicable to only a limited number of species. The great variability of ozone requires long measurement series to determine average mixing ratios, seasonal trends, and other features. Another open question is the understanding of how processes operating at the level of individuals translate into population and community dynamics. No doubt that the long-term effects on tree growth and forest composition and functioning are still controversial