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MOnitoring ozone injury for seTTing new critical LEvelS

The ozone produced by human activity is a harmful greenhouse gas, not only for human health, but also for forests. European standards to protect vegetation from this air pollutant are currently based on O3 concentration in the atmosphere, but ozone is up taken by leaf stomata. There is consensus in science that policy makers should implement new thresholds based on how much ozone, effectively, comes into the leaves. Hence, the LIFE project MOTTLES (LIFE15 ENV/IT/000183) establishes a long-term monitoring strategy in three EU countries (Italy, Romania and France) in order to produce new scientifically-based critical levels for forest protection against O3.


Climate change and air pollution are interlinked and are a challenge for European forest management. Ground-level background Ozone (O3) is a phytotoxic air pollutant formed from photochemical reactions of its precursors such as nitrogen oxides and volatile organic compounds. While O3 is a normal component of the troposphere, its background concentrations in the Northern Hemisphere have doubled since pre-industrial times, with negative effects on human and forest health.

Schematic ozone formation process

Ozone causes cellular damage on plants: inducing reduced stomatal conductance, decreasing CO2 assimilation rates and producing visible leaf injury. These effects often accelerate senescence, diminish leaf area and biomass, and reduce photosynthetic capacity productivity. Hence, O3 pollution has large impacts on plant function, and, consequently on forest ecosystem productivity, and other forest services.

European Directives sets acceptable limits for ozone concentration and exposure to protect human health and vegetation using the AOT40 index, calculated as sum of the hourly exceedances above 40 ppb, for daylight hours (8am-8pm) during the assumed growing season. Over these limits, national and regional regulatory agencies must provide countermeasures.


Rural concentration map of ozone indicator AOT40 for
forests (source: European Environmental Agency)

Even if, available data from air quality monitoring stations and passive samplers suggest that O3 levels regularly exceed the critical level for forests, relationships between O3 exposure and indicators (visible foliar injuries, radial growth and crown defoliation) often fail. The reason lies in the fact that ozone effect on vegetation does not only depend on atmospheric concentrations, but also on the O3 uptake through the stomata (small pores on leaves or needles)


Therefore, critical levels for O3 risk assessment should be define as the accumulated stomatal O3 flux above which may cause negative effects on the vegetation, taking into account O3 concentrations and the effects of multiple climate factors, characteristics of vegetation and local and phenological inputs.


Ozone visible injury on a poplar leaf

Hence, Mottles implements a network of monitoring stations capable to return in real time O3 concentrations together with meteorological parameters, modelling stomatal O3 flux and its responses to Climate Change. Plant response indicators of forest health and vitality are also assessed and related to a range of different thresholds in order to produce new criteria and usable legislative standards for protecting forests against O3.


Ozone uptake through a leaf stomata