Ozone is one of the most important pollutants in large areas of Europe. There is evidence that the ambient ozone concentrations found in Europe can cause a range of effects to vegetation, including visible leaf injury, growth and yield reductions, and altered sensitivity to biotic and additional abiotic stresses. Since ozone pollution leaves no elemental residue that can be detected by analytical techniques, visible injury on needles and leaves is the only easily detectable evidence in the field. Even though visible injury does not include all the possible forms of injury to trees and natural vegetation (i.e. pre-visible physiological changes, reduction in growth, etc.), observation of typical symptoms on above ground plant parts in the field has turned out to be a valuable tool for the assessment of the impact of the ambient ozone in sensitive species.

In the framework of the ICP-Forest, the Working Group on Air Quality has prepared a submanual for the assessment of ozone injury on European Forest Ecosystems (Intensive Monitoring Plots, Level II). The main objectives are the identification of ozone sensitive species from European forest ecosystems, the assessment of the injury, and to provide an extensive and harmonised photo-documentation of ozone symptoms for these species, and to give information on the spatial distribution of the symptoms.