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Tropospheric Ozone (O3) pollution leaves no elemental residue that can be detected by analytical techniques. Therefore, visible injury on leaves and needles is the only easily detectable evidence in the field. Until now, experiments have concentrated on explaining the mechanisms leading to the injury observed in the experimental studies, rather than to identify and characterize the symptoms observed in the field. The evidence we have today strongly suggests that ozone occurs at concentrations which cause visible foliar injury to sensitive plants. 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 ambient ozone on sensitive plant species.
In 1997, based on ten years of monitoring forest condition in Europe for estimating direct effects by air pollution on forest trees, the United Nations Economic Commission for Europe (UNECE) with the Convention on Long-range Transboundary Air Pollution (LRTAP) has concluded that ozone is the main parameter to be considered (UNECE, 1996).
Ozone risk assessment
In Europe, the development of air
pollution abatement strategies is founded on the effects-based
approach, which includes the ozone-induced plant injury as one of the
key effects to be minimized (Tuovinen 2009). Within this framework, the risk of ozone
damage to vegetation is related to concentration and dose indices.
Both types of index are presently defined within the risk assessment
methodology adopted within the Convention of Long-range Transboundary
Air Pollution of the United Nations Economic Commission for Europe. However, there is increasing evidence for the superior biological basis of the dose approach.
Besides plant-specific characteristics, factors including soil water content, vapor pressure deficit, wind speed, radiation, and temperature influence stomatal conductance and thus affect stomatal ozone uptake. Hence, a realistic estimate of the actual ozone impact is not feasible when only considering a concentration based approach. However, a flux-based approach for ozone risk assessment is data intensive. Thus, it is the long-term goal to gain additional data to develop a realistic ozone risk assessment based on a flux-based approach. Ozone inudecd visible symptoms may serve as a valuable response parameter to assess a critical flux for plant protection.
In conclusion, it is widely accepted that the general receptor-specific maximum leaf conductance is modified by environmental and phenological parameters: soil factors, such as soil moisture deficit and irrigation, plant development factors such as phenological stages, and factors influencing the instantaneous ozone uptake by plants, including temperature, leaf-to-air vapor pressure deficit, global radiation, wind speed. In order to improve our ability to identify areas potentially at risk for ozone impacts an extension of the existing database and a more detailed understanding of the flux-response relationships is needed.
There is evidence that ambient ozone concentrations as they are
monitored in Europe can cause a variety of effects to vegetation such
as visible foliar injury, growth and yield reductions, and altered
sensitivity to biotic and abiotic stresses. Our surveys (see references) have recorded ozone-like symptoms on
numerous native tree, shrub, and forb species. Open-top chamber studies (filtration studies) and research
using Continuously Stirred Tank Reactors (CSTR’s) (fumigation studies)
have confirmed that ozone is the cause of the visible injury seen on
seedlings of a variety of plant species. Innes and Skelly (1996) found
that seedlings varied markedly in symptom severity and several species
were found to develop ozone symptoms at exposures below the current
short- and long-term European air quality standards. There is a lack of
adequately documented evidence of ozone-induced foliar injury to tree
and shrub species in Europe.
Methods & tools
Many plant species respond to ambient levels of tropospheric ozone pollution with distinct species-specific ozone visible foliar injury. These symptoms can be diagnosed in the field only after adequate training (see Intercalibration Courses). In general, the guidelines of the ICP-Forests Sub-Manual on the Assessment of Ozone Injury are to be applied.
For further information on the applied tools, see the following links: