Tropospheric ozone (O3) concentrations are increasing all over the world. The troposphere extends to between 10 and 18 kilometers above the surface of the Earth and consists of many layers. Ozone is more concentrated above the mixing layer, or ground layer. Ground-level ozone is a serious problem because of its environmental effects. Ground level ozone pollution is pronounced in regions with strong photochemical activity, such as the Mediterranean Basin. For a general description of the Mediterranean basin climate and vegetation, see "Mediterranean conifer and mixed forests".
The physical and chemical processes affecting O3 formation vary greatly even within the Mediterranean Basin. In summer, the Western basin is under the influence of weak levels of Azores anti-cyclonic subsidence, low winds, and strong insolation. These conditions favor massive photochemical production of O3, with development of mesoscale processes and recirculation within air masses. During the same period, the Eastern basin is under conditions of weak ascent and strong advection, i.e. the Etesian winds, that largely inhibit the development of recirculation, even if peaks of 150-220 parts per billion (ppb) occur. The boundary between the two major O3 formation areas is located over Italy. Due to its central position in the Mediterranean, Italy may be considered as a hot-spot for O3 and representative of O3 impacts on Mediterranean vegetation.
Southern Europe is affected by dangerous ground level ozone concentrations. In 2006, the frequency of ozone level exceedances was higher than in previous years, though not as high as in the record year 2003. The European Environmental Agency reports that the highest one-hour ozone concentration occurred in Italy. Other high hourly ozone concentrations were reported in Austria, France, Italy, Portugal, Romania and Spain (Figure 1). North-western, central and eastern Europe did not escape either.
In the Mediterranean Basin, the detrimental impact of O3 on forests remains largely under-investigated. Detecting plant, or vegetative effects is necessary to give biological significance to O3 standards. Field evidence of direct effects of O3 on Mediterranean forests are controversial. Significant relationships between O3 exposure and effects (crown transparency, radial growth and foliar symptoms) often fail. Possible causes for this discrepancy are:
- The critical level established to protect Mediterranean forests against ozone is inappropriate. Ozone effects on trees have been mainly inferred from controlled-condition experiments on seedlings rather than multi-factorial analysis of forest conditions in the field. Extrapolating O3 sensitivity from young to mature trees creates substantial overestimations. A number of studies have raised doubts about conclusions drawn from O3 exposures in closed and open-top chambers. In addition, the current approach used by the European Union (EU) to assess and predict risk to vegetation is based on the concept of exposure of vegetation to air concentrations of ozone rather that on uptake of this substance by vegetation. The exposure-based approach is functionally wrong, as effects are caused by the amount of ozone uptaken into the leaf and detoxified inside the leaf (flux). The complexity of this approach, however, may interfere with an extensive use for risk assessment and still needs to be evaluated;
- Response indicators are improper or improperly investigated. Ozone effects on plants are aspecific. Thus, all indicators are ambiguous. Multivariate statistical analysis may help in decoding the role of different predictors. It is somehow surprising that we are searching for O3 effects on tree radial growth under field conditions, when experiments have not yet determined if ambient O3 levels actually impair it; and
- Site and plant characteristics increase O3 tolerance in Mediterranean vegetation. Mediterranean forest vegetation appears to be adapted to face oxidative stress factors, such as elevated O3 concentrations, drought and high radiation, including UV-B. Some reasons to explain why Mediterranean vegetation may tolerate potentially harmful O3 concentrations are: sclerophyllous leaves (little intercellular air space, thick cuticle and cell wall, high stomatal density); low gas exchange rates; emission of volatile organic compounds (VOC); and active antioxidant pool. The prevailing environmental conditions in the Mediterranean Basin (excess light, elevated temperature, reduced precipitation) reduce stomatal conductance (and thus the uptake of ozone) at the time of the highest O3 levels, and promote sclerophylly, VOC emission, and content and activity of antioxidants.
In conclusion, Mediterranean forests are at the highest ozone risk in Europe because of the high ozone concentrations they experience. Even if field injury is not as high as expected on the basis of concentration-based standards, visible ozone-like foliar injury has been observed for several years on a number of tree species in Spain, France and Italy at permanent monitoring sites where high O3 concentrations occur. Visible ozone-like injury to vegetation is often in the form of spotty brown discolorations on leaves (Figure 2). Two Mediterranean pine species – Pinus ponderosa and P. halepensis – are among the most symptomatic conifers under field conditions. This suggests that ozone affects Mediterranean forests, even if the extent of ozone impairment is still to be quantified.