Air pollution, forest condition and forest decline in southern Europe. An overview

Over the last decades much of the work on the impact of air pollution on forests in Europe has concentrated on Central and Northern countries. The Southern part of Europe has received far less attention, although air pollutants - especially the photochemical ones - can reach levels and doses likely to have adverse effects on forest vegetation. Although international forest condition surveys present serious problems where data consistency is concerned, they reveal considerable year-by-year species-specific fluctuations rather than a large-scale forest decline. Cases of obvious decline related to environmental factors are well circumscribed: the deterioration of some coastal forests due to the action of polluted seaspray; the deterioration of reforestation projects, especially conifers, mainly due to the poor ecological compatibility between species and site; and the decline of deciduous oaks in Southern Italy and of evergreen oaks in the Iberian peninsula apparently due to the interaction of climate stresses and weakness parasites. However, besides obvious deterioration, changes in environmental factors can provoke situations of more subtle stress. The most sensitive stands are Mediterranean conifer forests and mesophile forests of the Mediterranean-montane plane growing at the edges of the natural ecological distribution area. Evergreen sclerophyllous forests appear less sensitive to variations in climatic parameters, since they can adapt quite well to both drought and the action of UV-B rays. Several experiments were carried out to test the sensitivity of Mediterranean forest species to air pollutants. Most of those experiments used seedlings of different species treated with pollutant concentrations too high to be realistic, so it is difficult to derive adequate information on the response of adult trees in field conditions. Ozone has been proved to cause foliar injury in a variety of native forest species in different Southern Europe countries, while the effects of other pollutants (such as e.g. nitrogen, sulphur and acidic deposition) are less obvious and likely to be very localized. In the case of ozone, visible symptoms were almost completely missed by large scale surveys and - at the same time - non-visible symptoms are suspected to be even more widespread than the visible ones. Owing to this and to the complex relationships existing between species sensitivity, ozone exposure and doses, length of the vegetative periods, influence of climatic and edaphic condition on the tree’s response, the impacted areas are yet to be identified. Therefore, the large scale impact of air pollutants on the forests of Southern Europe remains largely unknown, until more specific investigation is carried out.