The results of the 1998 European forest health survey show a general deterioration of the crown condition of the main tree species; the trend of the forest condition is comparable with that of the past years. This long-term development must be seen differently for each species of tree and for each individual region. While the vitality of the Pine in parts of Eastern Europe has been recovering the condition of common and sessile Oak in Western Europe has clearly been deteriorating. It gets more and more apparent that photooxidants, with ozone as the main substance, can lead to visible damage in forests. This is particularly true for the Mediterranean. Ozone damage is to be suspected in other regions as well. On average the crown condition (defoliation) in Europe between 1992 and 1998 worsened on approximately 31% of the observation plots and improved on only 15% of the plots.
The response of terrestrial ecosystems to increased UV-B is evident primarily in interactions among species rather than in the performance of individual organisms. Recent studies indicate that increased UV-B affects the balance of competition among higher plants, the degree to which higher plants are consumed by insects and the susceptibility of plants to pathogens (UNEP 1998b). Increased UV-B can be damaging for crop varieties but this may be offset by protective and repair processes.