Assessing the potential impact of ozone on native vegetation: findings from the long-term pan-European monitoring program of ICP Forests

The assessment of air pollution levels and effects on European forest ecosystems is the main goal of the UNECE ICP Forests program. Currently, ground level ozone is of primary interest due to its phytotoxicity and the exceedance of critical levels over large parts of Europe. Within ICP Forests, monitoring of ozone concentrations and effects on vegetation have been carried out since 2000 according to standardized and quality assured methods on a number of intensive monitoring (Level II) sites purposively selected throughout Europe. Ground-level ozone concentrations have been measured from April to September by passive sampling, a method proved to be particularly useful at remote sites. Effects on vegetation have been evaluated once a year through the assessment of ozone-like visible foliar symptoms, considered one of the most specific and easily detectable indicator for ozone impact. Overall, data collected on 170 intensive monitoring Level II sites in 20 countries for 12 years reveal a downward temporal trend for both, ozone concentrations and frequency of ozone-symptomatic species. While a decreasing south-north gradient across Europe is obvious for ozone levels, a clear spatial pattern seems to be lacking for visible symptoms. There is a great potential of the ICP Forests long-term dataset for understanding ecosystem status and trends in response to ozone. Further analyses are being undertaken to better clarify temporal and spatial patterns and to better identify relationships among ozone, other important environmental drivers, and the response of forest ecosystems.