Monitoring the effects of air pollution on forest condition in Europe: is crown defoliation an adequate indicator?

Forest condition in Europe is monitored under the International Co-operative Program on Assessment and Monitoring of Air Pollution Effects on Forests (ICP Forests). The focus of the program is to provide an overview of the status of forest ecosystems in Europe with respect to air pollution and contribute to the understanding of the relationship between pollution and forest health. Crown condition parameters, i.e., defoliation and discoloration, are used as the principal indicators of forest condition within the ICP Forest network. Crown condition assessments have been successful in disproving the "forest die-back" hypothesis. However, there are a number of limitations associated with this approach given the current monitoring objectives. Data from crown condition assessments indicate that stand age and country differences explain most of the variation in crown defoliation. It is difficult to clearly discern the effects of air pollution from other site and stress factors. The results do however indicate the role of meteorology and other factors. A better understanding of the relationships between forest condition and air pollution may be achieved if more specific, quantitative response variables are selected. However, we suggest that the original focus of the program on air pollution effects may need to be re-evaluated.