Critical levels as applied to ozone for North American forests

The United States and Canada have used concentration-based parameters for air quality standards for ozone effects on forests in North America. The European critical levels method for air quality standards uses an exposure-based parameter, a cumulative ozone concentration index with a threshold cutoff value. The critical levels method has not been used in North America, but efforts are now being initiated in the United States and Canada to use the critical loads concept for nitrogen and sulfur deposition to forest and aquatic ecosystems. It is expected that if this effort is successful, the critical levels approach might also be used for ozone effects on forests in North America. Although the European exposure-based or the newer flux-based critical levels standard may seem to be an improvement over the concentration-based United States and Canadian standards in relating plant response to ozone, the most appropriate ozone critical level index for plant response includes a flux-based parameter that incorporates plant defensive mechanisms. Forest plots and monitoring protocols are operational in Europe to obtain data used in determining critical levels for ozone. Similar plots should be established in the United States and Canada to begin collection of field data needed for determining critical levels for ozone in North America. The plots should be established in coordination with current Forest Health Monitoring (FHM) plots in the United States and long-term forest health plot networks in Canada. Current North American ozone monitoring networks should be expanded to obtain initial ozone data for these forest health monitoring plots.