A national ozone biomonitoring program--results from field surveys of ozone sensitive plants in northeastern forests (1994-2000).

Ozone biomonitoring is a detection and monitoring technique that involves documenting ozone-induced visible injury to known ozone-sensitive species under conditions of ambient exposure. The USDA Forest Service administers a long-term, nationwide ozone biomonitoring program to address public and scientific concerns about ozone impacts on forest health. A systematic grid is used as the basis for biomonitoring site locations. At each site, trained field crews evaluate a maximum of thirty plants of up to six species and record the amount and severity of leaf-injury on individual plants. Injury from ozone was found more often on biomonitoring sites in the eastern Unites States than in the interior or west-coast areas. Further results from the northeast reveal that in any year, there is a higher percentage of ozone-injured plants with more severe symptoms in areas with relatively high ozone concentrations than in areas with relatively low ozone. In very dry years (e.g., 1999) the percentage of injured plants and injury severity estimates are both sharply reduced even though ambient ozone exposures are high. These findings demonstrate that biomonitoring data provide meaningful evidence of when high ozone concentrations during the growing season have biological significance. Any assessment of ozone stress in the forest environment must include both biomonitoring (i.e., plant response) and air quality data to be complete.