Abstract
Tropospheric ozone (O(3)) is among the most pervasive and harmful air pollutants known to affect ecosystems. In the United States, the Environmental Protection Agency and other agencies are tasked with protecting plants and ecosystems from harmful O(3) exposures. Controlled exposure experiments conducted in field open-top chambers (OTCs) with small tree seedlings have been used to estimate empirical models of tree growth in response to O(3) exposure for more than 16 species. While this experimental method makes it possible to obtain detailed exposure-response data, it remains uncertain whether predictions of empirical models parameterized using those data are sufficiently accurate when applied to trees grown in uncontrolled natural environments for long periods. We used O(3) exposure-response relationships developed from several OTC studies of trembling aspen (Populus tremuloides Michx.) seedlings to predict the growth of the same species in the Aspen FACE "free-air" O(3) exposure experiment in Rhinelander, Wisconsin, over 11 years. We acquired individual tree growth data and hourly O(3) exposure from the ambient and elevated O(3) plots in the Aspen FACE experiment, computed annual exposure using the same metrics of O(3) exposure as were used in the OTC seedling experiments, and generated predictions of growth in the Aspen FACE exposures. A simple empirical model parameterized using the OTC seedling data accurately predicted the percent above-ground biomass loss due to O(3) exposure in the Aspen FACE trees for all 11 years. In the Aspen FACE experiment, the effect of O(3) exposure was established in early years and continued to be observed in later years without worsening. Our study suggests that O(3) exposure-response relationships obtained from OTC seedling studies can be used to make inferences about effects on larger trees. These results imply that researchers can use these relationships with confidence when estimating risks of O(3) pollution across the United States.