Ecological effects of tropospheric ozone: A US perspective - Past, present and future

Understanding the effects of tropospheric ozone on vegetation, as called for in the U.S. Clean Air Act, has involved collection of experimental data at the species level and, in particular, at the level of the individual and populations. Frequently the studies have been regression designs involving single species and single pollutants resulting in quantitative exposure-response functions that characterize the effects on biomass or reproduction (crop yield). Recently, the 1996 EPA Oxidant Criteria Document reviewed the published research on oxidant effects on crops, forests, and ecological resources, and concluded that the current secondary National Ambient Air Quality Standard (NAAQS) for ozone is neither protective or appropriate. The subsequent discussion and decision on what is an appropriate form and level of the secondary NAAQS utilized results from single pollutant and single species studies. To estimate concentrations causing effects composite response-functions for crops and trees from a large number of species were developed, predicting crop yield loss or annual biomass loss in seedlings. This approach is useful based on available quantitative data on biological effects; however it assumes that individual plant response does Plot change in the presence of other stresses or in natural systems which are considerably more complex (e.g. species' assemblages and competition for resources). For example, the importance of biological complexity is illustrated when we examine the below-ground ecosystem, an often overlooked portion of the ecosystem. We have found that incorporating natural biological complexity into potting soils can result in carbon fluxes opposite those predicted from individual plant studies using artificial media lacking natural soil foodwebs. A future research approach to understanding O-3 effects on ecosystems is required that will develop the necessary linkages to extrapolate experimental data taken at the individual level, often in artificial conditions, to predict changes on individuals or populations in more complex native environments. We will present experimental and modelling activities from our laboratory that show how we are beginning to address the problems of scale, complexity, and multiple stresses in forested ecosystems exposed to ozone stress. Questions frequently posed in ecological risk assessments.