In many forested regions of North America, background O3 levels have been rising despite the fact that hourly maximum concentrations have been decreasing. Unlike Europe, where critical levels based on a response threshold are used to assess risk, Canada and the United States use the best available scientific knowledge balanced by social, economic and political considerations to establish ambient air quality standards for air quality compliance purposes. These ambient air quality standards do not assume the existence of a concentration threshold. The United States (1997) and Canada (2000) established the O3 standard as the 3-year average of the annual fourth highest daily maximum 8-hour O3 concentrations, with target values set at 65 ppb and 80 ppb, respectively. Here, we use 5 years of basal area growth in one control and one O3 Aspen FACE ring to evaluate the performance of five O3 indices including AOT40 and the North American air quality standard. With our data, this standard outperformed AOT40, SUM60, SUM00 and 1-hour maximum as a single predictor of growth response. We also evaluate the potential of a standard-based, simple dose response function developed from Aspen FACE to predict productivity in Populus tremuloides and Betula papyrifera.