||Concentrations of both CO2 and O3 have increased significantly in the atmosphere during the past century, and both have been projected to increase further in the coming decades. Alone, these two gases often affect trees in diametrically opposite ways, but very little is known about their interactive effects. Consequently, it is difficult to predict how forest trees and forest ecosystems will respond as the concentrations of both CO2 and O3 increase in the future. To address this problem, the FACTS-II Free-Air CO2 and O3 Enrichment (Aspen FACE) Project was established in 1997 near Rhinelander, Wisconsin. The experiment examines the impacts of elevated CO2 and O3, alone and in combination, on the structure, growth and functioning of aggrading aspen (Populus tremuloides Michx.), birch (Betula papyrifera Marsh.) and maple (Acer saccharum Marsh.) ecosystems. Many models used to predict possible positive effects of increasing atmospheric CO2 on tree growth and forest productivity do not include potential negative effects of simultaneous increases in O3. The scientific results being obtained in the DOE Aspen FACE user facility indicate that such predictions may be inaccurate, overstating the growth of future northern hardwood forests. Elevated O3 (1.5 times ambient) has offset the beneficial effect of elevated CO2 (200 ppm greater than ambient) on the growth of aspen in the aspen-only portions of the research plots and partially offset the beneficial effect of elevated CO2 on tree growth in the aspen-birch and aspen-maple sections. The two gases have also affected competitive interactions among the three species and among aspen clones of differing O3 tolerance. Ozone sensitive individuals are being lost from the model ecosystem in the O3 treatment at a higher rate than in the other treatments, reducing the genetic diversity of the forest ecosystem. The elevated O3 treatment also has delayed bud break, decreased leaf area, caused earlier fall senescence, and exacerbated the influence of pest outbreaks. For example, it was found that elevated O3 caused a 3-5 fold increase in Melampsora leaf rust of aspen and altered aphid and forest tent caterpillar survival and growth. Elevated CO2, in contrast, extended the length of the growing season by delaying autumnal senescence. These are just a few examples of the many important research topics that are being examined at Aspen FACE. Since 1997, over 110 scientists representing 10 countries, 30 universities and 24 non-academic institutions have used the Aspen FACE facility to study the effects of potential future atmospheric conditions on all attributes of the forest ecosystems.