[Illustrations, footnotes and references available in PDF version]
Karnosky et al. (1999)-1 described how they had grown O3-sensitive and O3-tolerant aspen clones in 30-m diameter plots at the Aspen FACE site near Rhinelander, Wisconsin, USA, where the young trees were maintained at atmospheric CO2 concentrations of either 360 or 560 ppm either with or without exposure to elevated O3 (1.5 times the ambient ozone concentration). And there, after one year of growth at ambient CO2, they determined that the elevated O3 had caused visible injury to leaves of both types of aspen, with the average percent damage in O3-sensitive clones being more than three times as great as that observed in O3-tolerant clones (55% vs. 17%, respectively). In combination with elevated CO2, however, the O3-induced damage to the leaves of these same clones was only 38% and 3%, respectively. And so they learned that elevated CO2 prevented much of the foliar damage that would otherwise have been induced by the high O3 concentrations.