What was done
From 1998 through 2001, the authors grew the most widely distributed North American tree species - trembling aspen (Populus tremuloides Michx.) - in twelve 30-m-diameter FACE rings near Rhinelander, Wisconsin, USA in air maintained at ambient CO2 and O3 concentrations, ambient O3 and elevated CO2 (560 ppm during daylight hours), ambient CO2 and elevated O3 (46.4-55.5 ppm during daylight hours), and elevated CO2 and O3 over the period of each growing season.  Throughout the experiment they assessed a number of the young trees' growth characteristics, as well as the responses of one plant pathogen and two insects with different feeding strategies that typically attack the trees.

Of the plant pathogen studied, the authors say that "the poplar leaf rust, Melampsora medusae, is common on aspen and belongs to the most widely occurring group of foliage diseases."  As for the two insects, they report that "the forest tent caterpillar, Malacosoma disstria, is a common leaf-chewing lepidopteran in North American hardwood forests" and that "the sap-feeding aphid, Chaitophorus stevensis, infests aspen throughout its range."  Hence, the rust and the two insect pests the scientists studied are widespread and have significant deleterious impacts on trembling aspen and other tree species.  As but one example of this fact, the authors note that, "historically, the forest tent caterpillar has defoliated more deciduous forest than any other insect in North America" and that "outbreaks can reduce timber yield up to 90% in one year, and increase tree vulnerability to disease and environmental stress."

What was learned
In the following enumerations of results, whenever elevated CO2 and O3, by themselves, had qualitatively different responses, we indicate which response is good, in terms of maintaining good tree growth and health, and which is bad.

By itself, elevated CO2: (1) increased tree height and trunk diameter (good), (2) stimulated the production of leaf cuticular wax, fatty acids, hydrocarbons and phenolic glycosides (good with respect to phenolic glycosides, since these substances, in the words of the authors, are "of singular importance as protective agents against pests"), (3) did not alter rust occurrence (good), (4) reduced female pupal mass of the forest tent caterpillar (good), (5) did not affect aphid abundance, but (6) increased the densities of natural enemies of aphids (good).

By itself, elevated O3: (1) decreased tree height and trunk diameter (bad), (2) stimulated the production of leaf cuticular wax, fatty acids and hydrocarbons, but decreased phenolic glycoside production (bad), (3) increased rust occurrence by nearly fourfold (bad), (4) improved tent caterpillar performance, increasing female pupal mass by 31% (bad), (5) did not affect aphid abundance, but (6) had a strong negative effect on natural enemies of aphids (bad).

When applied together:

1.  Elevated CO2 completely ameliorated the negative effects of elevated O3 on tree height and trunk diameter development.
2.  Elevated CO2 reduced the enhancement of rust development caused by elevated O3 from nearly fourfold to just over twofold.
3.  Elevated CO2 completely ameliorated the enhancement of female tent caterpillar pupal mass caused by elevated O3.
4.  Elevated CO2 completely ameliorated the reduction in the abundance of natural enemies of aphids caused by elevated O3.

What it means
Clearly, elevated O3 has a number of significant negative impacts on the well-being of North America's most widely distributed tree species, while elevated CO2 has a number of significant positive impacts.  In addition, elevated CO2 often completely eliminates the negative impacts of elevated O3.  Hence, if the tropospheric O3 concentration continues to rise in the future - and it is indeed expected to do so (the authors note that "damaging O3 concentrations currently occur over 29% of the world's temperate and subpolar forests but are predicted to affect fully 60% by 2100") - we had better hope that the air's CO2 content continues to rise as well.  Without it, we will be in a world of hurt; for the authors report that "since the mid-1990s, aspen use has increased almost exponentially in the US Great Lakes Region and in the boreal mixed wood region of Canada, where the resource is now almost fully utilized."