What was done
Noting that the "detrimental effects of ozone on plants are well known," and that "carbon dioxide [CO2] generally affects trees in opposite ways to ozone [O3]," the authors conducted a literature review that they describe as "the first quantitative analysis of the interactive effects of elevated O3 and elevated CO2 on tree chemistry and herbivore performance," based on the results of "63 studies conducted on 22 tree species and 10 insect herbivore species and published between 1990 and 2005."

What was learned
With respect to ways by which elevated O3 may benefit insect herbivores that tend to damage trees, Valkama et al. determined that "elevated O3 significantly shortened development time of insect herbivores [when they are exposed and vulnerable to attack by various enemies] and increased their pupal mass in the overall dataset." In addition, they found that the "relative growth rate of chewers was significantly increased by 3.5% under elevated O3." However, they discovered that "these effects were counteracted by elevated CO2," such that "elevated O3 in combination with CO2 had no effect on herbivore performance," with the exception that when elevated CO2 was added to the O3-enriched air, it not only counteracted the O3-induced increase in pupal biomass, it actually reduced it below what it was in ambient air by 7%.

What it means
This analysis of the vast majority of pertinent experimental data obtained prior to 2006 suggests that in the never-ending battle between insect herbivores and the trees on whose foliage they feast, the ongoing rise in the air's CO2 content likely plays an extremely important role in negating, and in some cases even more than negating, the damage otherwise capable of being done to earth's forests by voracious insect pests.