What was done
The authors performed several different experiments on young trees, including sessile oak (Quercus petraea), European beech (Fagus sylvatica), and sweet chestnut (Castanea sativa), to determine how their responses to ozone exposure are affected by elevated CO2 and other environmental variables.

What was learned
Elevated CO2 generally reduced the amount of ozone damage to plants by inducing various degrees of stomatal closure, which decreased the incidental uptake of this harmful air pollutant.  It was also learned that the degree of CO2-induced stomatal closure was influenced by the leaf-to-air vapor pressure gradient, with the greatest stomatal closure typically occurring at lower, rather than higher, air vapor pressure deficits.

What it means
As the atmospheric CO2 concentration rises, it is likely that plants will exhibit reductions in stomatal conductance in a species-dependent manner, which should reduce the negative effects of tropospheric ozone on their growth and development.