What was done
The authors grew northern red oak seedlings in controlled environment chambers for 8 to 11 weeks at atmospheric CO2 concentrations of 400, 530, and 700 ppm.  In addition, seedlings were repeatedly subjected to well-watered or water-stressed conditions to study the interactive effects of elevated CO2 and water stress on photosynthesis and growth.

What was learned
Elevated CO2 generally increased photosynthetic rates and water-use efficiency in all seedlings, regardless of soil moisture conditions.  In going from 400 to 700 ppm, for example, elevated CO2 boosted rates of net photosynthesis by 34 and 69% for well-watered and water-stressed seedlings, respectively.  Furthermore, elevated CO2 caused reductions in stomatal conductance for all treatment conditions, thus contributing to water-use efficiencies that were more than 100% greater than those observed in plants grown at 400 ppm CO2.  Ultimately, elevated CO2 significantly stimulated total biomass accumulation; and water-stressed seedlings grown at 700 ppm attained just as much biomass as well-watered seedlings grown at 400 ppm CO2.

What it means
As the CO2 content of the air rises in the future, it is likely that red oak seedlings will exhibit increased rates of photosynthesis and decreased values of stomatal conductance.  In tandem, these CO2-induced physiological modifications should offset the negative effects of water stress on the growth and development of this species, thus allowing them to persist during times of moderate to severe soil moisture deficits.