What was done
Eucalyptus seedlings were grown for seven months in glasshouses receiving atmospheric CO2 concentrations of 380 and 800 ppm, before water was withheld from half the seedlings in each CO2 treatment to study the interactive effects of elevated CO2 and drought on photosynthesis in this species.

What was learned
After eight months of differential CO2 exposure and one month of differential water application, well-watered seedlings grown at 800 ppm CO2 displayed net photosynthetic rates that were 120% greater than those of well-watered seedlings exposed to ambient CO2.  Water stress, however, decreased photosynthetic rates of all seedlings, regardless of atmospheric CO2 concentration.  Nonetheless, water-stressed seedlings grown at elevated CO2 still exhibited net photosynthetic rates that were 12% greater than those of their water-stressed counterparts.

What it means
As the CO2 content of the air rises, it is likely that Eucalyptus seedlings will exhibit increased rates of photosynthesis, even during times of water stress.  Thus, it is likely that CO2-induced increases in photosynthesis will lead to greater biomass production in this woody species, even when soil moisture is low, as during drought conditions.