What was done
The authors constructed open-top chambers in a native scrub oak community in Florida, USA, and exposed the enclosed ecosystems to atmospheric CO2 concentrations of 350 and 700 ppm for three growing seasons to determine the effects of elevated CO2 on gas exchange in the dominant species, Quercus myrtifolia.

What was learned
Elevated CO2 enhanced photosynthetic rates by approximately 68% when measured at the CO2 concentrations at which the plants were grown.  However, measuring photosynthetic rates at non-growth CO2 concentrations indicated that some photosynthetic acclimation had indeed been induced.  Nevertheless, stomatal conductances of CO2-enriched plants were about 42% lower than those of ambiently-grown plants; and this phenomenon, together with the increased rates of photosynthesis, contributed to the instantaneous water-use efficiencies of the CO2-enriched trees being 2.6 to 3 times greater than those of control trees exposed to ambient air.

What it means
As the CO2 content of the air rises, it is likely that scrub oak communities dominated by Quercus myrtifolia will exhibit enhanced rates of photosynthesis and concomitant reductions in stomatal conductance.  Thus, this species should display significant increases in its water-use efficiency, allowing it to better cope with intermittent droughts and reductions in soil moisture.