What was done
Several CO2-emitting springs occur in central Italy; and because they likely have been enriching the surrounding air with elevated levels of CO2 for several centuries, they provide unique opportunities for investigating responses of naturally-occurring vegetation to long-term atmospheric CO2 enrichment.  Hence, the authors of this paper studied the long-term effects of elevated CO2 on various leaf surface structures in mature white oak (Quercus ilex) trees growing in the immediate vicinity of some of these springs.

What was learned
Elevated CO2 significantly decreased the stomatal frequency of white oak leaves by a factor of nearly 1.5 over the atmospheric CO2 concentration range 350 to 750 ppm.  From that point on, however, there were no further reductions in this parameter, even for CO2 concentrations as great as 2600 ppm.

The amount of waxes comprising the leaf cuticle was also significantly increased by elevated CO2, rising nearly three-fold between 750 and 2600 ppm.  Between 350 and 750 ppm, however, there was no observable effect of elevated CO2 on cuticular waxes.  In addition, atmospheric CO2 enrichment had no impact on leaf "wetability," which is a measure of a leaf's ability to intercept and retain precipitation.

What it means
As the atmospheric CO2 concentration continues to rise, it is likely that white oak trees will reduce the density of stomata on their leaves, thereby reducing the total pore area by which they loose water to the atmosphere via transpiration.  This structural modification likely will act to enhance plant water-use efficiency, which should allow these trees to better cope with periods of drought.  In addition, it is likely that increasing CO2 concentrations will lead to greater wax deposition on leaf surfaces, which also should reduce the amount of water these trees lose to the atmosphere, thereby helping the trees to maintain a better internal water status for longer periods of time.