What was done
Stomatal density and stomatal conductance were measured on leaves of 17 different species of plants growing in the vicinity of a natural CO2-emitting spring in central Italy that has produced twice-ambient atmospheric CO2 concentrations for at least two centuries.  In addition, similar measurements were made on plants of the same species located further from the spring, where ambient CO2 concentrations have prevailed.  Thus, the authors studied the long-term effects of atmospheric CO2 enrichment on stomatal density and conductance in these species.

What was learned
Elevated CO2 decreased leaf stomatal conductances in all but one species.  The observed reductions ranged from 19 to 73%.  These reductions, however, were not accompanied by decreases in stomatal density, which remained unaffected by long-term atmospheric CO2 enrichment in all but three species.  Thus, elevated CO2 reduced plant water usage primarily by controlling leaf stomatal function, and not by changing leaf anatomical features (i.e., the number of stomata per unit leaf area).

What it means
As the atmospheric CO2 concentration rises, it is likely that most plants will respond by exhibiting reductions in stomatal conductance, which should enhance their ability to avoid water stress and maintain better turgor pressure during periods of low moisture availability.  This enhanced plant water status, in turn, should allow earth's vegetation to maintain greater carbon uptake during periods of water stress and enhance their overall production of biomass.