What was done
The authors studied the effects of naturally occurring elevated CO2 concentrations (500 to 1000 ppm) on summer water relations in mature oak trees growing in a Mediterranean climate in central Italy by measuring rates of leaf stomatal conductance (similar to transpiration) and sap flow, and by also measuring total leaf mass per tree at the end of two consecutive summers of research.  Control oaks growing at ambient CO2 concentration were located at a similar site three kilometers away.  At both locations, trees have grown for approximately 15 to 25 years under natural conditions.

What was learned
As both summers were characterized by severe drought, rates of water loss, as determined by measuring leaf conductance, were relatively high in both sets of trees.  Conductance rates were, however, significantly lower in trees growing near the high CO2 springs.  Measurements of sap velocity also indicated that less water flux occurred from trees near high CO2 springs relative to control trees at the ambient CO2 site.  Overall, these data show that trees near the high CO2 springs experienced less water loss, and maintained, therefore, a more favorable internal water status than trees growing at the ambient CO2 concentration.  In addition, after all their leaves were removed, trees near the high CO2 springs were found to possess less foliage area than that of control trees.  This reduction in foliage area, however, reduced the transpirational surface area of trees growing at elevated CO2 levels, and consequently allowed them to maintain a better water status than control trees during the long drought periods of the Italian summers.

What it means
As the CO2 content of the air continues to rise, Italian oak trees that experience summer droughts should better maintain their water status by reducing leaf conductance and transpirational water loss.  In addition, these oaks may adapt to drought conditions by reducing their foliage area as another means of regulating internal water status, while still allowing photosynthesis to occur.  Just as some plants respond to atmospheric CO2 enrichment by reducing leaf rubisco content, reductions in foliage area can occur without eliminating increased carbon gains resulting from elevated CO2.  And in the words of the authors, this foliage reduction is "equally, if not more, effective than stomatal closure in reducing transpiration and plant water use under elevated CO2."  So in the future, if drought situations occur during the Italian summers, the rising concentration of atmospheric CO2 should provide oaks with a number of mechanisms for sustaining their growth and development during those periods of reduced water availability.