What was done
The authors "investigated how a 40% increase above ambient values in CO2 and O3, alone and in combination, affect tree water use of pure aspen and mixed aspen-birch forests in the free air CO2-O3 enrichment experiment near Rhinelander, Wisconsin (Aspen FACE)," where "measurements of sap flux and canopy leaf area index (L) were made during two growing seasons, when steady-state L had been reached after more than 6 years of exposure to elevated CO2 and O3."

What was learned
Uddling et al. report that the 40% increase in atmospheric CO2 concentration increased tree size and L by 40%, while the 40% increase in O3 concentration decreased tree size and L by 22%, in both the aspen and aspen-birch stands. Hence, it was not surprising to learn that the combined effect of the two trace gas increases was an 18% increase in maximum stand-level sap flux in both types of tree stands. In addition, they observed that "elevated O3 predisposed pure aspen stands to drought-induced sap flux reductions, whereas increased tree water use in response to elevated CO2 did not result in lower soil water content in the upper soil or decreasing sap flux relative to control values during dry periods."

What it means
The negative effects of O3 enrichment on tree growth and leaf development were more than compensated by the positive effects of an equal percentage increase in atmospheric CO2 concentration; and although the net effect on sap flux was positive (so that the trees transferred more water to the atmosphere), when the aspen stands needed moisture most (during times of drought), the water they needed was available to them, possibly because they "were growing in soil with CO2-induced increases in litter build-up and water-holding capacity of the upper soil," whereas these latter two benefits and the extra water they could supply to the trees were lacking when the trees were exposed to elevated ozone.