Reduced stomatal conductance in sweetgum (Liquidambar styraciflua) sustained over long-term CO2 enrichment

Over 4 yr (1998-2001) we examined the effects of CO2 enrichment on stomatal conductance (g(s)) of sun and shade leaves of overstory sweetgum (Liquidambar styraciflua) grown at the Duke Forest Free Air Carbon CO Enrichment (FACE) experiment. Gas-exchange measurements were taken in June and September of each year and relationships between water stress and stomatal conductance were examined. Stomatal density was measured in June 2000. Relative stomatal limitation (I-g) was calculated from gas-exchange measurements. We found a 28% reduction of gs in elevated CO2 that was sustained over the study period and was similar in sun and shade leaves. Elevated CO2 reduced I-g by 26%. Stomatal density was not affected by CO2 enrichment. Elevated CO2 did not change the sensitivity of gs to soil moisture or vapor pressure deficit. The data illustrate that decreased gs of sweetgum leaves in CO2 enrichment is consistent over long periods of time and under varying environmental conditions.