Abstract, Ecological Society of America Annual Meeting, Spokane, WA, July, 1999.

WULLSCHLEGER*, S.D. and R.J. NORBY. Stomatal conductance and sap velocity for a closed-canopy sweetgum stand exposed to free-air CO₂ enrichment.

Questions concerning the hydrologic response of forests to rising CO₂ continue to be asked by the global change community. Leaf-level investigations provide useful insights in this regard, although studies that integrate leaf and whole-tree effects of elevated CO₂ on canopy transpiration are needed. Leaf and whole-tree measurements were, therefore, initiated in 1998 to quantify the response of stomatal conductance and sap velocity to elevated CO₂ (560 ppm) in a 10-year-old stand of sweetgum (Liquidambar styraciflua L.) exposed to free-air CO₂ enrichment. Sap velocity was measured by the heat pulse velocity method and will eventually be used along with sapwood area to estimate canopy transpiration. Stomatal conductance was 33 to 50% lower at elevated CO₂ concentrations. Mid-day rates of sap velocity also were reduced at elevated CO₂ and this response was easily observed by turning the CO₂ off and on. Sap velocity was 12 to 21% lower in trees exposed to elevated CO₂ concentrations. Increased leaf area at elevated CO₂ would lessen the impact of these observations at the canopy scale, although leaf area index was not different between the ambient and elevated CO₂ plots after the first year of this study. Continued measurements are needed to evaluate whether these leaf and canopy effects will persist as the stand adjusts to elevated CO₂ and to assess their significance to site water balance.