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.