Abstract
In 2007, an extreme drought and acute heat wave damaged ecosystems across the southeastern US, including a 19-year-old Liquidambar styraciflua L. (sweetgum) tree plantation exposed to long-term elevated CO2 treatments. Stem sap velocities in trees exposed to ambient (A) or elevated (E) CO2 were analyzed to assess potential interactions between CO2 and these weather extremes. Leaf temperature (Tleaf) and net carbon uptake (GPP) were modeled based on patterns of sap velocity to estimate indirect impacts of CO2-reduced transpiration on premature leaf senescence. Elevated CO2 reduced sap flow by 28% during early summer, and by up to 45% late in the drought during record-setting high air temperatures. Canopy transpiration and conductance declined more rapidly in ECO2 plots, resulting in ECO2 Tleaf up to 45 °C, which was 1-2 °C greater than ACO2 Tleaf. Pre-drought GPP was ~7% greater in ECO2 plots, then declined to 30% less than ACO2 GPP as the drought progressed. Leaf abscission peaked during this period, and was 30% greater for ECO2 trees. While ECO2 can reduce leaf-level water use under droughty conditions, acute drought or heat conditions may induce excessive stomatal closure that could offset benefits of ECO2 to temperate forest species during extreme weather events.
