In 2007, an extreme drought and acute heat wave impacted ecosystems across the southeastern U.S., including a 19-year-old sweetgum (Liquidambar styraciflua L.) plantation exposed experimentally to long-term elevated or ambient CO2 treatments. Stem sap velocities (i.e., tree water use) were analyzed to assess plant response to potential interactions between CO2 and these weather extremes. Canopy conductance and net carbon assimilation were modeled based on patterns of sap velocity to estimate the indirect impacts of observed reductions in transpiration under elevated CO2on 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 temperatures. Elevated CO2 reduced the capacity of leaves to exchange gases with the atmosphere (i.e., stomatal conductance) and reduced photosynthetic carbon gain. Premature leaf loss also increased rapidly during this period, and was 30% greater under elevated CO2 conditions. While elevated CO2 can reduce leaf-level water use under drought conditions, acute drought may induce excessive stomatal closure that could offset the overall productive capacity of temperate forest species during extreme weather events.
References: Warren J. M., R. J. Norby, and S. D. Wullschleger. 2011. “Elevated CO2 Enhances Leaf Senescence During Extreme Drought in a Temperate Forest,” Tree Physiology 31, 117–130. DOI: 10.1093/treephys/tpr002.
Contact: Mike Kuperberg, SC-23.1, (301) 903-3281, Daniel Stover, SC-23.1, (301) 903-0289
SC-33.1 Earth and Environmental Sciences Division, BER
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