04/18/2011

Elevated CO2 and Extreme Drought Too Much for a Temperate Forest To Handle

Summary

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.