Elevated CO₂ enhances leaf senescence during extreme drought in a temperate forest

In 2007, an extreme drought and acute heat wave impacted ecosystems across the southeastern USA, including a 19-year-old Liquidambar styraciflua L. (sweetgum) tree plantation exposed to long-term elevated (E_CO2) or ambient (A_CO2) CO₂ treatments. Stem sap velocities were analyzed to assess plant response to potential interactions between CO ₂ and these weather extremes. Canopy conductance and net carbon assimilation (A_net) were modeled based on patterns of sap velocity to estimate indirect impacts of observed reductions in transpiration under E _CO2 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 temperatures. Modeled canopy conductance declined more rapidly in E_CO2 plots during this period, thereby directly reducing carbon gain at a greater rate than in A_CO2 plots. Indeed, pre-drought canopy A_net was similar across treatment plots, but declined to ∼40% less than A_net in A_CO2 as the drought progressed, likely leading to negative net carbon balance. Consequently, premature leaf senescence and abscission increased rapidly during this period, and was 30% greater for E_CO2. While E_CO2 can reduce leaf-level water use under droughty conditions, acute drought may induce excessive stomatal closure that could offset benefits of E_CO2 to temperate forest species during extreme weather events.