In 2007, the southeastern US experienced record-breaking drought conditions that induced mortality of some shallowly-rooted plants and induced premature senescence of foliage in trees with deeper water access, including trees in a Liquidambar styraciflua (sweetgum) plantation in eastern
Total sap flux in elevated CO2 plots was 72% of sap flux in ambient CO2 plots during early and mid-summer, a greater difference than earlier measurements in 2004 (82%) or 1999 (88%). However, as the summer drought intensified, the effect of elevated CO2 increased, such that by late August CO2-enriched trees used only 55% that of ambient CO2 trees. Concurrently, drought-induced leaf senescence was 30% higher for elevated CO2 trees than for ambient CO2 trees (15% or 10% of total leaf area, respectively). Higher senescence of elevated CO2 foliage may be attributable to an increase in foliar heat stress caused by an interaction between increased leaf mass per area, and reduced latent heat loss through stomata in elevated CO2 foliage. In fact, modeled foliar T was diurnally up to 0.5 °C higher in elevated CO2 trees than in ambient CO2 trees, exceeding 43 °C for both treatments. While neither height nor diameter growth increment were affected by treatments during the drought, branch xylem vessel diameter was larger in elevated CO2 trees, perhaps due to reduced water limitations on cell expansion. This research suggests that plant response to seasonal dynamics in water availability may be modulated under a projected future CO2 scenario.