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 Tennessee concurrently exposed to free-air carbon enrichment (FACE) treatments.  During a 25-day period in August, the Oak Ridge National Laboratory’s FACE facility received <5 mm of precipitation, as canopy air temperature reached 38 °C.  Elevated CO₂ reduces stomatal aperture at this site, resulting in lower rates of evapotranspiration, and concurrently increases production of fine roots in deeper, moister soil.  Both of these physiological responses to elevated CO₂ should buffer trees against droughty conditions and thereby reduce potential growth limitations.  However, reduced foliar water loss also reduces latent heat loss, and under an extreme heat event foliar temperature may reach a critical threshold.  To assess if elevated CO₂ buffered tree drought response we analyzed transpiration (sap flux) data from Granier-style sensors installed in stem sapwood at the beginning of the growing season.  We also measured foliar senescence, modeled diurnal patterns of foliar T, and measured branch xylem vessel size, stem diameter and tree height increment to assess potential growth limitations.

Results/Conclusions

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.