Effects of drought and changes in vapour pressure deficit on water relations of Populus deltoides growing in ambient and elevated CO2

The means by which growth CO2 concentration ([CO2]) affects anatomy and water relations responses to drought and vapour pressure deficit (VPD) were studied for yearly coppiced, 4-year-old Populus deltoides clones that were grown in either 400 mu mol mol(-1) (ambient) or 800 mu mol mol(-1) (elevated) CO2 for 3 years. It was hypothesized that, during drought, trees growing in elevated [CO2] would have a lower volume flux density of water (J(V)), stomatal conductance (g(s)) and transpiration per leaf area (E), as well as a lower stomatal density and a greater stomatal response to drought and changes in VPD than would trees in ambient [CO2]. Trees in elevated [CO2] actually had higher J(V) values throughout the study, but did not differ from trees in ambient [CO2] with respect to g(s) or E under saturating light or E scaled from J(V) (E-scaled), all of which indicates that the higher J(V) in elevated [CO2] resulted from those trees having greater leaf area and not from differences in g(s). Furthermore, although plants in elevated [CO2] had greater absolute leaf loss during the drought, the percentage of leaf area lost was similar to that of trees in ambient [CO2]. g(s) and E under saturating light were affected by changes in VPD after the first 9 days of the experiment, which coincided with a large decrease in water potential at a soil depth of 0.1 m. Trees in elevated [CO2] had a greater stomatal density and a lower wood density than trees in ambient [CO2], both traits that may make the trees more susceptible to xylem cavitation in severe drought. Drought and VPD effects for the P. deltoides clone were not ameliorated by long-term growth in elevated [CO2] compared with ambient [CO2], and plants in elevated [CO2] possessed anatomical traits that may result in greater stress associated with long-term drought.