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 μmol mol^-1 (ambient) or 800 μmol 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 (JV), stomatal conductance (gs) 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 JV values throughout the study, but did not differ from trees in ambient [CO₂] with respect to gs or E under saturating light or E scaled from JV (Escaled), all of which indicates that the higher JV in elevated [CO ₂] resulted from those trees having greater leaf area and not from differences in gs. 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]. gs 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 [CO ₂], and plants in elevated [CO₂] possessed anatomical traits that may result in greater stress associated with long-term drought.