Elevated carbon dioxide concentrations and limited water supply have been shown to reduce the impact of ozone pollution on the growth and physiology of Quercus petraea in a long-term factorial experiment. These responses can be explained by observed reductions in stomatal conductance, and thus potential ozone exposure of 28% and 40% for CO2 and drought treatments respectively. However, parameterisation of a stomatal conductance model for Quercus robur and Fagus sylvatica grown under ambient and elevated CO2 concentrations in a separate experiment has demonstrated that elevated CO2 also reduces the responsiveness of stomata to both saturation deficit (LAVPD) and soil moisture deficit () in beech, and to a lesser extent, in oak. Season-long model simulations of ozone fluxes suggest that LAVPD and conductance parameters derived at ambient CO2 concentrations will lead to these fluxes being underestimated by 24% and 2% for beech and oak respectively at 615 ppm CO2.