Thirty-six F₂ hybrid poplar (Populus trichocarpa×P. deltoides) clones were fumigated with ozone to select for ozone sensitive and resistant clones. Fumigation was applied for 6 to 8 hours each day for approximately 3 months at ozone concentrations of 90 to 115 ppb using by open-top chambers. Height, diameter, number of leaves, total biomass, biomass components, root/shoot ratios, leaf drop and stomatal response were investigated. In summary, ozone generally reduced height, diameter, number of leaves, total biomass, and root/shoot ratios. Ozone stress induced leaf drop and foliar senescence in trees. This study showed very low relationship between total biomass and stomatal conductance. Increased plant resistant to ozone is not always correlated with stomatal behaviour. Probably, characterization of biochemical and other physiological responses to ozone exposure can provide a better understanding of tree response to ozone environment.