Natural selection for ozone tolerance in Populustremuloides: field verification

Field plantings of Populustremuloides Michx. cuttings in two areas with elevated levels of ambient ozone (New York and Michigan) were evaluated for differences in relative ozone sensitivity among five natural populations from areas that differed in air quality. At least fivefold differences in leaf injury were measured among population means in each planting, and the means were negatively correlated with ozone concentrations for each area. The relative sensitivity rankings among populations and among clones within populations agreed closely with rankings derived from laboratory fumigations of the same plant material. The hypothesis that ozone-sensitive clones of this species are less common in relatively polluted parts than they are in pristine parts of the eastern United States is therefore further supported by field verification of laboratory results. Data from a 10-year-old aspen planting in New York showing impaired growth of ozone-sensitive clones suggest that sensitive individuals can be eliminated from populations by the mechanism of competition.