NATURAL-SELECTION FOR OZONE TOLERANCE IN POPULUS-TREMULOIDES - AN EVALUATION OF NATIONWIDE TRENDS

Relative ozone sensitivity was evaluated among populations of Populus tremuloides Michx. from 15 locations, mostly in national parks, that spanned the United States. Seven to 15 clones were selected from each population, greenhouse grown, fumigated with 150 ppb ozone for 6 h, and evaluated for visible injury. Differences among populations were statistically significant, with the most tolerant populations being obtained from the West Coast, the northeast, and the industrialized portions of the Great Lakes. Populations sampled from areas that failed to achieve the National Ambient Air Quality Standard for ozone had significantly less injury than populations from areas that achieved this standard. There was a significant negative correlation between the amount of injury and maximum daily ozone averages at localities where the populations were collected. There were also significant correlations between the amount of injury and two climatic variables, annual precipitation and minimum temperature. Differences among clones within populations were highly significant, and clonal variance was negatively correlated with maximum daily ozone average for the area where the populations were collected. These findings support our hypothesis that ambient levels of ozone may be eliminating ozone-sensitive clones from natural populations of this species, but indicate that climatic variables play a role as well.