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 each of five national parks differing in air quality. Eleven to 15 clones from each population were greenhouse grown and fumigated twice during 2 years of screening, each time with 180 ppb ozone for 6 h. The average injury for clones was significantly less for the most polluted park than for the least polluted park and there was a high negative association between average injury and ambient ozone levels. Differences in ozone sensitivity among clones within populations were highly significant and larger than differences among populations. These results suggest natural selection for tolerance in P. tremuloides may have occurred in some areas of eastern United States