The false positive in bioindicators of air pollution

Yellow-poplar (Liriodendron tulipifera) seedlings were fumigated with a gradient of ozone to determine what effect ozone has on the species. The fumigated seedlings displayed physical symptoms of ozone injury but a large number of the individuals from the charcoal-filtered treatment showed physical symptoms of ozone when indeed no ozone was present. This is an indication that what appears to be ozone damage on yellow-poplar may not be caused by ozone.