Effects of tropospheric O-3 on trembling aspen and interaction with CO2: Results from an O-3-gradient and a face experiment

Over the years, a series of trembling aspen (Populus tremuloides Michx.) cl ones differing in O-3 sensitivity have been identified from OTC studies. Th ree clones (216 and 271[(O-3 tolerant] and 259 [O-3 sensitive]) have been c haracterized for O-3 sensitivity by growth and biomass responses, foliar sy mptoms, gas exchange, chlorophyll content, epicuticular wax characteristics , and antioxidant production. In this study we compared the responses of th ese same clones exposed to O-3 under field conditions along a natural O-3 g radient and in a Free-Air CO2 and O-3 Enrichment (FACE) facility. In additi on, we examined how elevated CO2 affected O-3 symptom development. Visible O-3 symptoms were consistently seen (5 out of 6 years) at two of the three sites along the O-3 gradient and where daily one-hour maximum concentration s were in the range of 96 to 125 ppb. Clonal differences in O-3 sensitivity were consistent with our OTC rankings. Elevated CO2 (200 ppm over ambient and applied during daylight hours during the growing season) reduced visibl e foliar symptoms for all three clones from 31 to 96% as determined by symp tom development in elevated O-3 versus elevated O-3 + CO2 treatments. Degra dation of the epicuticular wax surface of all three clones was found at the two elevated O-3 gradient sites. This degradation was quantified by a coef ficient of occlusion which was a measure of stomatal occlusion by epicuticu lar waxes. Statistically significant increases in stomatal occlusion compar ed to controls were found for all three clones and for all treatments inclu ding elevated CO2, elevated O-3, and elevated CO2 + O-3. Our results provid e additional evidence that current ambient O-3 levels in the Great Lakes re gion are causing adverse effects on trembling aspen. Whether or not elevate d CO2 in the future will alleviate some of these adverse effects, as occurr ed with visible symptoms but not with epicuticular wax degradation, is unkn own.