Samples of Parmelia sulcata from North American sites were sectioned and the algal layer thickness measured to test for differences from different national parks and other localities. Total thallus thickness and algal layer thickness and their ratio varied across the country, but the overall variability was low. Sites nearer urban areas tended to have lower algal layer ratios, indicating that the photobiont may be the more sensitive component to air pollution. Differences were observed even though no discoloration or other signs of stress were apparent.

Key words: air pollution, layer thickness, lichen, Parmelia sulcata, photobiont