Species richness of corticolous macrolichens was monitored at one- or two-year intervals on a total of 63 plots from 1997–2003 in a region of west-central Pennsylvania that included four coal-fired power generating stations and an industrial city. Lichen richness significantly increased from an average of 5.7 species/plot in 1997 to 9.3 species/plot in 2003. A linear mean rate of gain in species on regional monitoring plots was 0.56 species/yr. Plots along a major ridge top had a slower but significant gain in richness, and a localized area flanked by the city and two generating stations exhibited less lichen recolonization. Our results confirm the value of macrolichens as indicators of air quality and the importance of examining temporal as well as spatial changes in lichen richness to ascertain air-quality status.