In 1988 the macrolichen species richness and elemental concentrations in Hypogymnia physodes thalli were surveyed at four equally-spaced study areas along a 160-km west-to-east atmospheric deposition gradient in north-central Pennsylvania. There was significantly lower lichen species richness in the western, higher deposition half of the gradient and significantly higher elemental concentrations at the western study areas. The same study sites were revisited in 2004 to determine if spatial-temporal changes in lichen richness and elemental concentrations had occurred in the ensuing 16 years. By 2004 lichen richness had increased in the western portion of the gradient to be nearly equal that of the eastern portion of the gradient. Lichen elemental concentrations generally decreased at all study sites, with the greatest reduction in the western sites. These changes are likely a response to air quality improvements resulting from the Clean Air Act Amendments of 1990. Lichens are useful indicators of air quality and can provide strong biological confirmation of air quality improvement.