Improvement in the air quality of the upper Ohio River valley has led to lichen recolonization at previously depauperate sites. Between 1973 and 1996 the number of corticolous macrolichen species in the area increased from 6 to 20. During the same time the mean species richness per site increased from 0.8 to 6.5. An indicator species, Flavoparmelia caperata, was absent from all study sites in 1973, but present at 27 of 28 study sites in 1996. Lichen studies performed over time provide a good tool to document air quality improvement, but a single study in a changing air quality environment may give problematic results. There are many published accounts of the detrimental effects of air pollution, especially sulfur dioxide, on lichen species distribution and community success (Ferry et al. 1973; Nash & Wirth 1988). Recently, however, industrial modernization and clean air legislation have led to decreases in sulfur emissions with concomitant decreases in ground-level SO2 concentrations both in North America and Europe. One source (Placet & Streets 1987) estimated that total SO2 emissions in the United States decreased from 26.8 million metric tons in 1975 to 21.2 million metric tons in 1985. There have been further decreases to 16.6 million metric tons in 1995 (USEPA 1996). Ohio has shown similar decreases in emissions with 2.99 million metric tons in 1975, 2.43 in 1985 and 1.90 in 1995. The upper Ohio River valley was once reputed to have the dirtiest air in the nation (Showman 1990). This area was heavily industrialized with coal-burning power plants, steel mills, foundries, and coking ovens. However, the region was severely affected by the economic recession in the late 1970s that led to the decline of the steel industry. Old plants were closed and others were upgraded and modernized. At the same time, clean air legislation led to particulate and gaseous emission controls on most electric power generation and industrial plants. This decrease in emissions had a dramatic effect on the ambient SO2 levels measured at upper Ohio River valley locations. Monitoring data (North Ohio Valley Air Authority, pers. comm.) show that in 1975, Steubenville, Ohio had an annual average SO2 concentration of 118 iggm-3. In 1985, this had almost halved to 60 agm-3, and by 1995 the value was further reduced to 34 agm-3. Likewise, East Liverpool, Ohio reported annual average SO2 concentrations of 120, 44, and 31 agm-3 during the same years. For comparison, the National ambient air quality standard (annual average) for SO2 is 80 Igm-3. Data from the several monitoring sites in the area consistently show a sharp decline from high to much lower values about 1981 (North Ohio Valley Air Authority, pers. comm.). Lichen studies in the upper Ohio River valley were first conducted in 1973 around Cardinal Power Plant, located just south of Steubenville, Ohio (Fig. 1). After the initial study, it was apparent that the regional SO2 background overwhelmed any contribution to ground-level concentrations by the power plant. Lichens were depauperate throughout a 570 km2 study area with no discernible differences between sites upwind and downwind from the Cardinal Plant. Studies were continued in order to monitor the impact of regional SO2 on lichens. Lichen re-surveys were performed in 1983, 1988, and then yearly to the present. Lichen improvement was first seen in 1988 and a note reporting this was published in 1990 (Showman 1990). The purpose of this paper is to describe lichen recolonization up to 1996 in greater detail, and to relate the observed changes in the lichen flora to potential associations with improved air quality over the same timespan.
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Jan 1, 2007
E.R Davies 
Open Access