2007 NORTHEASTERN NATURALIST 14(1):15–26 Macrolichens as Biomonitors of Air-quality Change in Western Pennsylvania James R. McClenahen1,*, Donald D. Davis2, and Russell J. Hutnik3 Abstract - 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. Introduction Lichens are widely accepted indicators of air quality, and their use in biomonitoring has been extensively reviewed (e.g., Blett et al. 2003, Conti and Cecchetti 2001, Garty 2001). Lichen species exhibit a range of sensitivity to various air pollutants, although most studies have been conducted with sulfur dioxide and ozone (Hawksworth and Rose 1970, McCune 1988, Showman 1990, Showman and Long 1992, Sigel and Nash 1983). Lichens are also good accumulators of various trace elements in atmospheric deposition, and tissue analyses have effectively characterized their spatial and temporal deposition patterns (e.g., Garty 2001, Showman and Hendricks 1989, Walker et al. 2003). Surveys of the presence (richness) and/or abundance of lichen species have been used to define the geographic extent of pollution impact and temporal changes in air quality, especially in relation to coal-fired power generating stations (Giordani et al. 2002; Kauppi and Halonen 1992; McCune et al. 1997; Showman 1975, 1981; Showman and Long 1992; Van Dobben and De Bakker 1996; Will-Wolf 1980). One approach to lichen bio-monitoring is to observe species richness at spatially distributed favorable lichen habitats over time. This technique has been successfully applied to define the geographic extent of pollutant impacts, as well as to detect changing air quality (Showman 1981, 1990, 1997). It is important to include a temporal aspect to lichen bioindication to establish 1The Ohio State University, PO Box 272, Shreve, OH 44676. 2Department of Plant Pathology, The Pennsylvania State University, University Park, PA 16802. 3School of Forest Resources, The Pennsylvania State University, University Park, PA 16802. *Corresponding author - mcclenahen.l@osu.edu. 16 Northeastern Naturalist Vol. 14, No. 1 whether the present status of the flora is a reflection of earlier conditions and/ or as present air quality (Showman 1981, 1990, 1997). The objective of this study was to monitor spatial and temporal lichen species richness to assess air-quality changes in west-central Pennsylvania, particularly in relation to local power generation and industrial pollution sources. Methods Study area The study area (Fig. 1) centered on ca. 40.2oN, 79.0oW in west-central Pennsylvania included all or parts of Armstrong, Fayette, Cambria, Clearfield, Indiana, Jefferson, Somerset, and Westmoreland counties. The study area lies within the Allegheny Plateaus Physiographic Province, with Figure 1. Locations of regional lichen monitoring plots. 2007 J.R. McClenahen, D.D. Davis, and R.J. Hutnik 17 elevations varying from ca. 700–800 m along the crest of Laurel Ridge (a prominent anticlinal ridge sampled in this study), to ca. 360–500 m in the valley bottoms and lower hills. The annual growing-season temperature averages 20 oC, and mean annual precipitation is 104 cm (McClenahen et al. 1999). The landscape consists of mixed hardwood forests, hill farms, small villages, and the Greater Johnstown industrial area, which lies within complex terrain on the eastern base of Laurel Ridge. Thirty regional plots, in favorable lichen habitats, were distributed throughout the general study area (Fig. 1). Favorable habitat was defined as having adequate light for establishment and growth of corticolous macrolichens. Suitable sampling plots consisted of at least 10 mature hardwood trees in an open-grown condition: typically parks, picnic areas, churchyards, golf courses, cemeteries, tree-lined secondary and forest roadsides, and similar open areas. Included in the study area were an additional set of 22 favorable lichen habitat plots and 11 permanent forest-health monitoring plots located within oak forest on a northeast–southwest transect mostly along the top of Laurel Ridge (Fig. 2; Davis et al. 2001). Air quality The history of local industrial air pollution has been described elsewhere (Davis et al. 2001) and will be briefly reviewed here. Iron and steel manufacturing, including coke production, began in the mid-19th century within the Johnstown industrial area, near the center of the lichen study region and the Conemaugh Gap in Laurel Ridge (Fig. 2). Local industrial air emissions declined drastically around 1977 when a major flood halted most coke and steel production (Brown 1989); however, some coke production continued into the 1980s (V. Brisini, Reliant Energy Corp., Canonsburg, PA, pers. comm.). Coal-fired electric power generation began in the region with the construction of a 200 MW plant in 1921 near Conemaugh Gap. Three additional generating stations came on line from the mid-1960s through 1970–71, with a combined generating capacity of 5250 MW (Hutnik et al. 1989). Emissions scrubbers were put into operation on the three stations between 1995 and 2001. Local air quality is also impacted by long range transport of SO2 and associated pollutants from the Greater Pittsburgh area and the industrialized region of the Ohio River Valley (Knapp et al. 1988, Lynch et al. 1997, Pierson et al. 1989). Prevailing winds from the west and southwest transport pollutants into the study area. In addition, our biomonitoring data from an epiphytic moss indicate that these prevailing winds move pollutants northeastward of the generating stations and the Johnstown industrial region along Laurel Ridge (Davis et al. 2001, Hutnik et al. 1989, McClenahen et al. 1999). Data collection and analysis The biomonitoring network was structured in two ways. A regional lichen survey was conducted at 30 well-distributed plots representing favorable lichen habitat (Fig. 1). More spatially intensive monitoring was 18 Northeastern Naturalist Vol. 14, No. 1 conducted on a series of 22 favorable lichen plots, and on 11 permanent forest-interior plots (the 11 existing forest-health monitoring plots), mostly along the crest of Laurel Ridge. Lichen flora appeared to be impoverished along the ridge top, which receives relatively large amounts of acidic deposition (Lynch et al. 1997, Pierson et al. 1989). Favorable lichen plots were selected along a 53-km portion of Laurel Ridge top to sample intensively within the locality of our 11 permanent forest-health monitoring plots (Fig. 2). This northeast–southwest ridge is of particular interest in our biomonitoring program because two coal-fired Figure 2. Locations of lichen monitoring plots on Laurel Ridge. 2007 J.R. McClenahen, D.D. Davis, and R.J. Hutnik 19 power generating stations lie on the west flank, while the industrial city of Johnstown lies on the east flank (Fig. 2). The Laurel Ridge forest-health plots do not meet the criteria of favorable lichen habitat due to the shaded conditions within the interior of the forest stands, but were included in the survey to assess lichen richness and species distribution at the plots and document any changes. Lichen presence for each species was recorded for each of the numbered plot trees (􀂕 10 cm diameter at breast height within a 12-m radius of plot center) to detect changes in both plot richness and distribution in terms of numbers of trees occupied by each lichen species. The lichen survey consisted of a visual examination of at least 10 hardwood tree boles, but typically 20–50 trees, from the base to ca. 2 m height. The presence of each corticolous macrolichen species was recorded to genus and species. Species of the genus Cladonia were frequently seen only as squamules and were not recorded due to their ubiquity and the difficulty of species identification. Lichens of uncertain identity were collected for later verification or identification if adequate thalli were available for sampling; otherwise, “unknown” was recorded. The study design was based on similar studies conducted in Ohio and Pennsylvania (Showman 1981, 1997; Showman and Long 1992). Collected specimens were verified using Brodo et al. 2001, Flenniken 1999, and Hale 1969. Nomenclature generally follows that of Esslinger and Egan (1995). A reference collection of species found in the study has been archived in our collection. Lichen surveys were conducted at each of the monitoring plots in 1997, 1999, 2001, 2002, and 2003. On the 11 Laurel Ridge forest-health plots, the 1997, 2002, and 2003 surveys recorded lichen presence on numbered trees. This made it possible to capture data on changes in species distribution among particular trees within the plots. The total number of lichen species encountered at each plot was tabulated for each year. Two-factor ANOVA, with years as a fixed factor and plots as a random factor, were used to evaluate the significance of changes in species richness over years and among plots or plot groups for (1) the regional plots, (2) the Laurel Ridge favorable lichen plots, and (3) the 11 forest-health plots. For significant F-tests (p 􀂔 0.05), linear regressions of mean numbers of species over years were used to evaluate temporal trends. To examine potential geographical differences in lichen richness as related to local emissions sources, the 30 regional plots were grouped for ANOVA into 13 plots directly downwind (northeast) of the four generating stations, and 13 upwind (southwest) plots. Four plots near industrial Johnstown were omitted from this analysis. Results Twenty-seven species were recorded during the survey (Table 1), the most common being Flavoparmelia caperata, Parmelia sulcata, Physcia millegrana,, and Punctelia subrudecta These species were nearly ubiquitous on regional survey plots and were also common on Laurel Ridge. 20 Northeastern Naturalist Vol. 14, No. 1 Parmelia sulcata was the most widespread species encountered. It initially occurred on all of the regional survey plots, on 21 of 22 Laurel Ridge favorable plots, and on 7 of the 11 forest-health plots. Flavoparmelia caperata is well known to be among the most sensitive lichen species to atmospheric SO2 (Showman 1975, 1981, 1990). Its initial presence throughout the regional study area, and its increase on Laurel Ridge, suggest that SO2 concentrations are currently low and possibly decreasing (Table 1). Johnstown Air Basin monitoring data show that annual mean SO2 levels have remained below 0.01 ppm since 1997 (Pennsylvania Air-quality Monitoring Report 2002). Punctelia rudecta is also considered to be sensitive to ambient SO2 (Showman 1975, 1981, 1990). In contrast to F. caperata, P. rudecta was found at only 12 regional plots in the initial survey and was subsequently found just once (1999) at a plot on Laurel Ridge (Table 1). Punctelia rudecta may be less aggressive in colonizing new plots than is F. caperata (Showman 1990). Several lichen species exhibited large increases in distribution among regional plots. Notable among these were Phaeophyscia rubropulchra, Parmotrema hypotropum, Parmotrema stuppeum, Flavopunctelia flaventior, and Xanthoria ulophyllodes (Table 1). These species had initially low distributions, but are apparently capable of comparatively rapid colonization on favorable plots. On Laurel Ridge, species that showed the greatest increases at favorable plots differed somewhat from the regional picture, with Physcia millegrana, Imshaugia aleurites, and Phaeophyscia rubropulchra registering the largest gains. Flavoparmelia caperata was initially almost absent from the Laurel Ridge forest-interior plots, but subsequently increased in distribution more than any other species at these plots, suggesting that it is a rapid re-colonizer after air-quality improvement. There were no large decreases in distribution for any lichen species, although Physcia stellaris was found at none to four plots during the survey period (Table 1). Occasionally, a single thallus of a species occurred at a plot and then disappeared, possibly to re-colonize (or be re-discovered) later (e.g., Anaptychia palmulata, Canoparmelia crozalsianae, and Punctelia rudecta). Small thalli of Parmotrema hypotropum and Parmotrema stuppeum were difficult to discriminate in the field, and occasionally the species assignment was corrected in subsequent years as the maturing thalli provided clearer identifying characteristics. There was a consistent increase in average annual lichen richness on the regional monitoring plots throughout the survey period (Fig. 3). During the six-year study, richness significantly expanded by an average of 3.57 species/ plot (p = 0.001; Table 1). Annual ANOVA comparisons of lichen richness between the 13 plots directly downwind (northeasterly) of the generating stations with that of the 13 upwind plots failed to detect a significant difference (p = 0.093) in any year. Respective mean number of species per plot for upwind and downwind plots were 6.38 and 5.24 in 1997, 2007 J.R. McClenahen, D.D. Davis, and R.J. Hutnik 21 Table 1. Numbers of plots occupied by each lichen species and year. Plots were selected as favorable lichen habitat (open-grown trees), except for the Laurel Ridge forest-health plots, which are forest interior. 30 regional 22 Laurel Ridge 11 Laurel Ridge monitoring plots monitoring plots forest-health plots Species 1997 1999 2001 2002 2003 1997 1999 2001 2002 2003 1997 1999 2001 2002 2003 Allocetraria oakesiana (Tuck.) Randlane & Thell 11 12 15 17 18 3 3 3 4 7 1 1 1 2 3 Anaptychia palmulata (Michaux) Vainio 010221000000000 Canoparmelia crozalsiana (de Lesd. ex Harm.) Elix & Hale 221100000000000 Flavoparmelia caperata (L.) Hale 30 30 30 30 30 18 21 20 20 22 1 5 3 5 8 Flavopunctelia flaventior (Stirton) Hale 6101415171020100000 Flavopunctelia soredica (Nyl.) Hale 4 4 5 5 10 0 0 0 0 0 0 0 0 0 0 Heterodermia speciosa (Wulfen) Trevisan 011110000000000 Hypogymnia physodes (L.) Nyl. 366756778722112 Imshaugia aleurites (Ach.) S.F. Meyer 344543556901000 Melanelia subaurifera (Nyl.) Essl. 0 0 0 0 1 1 0 0 0 2 0 0 0 0 0 Myelochroa aurulenta (Tuck.) Elix & Hale 001120000000000 Parmelia squarrosa Hale 000100000000000 Parmelia sulcata Taylor 30 30 30 30 30 21 21 20 20 22 7 7 7 7 8 Parmeliopsis ambigua (Wulfen) Nyl. 0 0 0 0 1 0 2 2 2 4 0 0 0 0 2 Parmotrema hypotropum (Nyl.) Hale 3 10 9 14 16 1 2 2 3 4 0 0 0 0 0 Parmotrema stuppeum (Taylor) Hale 0 3 7 10 11 0 0 0 0 0 0 0 0 0 0 Phaeophyscia adiastola (Essl.) Essl. 001100000000000 Phaeophyscia pusilloides (Zahlbr.) Essl. 114360002200000 Phaeophyscia rubropulchra (Degel.) Essl. 7121521250113500012 Physcia adscendens (Fr.) H. Oliver 1 2 3 2 6 0 0 0 0 0 0 0 0 0 0 Physcia millegrana Degel. 23 29 29 29 30748111312211 Physcia stellaris (L.) Nyl. 130021000200000 Physconia detersa (Nyl.) Poelt 232340000000000 Punctelia rudecta (Ach.) Krog 12 8 12 14 12 0 1 0 0 0 0 0 0 0 0 Punctelia subrudecta (Nyl.) Krog 29 29 29 29 29 20 20 21 21 22 8 8 8 8 9 Pyxine sorediata (Ach.) Mont 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 Xanthoria ulophyllodes Rasanen 4 9 12 12 14 0 0 0 0 0 0 0 0 0 0 Unknown 0 0 0 0 2 0 0 0 0 0 0 0 0 0 0 Mean lichen species/site 5.73 6.97 7.67 8.43 9.30 3.77 3.95 4.14 4.55 5.55 1.82 2.36 2.00 2.27 3.18 22 Northeastern Naturalist Vol. 14, No. 1 and 9.62 and 9.12 in 2003. Thus, we found no indication that the lichen flora was reduced, or that lichen re-colonization differed, within the regional monitoring area with respect to the generating stations. The increase in mean lichen richness on regional plots was strongly linear over time (R2 adj = 0.973, p = 0.001), with an average gain of 0.56 species/plot/year. On Laurel Ridge, lichen richness showed smaller but significant gains (Fig. 3). Over the course of the survey, richness increased by an average of 1.78 and 1.36 species/plot at the favorable plots and forest-health plots, respectively (Table 1). The linear relationship of richness over time for Laurel Ridge favorable lichen plots (R2 adj = 0.656; p = 0.061) was weaker than for the regional plots, but revealed an average annual increase of 0.26 species/plot/year, about half the rate for regional plots. Lichen richness also increased significantly over time on the Laurel Ridge forest plots, but the temporal trend was not consistent. We found a spatially inconsistent pattern of lichen richness along the top of Laurel Ridge on the 11 forest-health plots. Unchanged or diminished lichen-species frequencies occurred on four plots in the immediate vicinity of, and mostly northeast (downwind) of, Conemaugh Gap (plots 2–5, Fig. 4). Furthermore, a block of favorable lichen plots in that locality exhibited Figure 3. Mean numbers of lichen species found on monitoring plots over time. Favorable lichen habitat consists of open-grown trees. There were 30 regional favorable lichen-habitat monitoring plots, 22 favorable lichen-habitat plots on Laurel Ridge top, and 11 Laurel Ridge top forest-health plots representing poorer quality (forest interior) lichen habitat. 2007 J.R. McClenahen, D.D. Davis, and R.J. Hutnik 23 reduced richness (plots 5–11, Fig. 5). The slower lichen recovery near, and immediately northeast of, the Conemaugh Gap area of Laurel Ridge may be a result of present local air emissions or simply a lag in lichen re-colonization following historic pollutant impact. However, the recorded increase and general presence of F. caperata, a highly SO2-sensitive species, in this area and elsewhere on Laurel Ridge suggests that present SO2 levels are not responsible for the lower richness. Figure 4. Change in distribution of lichen species between the 1997 and 2003 lichen surveys on the Laurel Ridge forest- health plot trees. Distribution change is expressed as the difference between 1997 and 2003 in the sum of plot trees occupied by each species present. Plots are at ca. 5 km intervals. See Figure 2 for plot locations. Figure 5. Change in the number of lichen species present on Laurel Ridge favorable lichen-habitat plots between the1997 and 2003 lichen surveys. 24 Northeastern Naturalist Vol. 14, No. 1 Discussion The question arises as to whether power generation and/or Johnstown industries have had a persistent impact on lichen flora near Conemaugh Gap. The two generating stations near the gap have upgraded air emissions controls over the years, especially in 1994. Shortly after 1994, annual foliar sulfur concentrations in natural vegetation showed a marked decline downwind of the two generating stations, including Laurel Ridge top (J.R. McClenahen, R.J. Hutnik, and D.D. Davis, unpublished data). Johnstown has had major iron and coke industries since its founding, but these have mostly disappeared due to the 1977 flood, except for a few coke batteries that operated into the 1980s. Lacking detailed data on emissions and wind transport patterns, it is difficult to say whether power generation and/or Johnstown industries are currently affecting lichen recolonization near Conemaugh Gap. Our regional survey recorded an average of 5.73 species/plot and a total of 18 species in 1997, which subsequently increased to an average of 9.30 species/plot and a total of 25 species in 2003. There is no indication that the rate of gain in richness is leveling off. A 1988 lichen survey along a gradient of decreasing sulfate and nitrate deposition from northwestern to east-central Pennsylvania revealed 15 lichen species and a mean of 5.8 species per plot in the western half of the gradient, directly north of our study area (Showman and Long 1992). In contrast, the eastern half of the gradient, where atmospheric pollutant deposition was lower, averaged 11.1 species per plot and a total of 27 species. A re-survey of the same plots in 2004 recorded 26 total species (averaging 11.7 species per plot) in the eastern half of the gradient, and 27 total species (averaging 10.0 species per plot) in the western half of the gradient (J.R. McClenahen, R.J. Hutnik and D.D. Davis, unpubl. data). Thus, 1997 lichen richness in our study area was similar to that in northwestern Pennsylvania in 1988. Further, lichen flora appears to be following a similar pattern of recovery in the two areas. Lichen recolonization has shown a steady increase throughout our study area in west-central Pennsylvania since 1997, nearly doubling in average number of species per plot by 2003. Slower but persistent recolonization occurred on Laurel Ridge, where mean richness was initially lower compared to regional plots. Lichen flora in our study area was similar to that in portions of the Ohio River Valley and northwestern Pennsylvania, but was lower than expected for an area of high air quality (Showman 1997, Showman and Long 1992). These meso-scale geographical patterns of lichen richness suggest that regional airpollution impact on lichen flora from emissions sources outside our study area has occurred in the past, and that air quality has subsequently improved. We detected no specific indication that local power generating stations have affected lichen richness or re-colonization within the region. Additional studies are needed to elucidate whether nearby emissions sources may have retarded lichen recovery within a localized area on Laurel Ridge. 2007 J.R. McClenahen, D.D. Davis, and R.J. Hutnik 25 Finally, our results confirm the utility of lichens as indicators of air quality, and the importance of examining temporal, as well as geographical changes in lichen richness to ascertain the status of air quality. Acknowledgments This study was funded by the Chestnut Ridge Power Center. The authors wish to thank Ray Showman of American Electric Power Company for his help and advice in designing the study, assistance in identification of lichen species, and suggestions for improving this manuscript. We are grateful for helpful suggestions made by three anonymous reviewers. Literature Cited Blett, T., E. Geiser, and E. Porter. 2003. Air pollution-related lichen monitoring in national parks, forests and refuges: Guidelines for studies intended for regulatory and management purposes. NPS D2292, US Department of the Interior, Air Resources Division, Denver, CO. 25 pp. Brodo, I.M., S.D. Sharnoff, and S. Sharnoff. 2001. Lichens of North America. Yale University Press, New Haven, CT. 795 pp. Brown, S.A. 1989. Historic resources study: Cambria Iron Company, US Department of the Interior, National Park Service, Washington, DC. 513 pp. Conti, M.E., and G. Cecchetti. 2001. Biological monitoring: Lichens as bioindicators of air pollution assessment: A review. Environmental Pollution 114:471–492. Davis, D.D., J.R. McClenahen, and R.J. Huntik. 2001. Use of an epiphytic moss to biomonitor pollution levels in southwestern Pennsylvania. Northeastern Naturalist 8:379–392. Esslinger, T.L., and R.S. Egan. 1995. A sixth checklist of lichen-forming, lichenicolous, and allied fungi of the continental United States and Canada. The Bryologist 98:467–459. Flenniken, D.G. 1999. The macrolichens in West Virginia. Carlisle Printing Co., Sugarcreek, OH. 231 pp. Garty, J. 2001. Biomonitoring atmospheric heavy metals with lichens: Theory and application. Critical Reviews in Plant Sciences 20:309–371. Giordani, P.G. Brunialti, and D. Alleteo. 2002. Effects of atmospheric pollution on lichen biodiversity (LB) in a Mediterranean region (Liguria, northwest Italy). Environmental Pollution 118:53–64. Hale, M. 1969. How to Know the Lichens, 2nd Edition. Wm. C. Brown Company, Dubuque, IA. 246 pp. Hawksworth, D.L., and F. Rose. 1970. Qualitative scale for estimating sulphur dioxide air pollution in England and Wales using epiphytic lichens. Nature 227:1455–148. Hutnik, R.J., D.D. Davis, and J.R. McClenahen. 1989. Evaluation of vegetation near coal-burning power plants in southwestern Pennsylvania: Part I. Sulfur content of foliage. Journal of the Air Pollution Control Association 39:1440–1443. Kauppi, M., and P. Halonen. 1992. Lichens as indication of air pollution in Oulu, northern Finland. Annales Botanici Fennici 29:1–9. Knapp, W.W., V.C. Bowersox, B.I. Chevone, S.V. Krupa, J.A. Lynch, and W.M. McFee. 1988. Precipitation Chemistry in the United States, 1: Summary on Ion Concentration Variability 1979–1984, Center for Environmental Research, Water Resources Institute, Cornell University, Ithaca, NY. 239 pp. 26 Northeastern Naturalist Vol. 14, No. 1 Lynch, J.A., J.W. Grimm, and K.S. Horner. 1997. Atmospheric deposition: Spatial and temporal variations in Pennsylvania: 1996. Annual Technical Report to the PA Department of Environmental Protection, Pennsylvania State University, ERRI, Report E9811, University Park, PA. McClenahen, J.R., D.D. Davis, and R.J. Hutnik. 1999. Northern red oak growth response to climate and industrial air pollution in western Pennsylvania. Pp. 386– 399, In J.W. Stringer and D.L. Loftis. Proceedings of the 12th Central Hardwoods Forest Conference, USDA North Central Forest Experiment Station, Lexington, KY. General Technical Report NC-188. McCune, B.1988. Lichen communities along O3 and SO2 gradients in Indianapolis. The Bryologist 9:223–228. McCune, B., J. Dey, J. Peck, K. Heiman, and S. Will-Wolf. 1997. Regional gradients in lichen communities of the southeastern United States. The Bryologist 100:145–158. Pennsylvania Air-quality Monitoring. 2002. 2001 Annual Report. PA Department of Environmental Protection, Bureau of Air Quality, Harrisburg, PA. 125 pp. Pierson, W.R., W.W. Brachaczek, R.A. Gorse, S.M. Japar, and J.M. Norbeck. 1989. Atmospheric acidity measurements on Allegheny Mountain and the origins of ambient acidity in the northeastern United States. Atmospheric Environment 23:431–459. Showman, R. 1975. Lichens as indicators of air quality around a coal-fired power generating station. The Bryologist 78:1–6. Showman, R. 1981. Lichen recolonization following air-quality improvement. The Bryologist 84:492–497. Showman, R. 1990. Lichen recolonization in the upper Ohio River Valley. The Bryologist 93:427–428. Showman, R. 1997. Continuing lichen recolonization in the upper Ohio River Valley. The Bryologist 100:478–481. Showman, R., and J.C. Hendricks. 1989. Trace element content of Flavoparmelia caperata (L.) Hale due to industrial emissions. Journal Air Pollution Control Association 39:317–320. Showman, R., and R. Long. 1992. Lichen studies along a wet sulfate deposition gradient in Pennsylvania. The Bryologist 95:166–170. Sigel, L., and T.H. Nash. 1983. Lichen communities on conifers in southern California mountains: An ecological survey relating to oxidant air pollution. Ecology 64:1343–1354. Van Dobben, H.F., and A.J. De Bakker. 1996. Re-mapping epiphytic lichen biodiversity in The Netherlands: Effects of decreasing SO2 and increasing NH3. Acta Botanica Neerland 45:55–71. Walker, T.R., P.D. Crittenden, and S.D. Young. 2003. Regional variation in the chemical composition of winter snow pack and terricolous lichens in relation to sources of acid emissions in the Usa river basin, northeast European Russia. Environmental Pollution 125:401–412. Will-Wolf, S. 1980. Structure of corticolous lichen communities before and after exposure to emissions from a “clean” coal-fired generating station. The Bryologist 83:281–295.