2007 NORTHEASTERN NATURALIST 14(1):15–26
Macrolichens as Biomonitors of Air-quality Change in
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.
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 - firstname.lastname@example.org.
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
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).
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.
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
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
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
and 2003 lichen
surveys on the
Laurel Ridge forest-
change is expressed
as the difference
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
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
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.
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
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