2007 NORTHEASTERN NATURALIST 14(1):1–14
Macrolichen Indicators of Air Quality for Nova Scotia
Robert P. Cameron1,*, Thomas Neily2, and David H.S. Richardson3
Abstract - Presence and frequency of epiphytic macrolichens were measured along
an air-quality gradient in Halifax City, NS, Canada. Species frequency plots over
distance and multidimensional scaling (MDS) suggested lichen-community changes
consistent with expected air-quality changes. A provisional list of air-quality indicators
was selected based on: 1) demonstrated variation along the air-quality gradient,
2) frequency across the province, 3) literature values of air-quality sensitivity, and
4) ease of field identification. Indicators were placed in one of three classes:
1) pollution tolerant, 2) intermediate pollution tolerance, and 3) pollution sensitive.
MDS analysis suggests an elevation gradient in Nova Scotia and this should be
investigated with a further study.
Despite initiatives to reduce airborne pollutants, air-quality problems are
anticipated to be an issue for the next 20 to 50 years. The growth in airpollution
sources has the potential to outpace the gains in pollution abatement
made in recent years. The effects of acid rain continue to impact ecosystems;
for example, modeling studies have shown that up to one quarter of lakes in
eastern Canada will be damaged even if 2010 emission targets are reached
(Environment Canada 2003). High levels of mercury have also been found in
Nova Scotian ecosystems (Cox et al., no date).
Nova Scotia provides an ideal region to monitor impacts of pollutants
because as one of the Maritime Provinces of Canada, it is situated within
storm tracks and the prevailing winds bring industrial emissions from central
and eastern North America (Beattie et al. 2002). Nova Scotia receives the
brunt of its pollutants from industrialized areas of North America and, from
an ecosystem standpoint, can provide an early warning system for the rest of
The value of lichens as indicators of air pollution, particularly acid rain,
fertilizers, sulphur and nitrogen oxides, and metals, has been documented
in thousands of scientific papers (Henderson 2000). Air-quality monitoring
studies have been done worldwide, and permanent monitoring programs
using lichens exist in the US, Netherlands, and Switzerland (McCune
2000). The sensitivity of lichens to air quality stems from their reliance on
airborne nutrients and water, as well as lack of protective structures such as
cuticles found in vascular plants. Trees and other vascular plants are affected
by pollution, but are generally slower to show impacts than lichens
(Muir and McCune 1988).
1Nova Scotia Environment and Labour, Protected Areas Branch, PO Box 697,
Halifax, NS, Canada B3J 2T8. 2RR 2, Middleton, NS, Canada B0S 1P0. 3Faculty of
Science, Saint Mary’s University, 923 Robie Street, Halifax, NS, Canada B3J 2T8.
*Corresponding author - firstname.lastname@example.org.
2 Northeastern Naturalist Vol. 14, No. 1
One of the challenges to using lichens as indicators is the difficulty of
identification of species. Many crustose species require examination of
spores under a microscope or the use of chemical spot tests. Indeed, some
species can only be identified after testing for the contained lichen substances
with thin layer chromatography or liquid chromatography coupled
with mass spectrometry. Usually there are few experts within a region able
to identify lichens and often they do not have the time to assist communitybased
monitoring projects. However, non-experts have helped successfully
with air-quality studies using lichens. For example, schoolchildren assessed
air quality in the UK in 1971 by mapping the distribution of particular
lichens across Britain (Richardson 1975), and this type of study was later
extended to Ireland (Richardson 1992). Similar projects have been carried
out in North America including Pennsylvania and Oregon (Richardson
2004). The general approach has been to use a suite of indicator species or
groups of lichens (e.g., crustose, foliose, or fruticose), thereby reducing the
need for expert identification of all species. Indicator species are selected on
the basis of sensitivity to air quality and ease of field identification. The
challenge is to select an appropriate number so that identification is not too
difficult for non-experts, but to ensure that background “noise” does not
The pollution sensitivity of some lichens seems to vary from region to
region and this variation necessitates local studies to determine the sensitivity
rating. For example Hypogymnia physodes tolerance has been rated as
sensitive to sulphur dioxide in Greece (Diamantopoulus et al. 1992), intermediate
in Britain (Hawksworth and Rose 1970), and tolerant in Denmark
(Johnsen and Søchting 1976); for other examples, see the USDA Forest
Service website (http://www.fs.fed.us/r6/aq/lichen/images.htm).
In the present study, macro-lichen presence and frequency were measured
across an air-quality gradient in Nova Scotia to determine species
sensitive to air quality. A suite of indicator lichen species was selected as a
basis for an air-quality monitoring protocol for non-experts in Nova Scotia.
The study also provides data on macro-lichen occurrence and frequency in a
series of plots in rural, suburban, and urban areas that can be reassessed
periodically to monitor the affects of a growing regional municipality or
changing patterns of air pollution.
Halifax city and surrounding urbanized areas, the Halifax Regional Municipality,
has a population of about 350,000 people and is the largest urban
center in Atlantic Canada (Statistics Canada 2000). The city has a temperate
climate with an average July temperature of 17 oC and an average January
temperate of -4 oC. Average annual precipitation is 1400 mm and the prevailing
winds are from southwest to northeast (Davis and Browne 1998). The
major local sources of air pollution are motor vehicles, coal-fired electrical
2007 R.P. Cameron, T. Neily, and D.H.S. Richardson 3
power generation, oil-based domestic heating, and oil refining (Nova Scotia
Department of the Environment 1998). Average annual sulphur dioxide
levels for Halifax for 2004 were 0.007 mg kg-1,with a range of less than
0.0004 (limit of detection) to 0.123 mg kg-1. Nitrogen oxide data for 2002
ranged from 0.0004 (limit of detection) to 0.306 mg kg-1, with an annual
average of 0.028 mg kg-1 (nitrogen oxide means are not available for years
2003 and 2004). Halifax has the highest average annual atmospheric sulphur
dioxide and nitrogen oxide levels in the province (Nova Scotia Environment
and Labour, Halifax, NS, Canada, unpubl. data).
Transects and plots
A southwest to northeast transect, the direction of the prevailing wind, was
established through Halifax City such that variation in geology, climate, and
elevation were minimized (Fig. 1). Twelve plots were established at various
distances along the transect up to 16.8 km upwind (southwest) and 22.3 km
downwind (northeast) of the city center (Table 1). The transect dissected 3
Figure 1. Map of Halifax City indicating locations of lichen air-quality plots.
4 Northeastern Naturalist Vol. 14, No. 1
geological formations with granitic rocks to the southwest and slate, quartzite,
greywacke, and schist to the northeast (Keppie 2000). Granites, quartzite, and
greywackes produce acidic soils and are resistant to erosion, while slate and
schists are also acidic, but less resistant (Davis and Browne 1998). All plots
were within the Atlantic climate region (Davis and Browne 1998) with the
exception of two, which were less than 15 km north of this climate region.
Elevation within plots ranges from 30 to 110 m asl.
Four additional lichen plots were established, which varied in geology,
climate, and elevation, and these were used to assess how these factors
affected lichen abundance in Nova Scotia. The additional plots were located
in two climate regions: two with underlying carboniferous bedrock and two
in a highland region (300 m asl). Plots along the transect were placed, where
possible, within forests in suburban and urban locations, heavily wooded
areas, or parks. The aim was to identify lichen indicator species that were
affected by air quality rather than other variables.
Cameron (2002) determined that 16 trees were required to sample a plot
adequately for lichens in the woodlands of Nova Scotia. The method used to
assess lichen frequency was based on that of Asta et al. (2002). The center
point of each plot was chosen systematically along the transect, taking into
account access and site suitability. The plot size was variable. A north–south
and an east–west line were placed through the plot center to divide the plot
into four quadrats. The four closest suitable trees to the plot center, in each
quadrat , were selected for study. Plot centers were permanently marked by a
5-cm diameter, 1.5-m long PVC stake driven into the ground and sprayed
with fluorescent paint, except for in urban plots. The trees selected for
sampling were Acer saccharum L. (sugar maple), Acer rubrum L. (red
Table 1. Locations of lichen air-quality study plots near Halifax, NS.
Distance from UTM zone 20
Plot Land city center coordinates
# Name use Land status (km) Easting Northing
1 Leaman Street Urban Urban 2.79 451706 4946325
2 Long Lake Forest Provincial Park 5.50 447969 4942155
3 Spruce Hill Forest Provincial Park 6.91 448971 4938468
4 Terence Bay Forest Wilderness Protected Area 10.84 448706 4934126
5 Beaver Creek Forest Wilderness Protected Area 200+ 294848 4883230
6 Lancaster Street Urban Urban 4.71 454730 4948399
7 Shubie Park Forest Urban park 6.79 456115 4950045
8 Lake Major Forest Wilderness Protected Area 14.35 460834 4956060
9 Millar Lake Forest Private forest 20.71 453217 4964685
10 Waverley Forest Wilderness Protected Area 22.28 462936 4964111
11 Gully Lake Forest Wilderness Protected Area 102.31 490582 5039279
12 East Gully Lake Forest Wilderness Protected Area 105.03 498127 5039168
13 Camphill Urban Cemetery 0.60 453598 4943542
14 MacPhee Corner Forest Private forest 51.63 459187 4995129
15 Soldier Lake Forest Private forest 17.88 453897 4962094
16 Brookside Forest Wilderness Protected Area 16.75 443824 4930200
2007 R.P. Cameron, T. Neily, and D.H.S. Richardson 5
maple), and Acer platanoides L. (Norway maple) with a diameter at breast
height (DBH) greater than 10 cm.
Each selected tree was examined using four independent quadrats 10-cm
wide by 50-cm long consisting of five segments, each a 10-x 10-cm square
(Asta et al. 2002). Each quadrat was attached vertically to the trunk by its
10-cm edge, so that the lower edge was 1 m above the ground level. This was
repeated for each section of the trunk facing the primary cardinal directions.
All macro-lichen species present in any of the quadrat segments were identified,
and the frequency of occurrence by segment was recorded. Thus,
species frequency per tree was calculated as the number of squares out of
twenty in which a species occurred. Voucher specimens were collected and
deposited at the Nova Scotia Museum of Natural History Herbarium. Nomenclature
for lichen species followed Esslinger (1997).
The frequency of each lichen species was plotted against distance from city
center. An air-quality gradient was determined using multidimensional scaling
(MDS). Multidimensional scaling is part of a family of multivariate ordination
methods used to arrange communities along environmental gradients based on
community composition (ter Braak 1987). Differences (or similarities) between
communities are calculated and then plotted in such a way that the
distances between sites are maximally correlated with ecological distances.
Multidimensional scaling is one of the most vigorous methods of multivariate
analysis and has been used in determining air-quality and other environmental
gradients in several lichen community studies in the US (McCune et al. 1997,
1998; Neitlich et al. 2003). Spearman’s correlation was used without standardization
or transformation of data. Lichens which occurred at fewer than 3 sites
were not used in the MDS analysis. To obtain a stable solution of best possible
fit, ten runs with one hundred iterations were used.
In selecting suitable indicators, the following criteria were used:
1) demonstrated variation along the air-quality gradient; 2) widespread
distribution across the province, as assessed by the four plots outside the
transect and by other studies (Cameron 2002, Cameron and Richardson
2006, Casselman and Hill 1995, Selva 1999); 3) information in the literature
with respect to known pollution sensitivity; and 4) ease of field identification.
In some genera, all species seem to have a similar pollution sensitivity
(McCune et al. 1997, Neitlich et al. 2003). Identification to genus rather than
to species increases the ease and speed of determination in the field.
Comparisons were therefore made between results of analyses using all
individual species with those that combined species in each genus.
Forty macro-lichen species were found (Table 2). Parmelia squarrosa was
the highest frequency and was found at all fifteen study plots. Lobaria
pulmonaria was the next most frequent species, even though it occurred at
6 Northeastern Naturalist Vol. 14, No. 1
Table 2. Frequency occurrence from 0 to 10 for each plot, mean by lichen taxa and number of species for sixteen plots in Nova Scotia. Frequency occurrence is
calculated as the number of grids a lichen taxa occurs in divided by the total number of grids sampled per plot (20 grids per tree x 16 trees per plot = 320 grids) and
multiplied by ten.“ - ” indicates lichen taxa not present. Mean = mean frequency per plot.
Species 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Mean
Bryoria nadvornikiana (Gyelnick) Brodo & Hawksw. - - - - - - - 0.06 - - - -----0.00
Bryoria spp. - 0.03 - - - - - - - - - - 0.13 - - - 0.01
Bryoria trichodes (Michaux) Brodo & Hawksw. 0.09 0.03 0.94 - - - - - - - - -----0.07
Cladonia coniocrea (Flörke) Sprengel - - 1.00 0.53 0.28 - - - - - - 0.00
Cladonia fimbrata (L.) Fr. - - - - 0.03 - - - - - - - - 0.56 - - 0.11
Cladonia humilis (With.) J.R.Laundon - - - - - - - - - - - - - 0.22 - - 0.04
Cladonia parasitica (Hoffm.) Hoffm. - - - - - - - - - - - - - 0.06 - - 0.01
Cladonia spp. - 1.25 2.59 3.97 0.19 - 0.50 2.69 0.91 3.03 0.03 0.00 0.16 0.22 2.41 3.41 0.00
Cladonia squamosa Hoffm. - - 0.34 - - - 0.34 - - - - -----1.33
Collema subflaccidum Degel. - - - - - - - 0.09 0.31 - 0.53 0.06 - - - - 0.04
Dendriscocaulon umhausense (Auersw.) Degel. - - 0.06 - - - - 0.03 - - - -----0.06
Evernia mesomorpha Nyl. - - - - - - - - - - - - 0.06 - - - 0.01
Heterodermia obscurata (Nyl.) Trevisan - - 0.06 - 0.41 - - - - - 0.03 -----0.00
Hypogymnia physodes (L.) Nyl. 2.47 0.25 0.97 0.22 0.13 1.06 1.75 1.22 - 0.09 - - 0.03 - - 0.16 0.03
Leptogium cynanescens (Rabenh.) Körber - - 0.06 0.34 - - - 0.47 0.28 - - - - 0.47 - - 0.52
Leptogium laceroides (B. deLesd.) P.M. Jørg. - - 0.00 - - - - - - - 0.09 -----0.10
Lobaria pulmonaria (L.) Hoffm. - - 1.41 0.97 1.69 - - - 2.59 - 3.75 3.78 - 4.84 5.78 2.59 0.01
Lobaria quercizans Michaux - - 0.66 0.63 1.38 - - 0.03 2.53 - 2.00 0.78 - 3.69 0.72 0.84 1.71
Lobaria scrobiculata (Scop.) DC. - - 1.06 1.22 1.50 - - 0.03 0.41 - - 0.56 - 0.06 0.22 1.13 0.83
Melanelixia subaurifera (Nyl.) O. Blaco et al. 0.03 - - - - - - - 0.28 - 0.19 0.69 0.03 - - - 0.39
Pannaria rubiginosa (Ach.) Bory - - - - - - - - - - 0.03 -----0.08
2007 R.P. Cameron, T. Neily, and D.H.S. Richardson 7
Table 2, continued.
Species 1 2 3 4 5 6 78 910111213141516Mean
Parmelia squarrosa Hale 5.09 9.88 5.56 7.00 6.03 0.75 5.53 7.00 1.50 9.44 1.78 3.50 2.94 3.00 1.34 - 0.00
Parmelia sulcata Taylor 4.31 - - - - 6.84 - - 0.06 - 0.13 2.25 1.25 0.13 - 4.19 4.40
Peltigera praetextata (Flörke ex Sommerf.) Zopf - - - 0.06 - - - - - - - ----0.13 1.20
Phaeophyscia pusilloides (Zahlbr.) Essl. - - - - - 0.03 - - - - - -----0.01
Phaeophyscia rubropulchra (Degel.) Essl. 0.59 - - - - - - 0.03 0.38 - 0.03 0.00
Physconia detersa (Nyl.) Poelt Syn. 0.16 - - - - - - - - - 0.16 0.09 - - - - 0.06
Physica spp. - - - - - - - - - - 0.09 -----0.03
Physicia adscendens (Fr.) H. Olivier 0.59 - - - - - - - - - - 0.01
Physicia millegrana Degel. 1.38 - - - - - - - - - - -----0.04
Platismatia glauca (L.) Culb. & C. Culb. 0.28 2.13 0.78 - - - 1.84 0.38 - - - - 0.13 - - - 0.09
Pseudocyphellaria crocata (L.) Vainio - - 0.31 0.13 0.13 - - - - - - -----0.35
Punctelia rudecta (Ach.) Krog 0.06 - - - 1.09 - - 0.16 1.22 0.13 0.25 0.34 - - 0.16 0.69 0.04
Pyxine sorediata (Ach.) Mont. - - - - - - - - - - 0.13 0.34 - - - 0.09 0.26
Ramalina Americana Hale - - - - - - - - - - - - - 0.13 - - 0.04
Ramalina dilacerata (Hoffm.) Hoffm. - 0.16 - - - - - 0.03 - - - -----0.01
Ramalina farinacea (L.) Ach - - - - - - - - - - 0.25 0.01
Ramalina roseleri (Hochst. ex. Schaerer) Hue 0.03 0.03 0.13 - 0.34 - - - - - - -----0.02
Usnea filipendula Stirton - 0.06 0.03 0.13 0.25 - - 0.16 - - - 0.03 0.13 - - - 0.03
Usnea lapponica Vainio - - - - - - - - - - - - - 0.06 - - 0.05
Usnea spp. Dill. ex. Adans. 0.38 0.25 0.22 - 0.66 - - - - - 0.03 - 0.44 - - 0.41 0.00
Usnea strigosa (Ach.) Eaton - - - - 0.03 - - - - 0.09 - - - 0.03 - - 0.15
Usnea subfloridana Stirton 0.75 0.75 0.44 - - - 0.06 - - - - -----0.01
Number of species 12 9 16 10 13 4 5 14 11 5 17 11 10 12 6 9
8 Northeastern Naturalist Vol. 14, No. 1
only six study plots. Species richness was highest at 16.8 and 22.3 km upwind
and downwind of city center, respectively, and the lichen frequency generally
increased with distance from the city center. Ten cyanolichens were found,
but none occurred less than 10 km from city center. Five Usnea species were
recorded; several species being discovered less than 2 km from city center.
When the frequency of particular lichen species was plotted against
distance from city center, four general patterns emerged (Fig. 2): 1) species
whose frequency was highest nearest the city center, 2) species whose
frequency was highest immediately surrounding the city center, 3) species
whose frequency increased with increasing distance from city center, and 4)
species which showed no particular pattern with respect to distance from
city center. Pattern-1 species—Melamelixia subaurifera (not shown),
Parmelia sulcata (not shown), Physcia adscendens, and Physcia
millegrana—peaked in frequency at 4.7 km downwind of city center, and
none of these species occurred greater than 0.6 km upwind of city center.
Species whose frequency peaked just outside the city center tended to peak
at either 6.8 km or 11.8 km downwind and 6.91 km to 0.6 upwind. Pattern-2
species included Hypogymnia physodes (not shown), Parmelia squarrosa
(not shown), Platismatia glauca, Ramalina dilacerata (not shown), Usnea
filipendula (not shown), Usnea subfloridana (not shown), and Usnea spp.
Figure 2. Frequency of six lichen taxa along an air-quality gradient in Halifax, NS,
Canada. Horizontal axis is distance from city centre. Negative integer indicates upwind
of city centre. Vertical axis is lichen frequency occurrence within 10 x 10-cm grid.
2007 R.P. Cameron, T. Neily, and D.H.S. Richardson 9
Species which increased in frequency from city center (pattern 3) included
Cladonia spp., Leptogium spp. (not shown), Lobaria pulmonaria, L.
quercizans (not shown), and L. scrobiculata (not shown). Pseudocyphellaria
crocata (not shown) increased in frequency away from the city center on the
upwind side but was not found on the downwind side. Individual species of
the genera Leptogium and Cladonia showed no particular pattern, although
none occurred close to city center. However, when species of Cladonia and
Leptogium were lumped into genera, a pattern 3 was evident (Table 2).
Combining species in the genera Parmelia, Physcia, and Ramalina resulted
in all genera showing a lack of pattern with respect to city center, thus
masking patterns demonstrated using particular species, notably, Parmelia
squarrosa, Parmelia sulcata, Physcia adscendens, Physcia millegrana, and
Multidimensional scaling analysis suggests an air-quality and elevation
gradient (Fig. 3). The best stress value (0.075) obtained for the representative
Figure 3. Multidimensional scaling map of lichen communities across an air-quality
gradient in Halifax, NS. Numbers next to dots indicate distance from city center with
negative intergers indicating upwind plots. All plots occur at less than 250 m asl,
except plots 105.03 and 102.31, which have elevation greater than 250 m asl.
10 Northeastern Naturalist Vol. 14, No. 1
space was with 3 dimensions, and additional dimensions did not significantly
decrease stress. Patterns in frequency plots are reflected in the air-quality
gradient of the MDS plots, which show three zones of influence. Downwind of
city center consists of zone 1 (city center to 4.7 km), zone 2 (6.8 km to about 20
km), and zone 3 (greater than 20 km). Upwind of city center air-quality zones
are reduced with zone 1 (city center to 5.5 km), zone 2 (5.5 km to 6.9 km), and
zone 3 (greater than 6.9 km). The MDS map suggests different lichen communities
above and below about 250 m asl.
The air-quality gradient found in this study is consistent with several
other studies in Nova Scotia. Ward (1968) was the first to find a difference in
lichen communities in downtown Halifax compared with the outer parts of
the city. Cameron (2004) showed a significant difference in species richness
in northern Nova Scotia compared with southern Nova Scotia, where since
1990, there has been a higher deposition of acid rain and non-marine sulphate.
A provisional suite of suitable lichen indicators of air quality in Nova
Scotia are given in Table 3. The value of lichens as indicators of air quality
has been well established (Hawksworth and Rose 1970, Kauppi and
Mikkonen 1980, Richardson 1992, Zobel 1988). However, the use of a
limited number of indicator species has been criticized by Wirth (1988), who
considers that phytosociological methods are better able to distinguish the
effects of air pollution from influences such as topography and climate
(Wirth 1988). This is important for areas with varied relief or climate, but
Nova Scotia has a gentle topography and limited climatic variation.
In the present study, the proposed pollution-sensitive indicator species for
Nova Scotia are almost all cyanolichens. This finding is consistent with
studies in Europe and North America that indicate that cyanolichens are
particularly sensitive to acid rain, sulphur dioxide, and nitrogen oxides (Gilbert
1986, Hallingback 1989, Hawksworth and Rose 1970, Sigal and Johnston
Table 3. Provisional lichen indicators of air quality for Nova Scotia.
Cladonia spp. Lobaria quercizans
Leptogium spp. Lobaria scorbiculata
Lobaria pulmonaria Pseudocyphellaria crocata
Intermediate pollution-tolerant lichens
Hypogymnia physodes Ramalina dilacerata
Parmelia squarrosa Usnea spp.
Pollution- tolerant lichens
Melamelixia subaurifera Physcia adscendens
Parmelia sulcata Physcia millegrana
2007 R.P. Cameron, T. Neily, and D.H.S. Richardson 11
1986). This also seems true for northeast North America where Maass and
Yetman (2002) found a 90% decline in the number of sites where Erioderma
pedicellatum occurred over the last two decades in Nova Scotia. They attribute
this decline, in part, to acid precipitation. Cyanolichens are especially
affected because nitrogen fixation, essential for their survival, is more sensitive
to acid rain than photosynthesis (Gries 1996).
The only taxa that seemed to be pollution-sensitive but was not associated
with cyanobacteria, was Cladonia. The tolerance of this genus to air pollution
varies. Neitlich et al. (2003) found Cladonia to have an intermediate sensitivity
to pollution in Idaho, whereas McCune et al. (1998) found Cladonia only
in non-urban/industrial plots in Colorado. In Quebec, Cladonia occurred in
the range of air-quality zones in an area subject to fluoride pollution (LeBlanc
et al. 1972a), but in Montreal these lichens only occurred in the intermediate
to pure air-quality zones (LeBlanc and De Sloover 1970). Finally, Brodo
(1966) found Cladonia coniocraea to be tolerant to pollution in New York
City. Thus, the pollution sensitivity of Cladonia may vary between species,
and its occurrence may reflect microclimate. In this study, it may be responding
to moisture regimes, which may be wetter in rural forest than in suburban
woods or more scattered urban trees.
Many of the lichen species categorized as intermediate air-quality indicators
in this study, have been noted as having this level of sensitivity in
other studies. Thus, Hypogymnia physodes was found to be of intermediate
sensitivity in field studies in Europe (Hawksworth and Rose 1970) and
Canada (LeBlanc et al. 1972a), as well as under laboratory conditions (Marti
1983). Usnea has been rated as intermediate in terms of sensitivity to air
pollution in Idaho (Neitlich et al. 2003), Colorado (McCune et al. 1998), and
in Quebec (LeBlanc et al. 1972a), but as sensitive in the United Kingdom
(Hawksworth and Rose 1970). Platismatia glauca is assessed as an intermediate
indicator of sulphur dioxide pollution in England (Hawksworth and
Rose 1970), but the genus as a whole was considered intermediate to tolerant
in Idaho (Neitlich et al. 2003). Finally, Hawksworth and Rose (1970) found
Parmelia saxatilis to be intermediate to tolerant to sulphur dioxide pollution
in England, and this seems to be true of its close relative P. squarrosa, which
is common in Canada. Although less studied, Melamelixia subaurifera is
known as intermediate to air pollution in North America (Denison 1973, Le
Blanc and De Sloover 1970, LeBlanc et al. 1972a). Little work has been
done on the sensitivity of Ramalina dilacerata to air pollution. Studies on
other species within the genus indicate varying sensitivity (Hawksworth and
Rose 1970, Marti 1983).
Species identified as pollution tolerant in this study have all been well
documented in other studies. Parmelia sulcata, Physcia millegrana, and
Physcia adscendens have been identified as pollution tolerant to sulphur
dioxide and air pollution in Europe and North America (Diamantopulos et al.
1992; Hawksworth and Rose 1970; Hoffman 1974; Le Blanc and De Sloover
1970; Le Blanc et al 1972a, 1972b; McCune 2000; McCune et al. 1997).
12 Northeastern Naturalist Vol. 14, No. 1
Several species documented in this study were found to be poor indicators
of air quality, but have been useful indicators in other studies. For
example, Phaeophyscia rubropulchra and Punctelia rudecta have been used
as indicators in several studies in North America. Brodo (1966) found
Punctelia rudecta to be an intermediate indicator of air pollution in New
York State. McCune et al. (1997) indicated Phaeophyscia rubropulchra was
tolerant of air pollution in the southeastern US. McCune et al. (1998) and
Neitlich et al. (2003) suggested species of the genus Phaeophyscia were
generally tolerant of air pollution in Colorado and Idaho. Collema
subflaccidum is a cyanolichen and might be expected to be intolerant of air
pollution. Although this species did not occur in Halifax City, it occurred
only sporadically in plots outside the city. In the present study, between-plot
variation was high for all three species, with no consistent response to
environmental variables. These species may occur too sporadically in Nova
Scotia to be useful indicators.
Elevation gradients can affect lichen communities, but where this has been
observed, there is a much greater elevation change than in Nova Scotia. For
example, elevation ranged from 1020 m to 4399 m asl in Colorado (McCune et
al. 1998), a difference of 3378 m, and from sea level to over 2000 min the
southeast US (McCune et al. 1997). In contrast, elevation in Nova Scotia
ranges from sea level to 500 m, but highland regions are generally only about
300 m asl (Davis and Browne 1998). No clear effect of elevation on macrolichen
communities was observed in the present study, and more data would
be needed to help distinguish elevation from other environmental parameters.
We would like to thank Nova Scotia Environment and Labour Air Quality Branch
for financial and technical support and Protected Areas Branch for providing staff
time. We would also like to thank Julie Towers, Irwin Brodo, and two anonymous
reviewers for helpful comments on the manuscript.
Asta, J., W. Erhardt, M. Ferretti, F. Fornasier, U. Kirschbaum, P.L. Nimis, O.W.
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