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Macrolichen Indicators of Air Quality for Nova Scotia
Robert P. Cameron, Thomas Neily, and David H.S. Richardson

Northeastern Naturalist, Volume 14, Issue 1 (2007): 1–14

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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. Introduction 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 North America. 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 - 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 obscure trends. 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 ( 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. Methods Study area 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. Frequency estimates 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). Data analyses 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. Results 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. Plot number 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. Plot number 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 Ramalina delicerata. 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. Discussion 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. Pollution-intolerant lichens Cladonia spp. Lobaria quercizans Leptogium spp. Lobaria scorbiculata Lobaria pulmonaria Pseudocyphellaria crocata Intermediate pollution-tolerant lichens Hypogymnia physodes Ramalina dilacerata Parmelia squarrosa Usnea spp. Platismatia glauca 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. Acknowledgments 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. Literature Cited Asta, J., W. Erhardt, M. Ferretti, F. Fornasier, U. Kirschbaum, P.L. Nimis, O.W. Purvis, S. Pirintsos, C. Scheidegger, C. Van Haluwyn, and V. Wirth. 2002. Mapping lichen diversity as an indicator of environmental quality. Pp. 273–279, In P.L. Nimis, C., Scheidegger, and P.A Wolseley (Eds.). Monitoring with Lichens - Monitoring Lichens. 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