AIR HYGIENE REPORT no. 10
Contents

III GASEOUS POLLUTANTS

1 Bryophytes

  1.1 Introduction
  1.2 Sulphur dioxide (SO₂)

1.2.1 Species distribution
1.2.1.1 Regional
1.2.1.2 Urban and industrial
1.2.2 Transplants and effects

  1.3 Nitrogen and its compounds (N, NO_x and NH₃)
  1.4 Other gases

1.1 Introduction

Bryophytes are generally easier to identify and are as equally susceptible to air pollution as lichens, yet less attention has been paid to their use in gaseous air pollution monitoring. A reason for this may be the larger number of lichen species (particularly epiphytic species) available for air pollution monitoring (Adams and Preston, 1992). Most studies are associated with regional and urban sulphur dioxide (SO₂) contamination. On a national and multi-national monitoring scale, bryophytes are used more as bioaccumulative indicators of aerial metal contamination than as bioindicators of gaseous air pollution. References

Most literature regarding gaseous pollutants and bryophytes is concerned with measuring and assessing impacts of pollution on bryophyte communities as a valuable ecological group in their own right. Hallingback and Tan (1996) discussed the IUCN/IAB Bryophyte Committees' commitment to endangered bryophytes and presented a skeleton action plan for these species. The conservation and protection of these 'key-stone' species was emphasised and air pollution was listed as one of the major threats to species. Biomonitoring of air pollution impacts on bryophytes therefore plays a role in biodiversity and conservation. Most of the published literature on research with regard to bryophytes and gaseous air pollutants concentrates on pollutant effects and harm and little is available in terms of their role as biomonitors and bioindicators. However, such work may still be used to ascertain pollution problems and indicate potential threats to other biological systems including humans.

Standard practices such as sampling, analysis and species selection in bryophyte monitoring are underdeveloped in comparison to lichen monitoring.

1.2 Sulphur dioxide (SO₂)

1.2.1 Species distribution

1.2.1.1 Regional

Adams and Preston (1992) presented comprehensive evidence of long-term effects of gaseous pollutants, particularly SO₂, on bryophyte distribution in the UK. The methods, patterns and correlations discussed highlight air pollution trends and much of the information is highly applicable to gaseous pollutant biomonitoring using bryophytes. In order to draw their conclusions, the authors collated evidence from various sources. Analysis of the quaternary sub-fossil record demonstrated that the reduction in Sphagnum cover in peat profiles corresponded with the presence of soot deposits due to the advent of the industrial revolution. Bryophyte distributions determined from the British Bryological Society's mapping scheme from 1960 onwards and herbarium collections at national and local scales appeared to be correlated with long-term direct air monitoring data. References

The national recording scheme revealed bryophyte species which had been adversely affected by atmospheric pollution on a national scale (Table 3.1).

More intensive distribution studies in the heavily polluted London and Essex areas enabled the development of an epiphytic bryophyte sensitivity scale (Table 3.2), similar to the highly recognised lichen zonation scale drawn up by Hawksworth and Rose (1970). It is important to note that the position of a species on the scale will vary with humidity in the area and acidity of the bark substrate. Epiphytic species may be indirectly affected by the soil through influences on the bark chemistry, as demonstrated by Gustafsson and Eriksson (1996) in aspen Populus tremula in Sweden. Some species may prefer limestone to trees. In addition, it would be difficult for this scale to consider the effects of events such as short seasonal elevations in SO₂ associated with prevailing winds. It is difficult to distinguish between the response of moss communities subjected to a few severe air pollution events or continued chronic exposure. It would prove difficult to develop a scale to include mosses in polluted environments, which may be protected against the effects of air pollutants if located in sheltered areas such as deep valleys (Winner, 1988). (Details of Hawksworth and Roses' scale are discussed in Section 2). Although not as detailed or as accurate as the lichen scale, the proposed bryophyte scale lends itself as a positive biomonitoring tool. References

National and local lists (Table 3.1 and 3.2) showed relatively good correlation. Disparities may be due to factors other than SO₂, which may cause local variation. Furthermore, local species decline due to localized high SO₂ levels may not be reflected on the national scale.

The same authors also reported similar reductions in the distribution of terrestrial and saxicolous bryophytes in relation to high SO₂ levels.

As is the case with lichens, recent declines in SO₂ levels are reflected in bryophyte recolonisation to formerly polluted areas. However, the sequence of decline in species associated with SO₂ pollution may not be as marked in bryophyte recolonisation for several reasons:

• Inconspicuous species may have simply been missed in previous studies.
• Bryophytes have shown a slower response to reduced SO₂ levels than lichens.
• Species may colonise areas where they were not previously found.
• Species may not recolonise areas where they were previously found because of changes in habitat. In a study of epiphytic bryophytes in Groningen in the Netherlands, (van Zanten, 1992) certain species (e.g. Leucodon sciuroides and Orthotrichum lyelli) became scarce in areas of relatively good air quality due to Dutch Elm Disease. These species rely on vulnerable Ulmus sp. for survival, illustrating the importance of available habitats even in improved air pollution conditions.
• Increasing levels of other atmospheric pollutants such as nitrogen oxides and ozone may reach threatening levels.
• Recolonisation processes depend on the mobility of the species and the proximity of source populations.

Changes in bryophyte cover in forest ecosystems in response to air pollution have been reported. In Estonia, two groups of moss in Scots pine forests were observed: moss whose distribution depended on the level of air pollution and those whose distribution was more dependent on other factors such as climate and soil (Vilde and Martin, 1996). Bryophyte cover also increased in thickness along a pollution gradient from the polluted north-east to less polluted south-west. In a study of bryophyte communities in two Atlantic forests in Brazil, higher percentage cover and biomass of bryophytes were observed in the least polluted forest (Rebelo et al., 1995). References

1.2.1.2 Urban and industrial

In a Finnish study of effects of metal, chemical and fertiliser plants on forest floor vegetation in the vicinity of the plants, the common forest bryophytes (Hylocomium sp. Pleurozium dicranum) were more sensitive than lichens. Further away from the factories Ceratodon purpureus and Pohlia nutans became frequent (Vaisanen, 1986).

Huber (1992) found that bryophyte species distribution in the Swiss Canton of Basel-Stadt was similar to that in the proximity of a cellulose factory. Furthermore, the area of suspected good air quality in the Canton contained similar species distribution to an area 200 m above the cellulose factory.

In Romania, the zone of influence on bryoflora of the industrial Alba district extended to a distance of 20 km (Plamada, 1986). The main atmospheric burden was from SO₂.

Winner (1988) reviewed the responses of bryophytes to air pollution by presenting an overview of North American studies in this field. Types of studies include determination of impact zones, Indices of Atmospheric Purity (IAP) methods, bioaccumulation and physiological responses to gaseous air pollution.

IAP methods have been applied to moss communities in much the same way as to lichens (Section 2.2.1.2). IAP values have been calculated and mapped with respect to urban areas and point emission sources. Zones have been determined representing changes in bryophyte communities relative to the location of an air pollution source. According to Winner (1988), IAP values should be regarded tentatively and cannot be compared between different sites for the following reasons:

• All changes in moss communities are too often attributed to air pollution alone, whereas other environmental factors such as elevation, humidity and temperature may have an effect.
• Cities generally have higher temperatures and fewer appropriate substrates than rural areas.
• IAP may not be an accurate measure of the complexity of bryophyte communities.

1.2.2 Transplants and effects

The biochemical and physiological effects of SO₂ on bryophytes will not be discussed here. Recent reviews in this area are presented by Brown (1995), Kovács (1992a) and Winner (1988). A recent review of transplantation exercises using bryophytes can be found in Brown (1995). These studies are concerned with assessing effects of air pollutants on bryophytes rather than their use as biomonitors.

A technique that was not mentioned by Burton (1986) was bryometers. Bryometers were developed in Japan to measure phytotoxic air pollution (Taoda, 1973). Mosses were placed in small, transparent plant chambers and two chambers were placed at a study site. One chamber was filled with ambient air and the other exposed to filtered, pollution free air. In this way the presence or absence of ambient air pollutants could be detected. References

1.3 Nitrogen and its compounds (N, NO_x and NH₃)

Much of the published literature in relation to inorganic nitrogen (N), acid rain and bryophytes is concerned with the effects of this type of deposition on bryoflora. This research is gaining profile in response to the coincidence of reduced atmospheric SO₂ levels and increases in nitrogen deposition and acid rain. However relatively few attempts have been made to use bryophytes to directly monitor nitrogen deposition.

The following paragraphs attempt to highlight aspects which may be suitable for monitoring purposes in the future. Due to their dependence on atmospheric inputs of nutrients, total N content in bryophyte tissues can reflect atmospheric inputs of N.

Press et al. (1986) chose two ombrotrophic mires dominated by bryophytes to study the ecological significance of increased atmospheric nitrate deposition in the UK Sphagnum sp. transplanted to a relatively polluted site from a non-polluted site showed an increase in tissue nitrogen concentration and reduced growth in the polluted site. The authors concluded that elevated nitrogen deposition in the polluted area might be affecting the growth and metabolism of ombrotrophic Sphagnum sp.

Comparison of bryophyte tissue N content in herbarium samples and field samples collected in 1989 illustrated temporal N deposition trends in the UK (Pitcairn et al., 1995). The study demonstrated increasing N deposition levels apparent throughout the UK and potential harm to the ecosystems themselves. Percentage increase in tissue N content at a range of sites of varying pollution climates corresponded with atmospheric increases in N levels. Certain sites remained unpolluted over the thirty-year period (e.g. Beinn Eighe National Nature Reserve in Scotland) and showed insignificant elevations in bryophyte tissue concentrations. A 62% increase in tissue N content of ombrotrophic Sphagna at Moor House, Cumbria, is of concern. The relationship between tissue N content with atmospheric N inputs was expressed as: N_Bry = 0.62 +0.022 N_Dep i.e. tissue N increases at 0.022 mg g^-1dry weight per kg total N deposited. The authors recommended the application of this equation as a rough measure of atmospheric N deposition in areas where monitoring equipment is impractical. References

Mäkipää (1995) proposed changes of forest floor moss biomass as an early warning indicator of atmospheric N and S deposition in boreal forests. In a study of acidic deposition, experimental plots of boreal forest in Finland were exposed to annual treatments of ammonium sulphate over a four year period. The forest floor mosses, dominated by Pleurozium schreberi and Dicranum polysetum, were more sensitive to N and S deposition than vascular plants. Biomass of bryophytes decreased by 60% over the study period and N content in moss tissues was greater than in the control areas. Although this experiment was not undertaken under natural conditions, such experiments are valuable in enhancing our knowledge of the response of bryophytes to N deposition if they are to be used as biomonitoring tools.

Woolgrove and Woodin (1996) analysed tissue-N content in the late snowbed bryophyte, Kiaeria starkei, collected from sites in the Scottish Highlands. Results corresponded with the atmospheric N loads to which the bryophyte was exposed, which due to acid flushes during snowmelt and the sensitivity of the species pose a threat to these snowbed species.

Urban and industrial point source studies in relation to nitrogen oxide pollution gradients are limited. A study of the distribution of epiphytic bryophytes in Naha City in Japan showed that few species were found in the city centre where NO₂ levels were > 0.02 ppm (Inui and Yamaguchi, 1996). An interesting observation of this study was the lack of correlation between IAP values and NO₂ concentration. If bioindication of NO₂ pollution using bryophytes and lichens is to develop, additional research into the applicability of indices and methods used in SO₂ monitoring should be encouraged.

1.4 Other gases

In contrast to Burton's (1986) review, little published literature was found in relation to fluoro-compounds and bryophytes. With regard to ozone (O₃), literature is concerned with the effects of this gas on bryophytes since they are regarded as an important ecological group. An open chamber fumigation experiment of the effects of elevated O₃ concentrations on forest floor moss cover was undertaken by Stanosz et al. (1990). Significant negative correlations between moss cover (predominantly Ditrichum pusillum) and O₃ concentrations were demonstrated. In the UK, Sphagnum recurvum and Polytrichum commune were exposed to long-term chronic O₃ concentrations (Potter et al., 1996). Both species showed a reduction in growth when exposed to O₃ in comparison to control experiments. Sphagnum appeared to be more sensitive to fumigation than Polytrichum. Fumigation studies will enable effects to be determined which could possibly be used as bioindication responses, since bryophytes may be more sensitive to O₃ than higher, economically important plants. References

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