Impact Type: Direct exposure to pollutant
Little has been published as to the precise effects of air pollution on epiphytic bryophytes and even less on terrestrial species. It would appear that SO2 is toxic to many, but not all, bryophyte species. As well as being sensitive to SO2, some species may also be affected by other pollutants (Adams & Preston 1992, Mulgrew & Williams 2000). Most of the work published used fumigation of established gametophytes or reciprocal transplant experiments in the field (Gimeno & Deltoro 2000, Hallbdcken & Zhang 1998, Mulgrew & Williams 2000). It has been shown that SO2 can acidify and the buffering capacity of bark (Coker 1967). However, rotten bark has a greater buffering capacity, and this provides a possible explanation as to why it is preferentially colonised in areas of higher air pollution.
Work carried out by Gimeno & Deltoro (2000) has shown that exposing Frullania dilatata (a liverwort) to high SO2 concentrations (1.7 ppm SO2 for 4 h daily during 2 consecutive days) results in severely impaired photosynthetic performance. However, it should be noted that the concentrations used where much higher than those encountered by plants in the field. Adams and Preston (1992) report that F. dilatata, Radula complanata, Porella platyphylla and Isothecium myurum are absent from areas of high SO2 pollution. Takaoki & Mitani (1986) also found photosynthetic impairment in the liverworts Marchantia polymorpha and Conocephalum conicum when exposed to SO2, although these were far less sensitive to than F. dilatata.
Other bryophytes such as the liverwort Lophocolea heterophylla and the mosses Hypnum cupressiforme, Mnium hornum, Isothecium myosuroides, Dicranum scoparium and Pleurozium schreberi are more tolerant of exposure to SO2 (Adams & Preston 1992, Hallbdcken & Zhang 1998).
Adams & Preston (1992) have produced a scale of bryophyte sensitivities to SO2 similar to the Hawksworth & Rose (1970a,b) scale for lichens, by comparing the sequence of extinctions (using historical records for England) against rising levels of SO2, and recent distribution of species compared to the known SO2 levels between 1950 and 1970. Bryophytes have been much slower to respond to reductions in the level of SO2 than lichens (Adams & Preston 1992).