Savaria - A Vas Megyei Múzeumok értesítője 15. (1981) (Szombathely, 1988)

Természettudomány - M. R. D. Seaward: A zuzmók alkalmazása a levegő szennyezettségének indikálására

the rise and fall of pollution levels (SEAWARD 1976, BRIGHTMAN and SEAWARD 1977). Detailed maps showing the reinvasion of L. muralis into the West Yorkshire conurbation, England (at a rate of с 9 km per annum) have been equated to S0

2 data determined from instrumentation at 104 sites. The importance of time-lag between pollution levels dropping below an identifiable threshold has been investigated, since no effective use can be made of lichen maps for predicting S0

2 concentration if this factor has not been credibly established. Therefore computer simulations based on detailed long-term field mapping data in digital form were used to compare rates of invasion under different time sequence regimes whilst holding a constant threshold of S0

2

. From this work it was possible to show, for example, that L. muralis requires five years at a threshold pollution level of 120 ug пГ

3 before responding to ameliorating conditions; a sudden increase in pollution produces a much more rapid effect. Further computational models based on this field data have been used to predict the response of L. muralis to ameliorating conditions with the West Yorkshire conurbation (HENDERSON-SELLERS and SEAWARD 1979). The results underline the fact that S0

2 level alone is no longer the major factor affecting lichen distribution within urban areas implementing clean air legislation. At certain sites in the West Yorkshire conurbation where the presence of L. muralis was restricted to asbestos-cement roofing (pH с 9.0-12.0) at the beginning of the investigation, it had spread to cement-tile roofing (pH с 7.5-8.5) at a later date, indicating a reduction in mean daily S0

2 concentration of с 10 ug m~

3 per annum over a 10-year period. Its rate and mode of spread at one of these sites is being carefully monitored : a detailed time and space analysis of propagule influx and thallial growth measurements has been undertaken annually from 1971 to 1980 (SEAWARD 1980, 1982). Although there is an influx of propagules at a reasonably constant rate throughout the period, there is little or no spread of propagules from existing thalli within the first three years (cohorts A-C) but there is an increase in aggregation thereafter (cohorts D-G). Furthermore, the growth performances of these earlier cohorts are dissimilar to those of the letter cohorts : annual increase in diameter (mm) A 3.92 В 4.01 С 3.68 D 2.66 E 2.67 F 2,38 G 2,29 It is difficult to make valid hypotheses from above observations, but nevertheless some tentative conjectures may reasonably be made. Although L. muralis is a successful lichen in urban environments and has a relatively rapid growth rate, it seems likely that in the long term it would be unsuccessful in competition with other slower growing species. It is remarkable that the earlier cohorts constantly maintained a higher growth rate, whereas the later cohorts constantly maintained a lower growth rate during the 10-year analysis, despite a gradual fall in the S0

2 level throughout the entire period and climatic variation from year to year. Since the roofing tiles were known to be more than 40 years old, any change in the physicochemical nature of the substrate, as a result of cohort 47