62 Allan Pentecost & J. H. Williams GRAPHIS SCRIPTA 13 (2002) Figure 2. Climatic data for the Snowdon Thamnolia sites during 1999 Precipitation (3-day totals) at the Snowdon summit weather station. dry and total acid deposition than did the 20 km × 20 km Cadair Idris square (Stevens et al. 1997). Another possible explanation is grazing pressure. During summer, sheep graze the low turf of Crib y Ddysgl and are bound to disrupt thalli by trampling. There was clear evidence of grazing adjacent to the sites so some damage appears inevitable, but it is not clear whether the intensity of grazing has varied historically, or how trampling affects reestablishment of the lichen fragments. Cooper et al. (2001) noted damage to High Arctic lichens and its amelioration by sustained periods of precipitation. However at our Snowdon sites there was no evidence of regeneration of damaged thalli and the population as a whole appears to be declining rapidly. Another possible cause of these changes is the global increase in temperature over the past 30 years. Since Cadair Idris and Snowdon receive almost identical weather, but only Snowdon has suffered a decline in Thamnolia, such changes are unlikely to be the cause of decline. Acknowledgements We thank Dr. M. O. Hill for some of the bryophyte identifications and Jack Grasse for information on the distribution of Thamnolia on Cadair Idris. The senior author is grateful for a small travel grant from the British Lichen Society. References Barry, R. G. 1981: Mountain weather and Climate. Methuen, London and New York.
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