The Lichens and Bryophytes of Rhyolite and Pumice-Tuff Rock Outcrops in Snowdonia, and Some Factors Affecting Their Distribution

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Abstract
The lichen and bryophyte flora of rhyolite and pumice-tuff rock outcrops in Snowdonia, Gwynedd, is described and compared using indicator species analysis and reciprocal averaging ordination. The rocks possessed distinct and contrasting floras. The rhyolite was dominated by Lecidea tenebrica, but the pumice-tuff was not dominated by any single species and the flora was variable. Factors affecting the flora included nutrient-enrichment by birds, the degree of exposure, water seepage and the physicochemical characters of the individual rocks. Experiments designed to compare the water-holding capacity and water-retaining power of the rocks showed that the pumice tuffs were more porous, but lost water more rapidly than the rhyolites. This resulted in both rocks drying out in approximately the same time under a steady air current. The weathered skin of the rhyolite acts as a water reservoir for the lichen, with > 75% of the absorbed water attributable to the rock. The unweathered rhyolites were virtually impervious to water, and the consequences of this are discussed. A method for measuring and analyzing rock microtopography using the light microscope is described.

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