The Influence of Metal Availability on the Bryophyte and Macrolichen Vegetation of Four Rock Types on Skye and Rhum

Abstract

SUMMARY (1) A quantitative survey of the bryophyte and macrolichen vegetation of outcrops of Durness limestone, Torridonian sandstone, basalt and ultrabasic rocks on Skye and Rhum was undertaken to examine correlations between flora and rock type. (2) Analysis of rock leachates confirmed that the solubility of calcium from Durness limestone greatly exceeded that of all other elements from all rocks. Detectable quantities of Ca, Mg, Fe, Al and K were leached in 24 h from basalt and ultrabasic rocks by distilled water and EDTA solution, but levels of these elements in Torridonian sandstone leachates were extremely low. The major trend in the non-calcareous rocks was an increase in availability of Fe, Al and Mg in the order sandstone < basalt < ultrabasic rocks. (3) The major floristic feature, revealed by comparison of species-abundances on each rock type using a modified principal coordinates analysis of the vegetation samples, was the uniqueness of the limestone flora compared with that of the other rocks. Overlap of the limestone flora with that of the non-calcareous substrata was due to a few relatively uncommon species, and no taxon was abundant on both calcareous and non-calcareous rocks. (4) The total Ca concentration of limestone bryophytes was consistently higher than that of the calcifuges, on average by a factor of 17. Fe and Al, although generally present at low concentration in calcicoles, were not consistently high in calcifuges, indicating that elevated levels of these elements may not be a general feature of the chemical environment on non-calcareous rocks. (5) Floristic ordination also partially separated vegetation samples on the Torridonian sandstone from those on basalt and ultrabasic rocks. This separation was mainly due to differences in the relative frequencies of the calcifuge species, and is thought to reflect chemical rather than physical and climatic factors. The basalt and ultra-basic rocks may provide generally higher levels of mineral nutrients, more potentially toxic metals, and/or a higher Mg: Ca balance than the sandstone.