Ecological and Historical Factors in the Distributions of the British Helianthemum Species

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Abstract
An account is given of the habitats occupied by the 3 British species of Helianthemum s. str. H. chamaecistus is widely distributed in short calcareous grasslands in England and Wales, and in Agrostis-Festuca grasslands on more acid soils in Scotland. H. apenninum and H. canum have disjunct distributions confined to small areas of open xeric grasslands on hard limestones near the west coast of Britain. Their ecological behavior and distribution pattern appears to accord closely with that found on the Continent. Field evidence suggests that the distribution of H. chamaecistus is determined mainly by contemporary ecological factors; while the distributions of the 2 rarer species are further limited by the occurrence of suitable exposed limestone cliff edges, on which they survived through the post-glacial forest cover, and from which they have attained only a limited secondary spread in a disjunctly distributed habitat.