The frost resistance of ericoid heath plants in the British Isles in relation to their biogeography

Aim To determine whether rare British ericoid heaths are restricted to the oceanic fringes of south‐western England and western Ireland because of their sensitivity to frost. Location The Wirral Peninsula, Cheshire, England. Shoots of the common heaths ( Calluna vulgaris (L.) Hull, Erica tetralix L., Erica cinerea L.) were obtained from Thurstaston Heath and those of the rarer heaths ( E. mackaiana Bab., E. ciliaris L., E. erigena R. Ross and E. vagans L.) from collections of known provenance in Ness Gardens. Methods Frost resistance of cut shoots was determined at approximately monthly intervals between February 1994 and March 1995. Shoots were exposed to a range of freezing temperatures and scored for damage. Freezing resistance was determined as the temperature that caused 50% damage to the shoots. Results The frost resistance of the common heaths was in accord with their geographical distribution. Calluna vulgaris and E. tetralix extend furthest north and east in Europe and developed the greatest frost resistance in winter (< –20 °C). Erica cinerea is widespread throughout the British Isles with an oceanic south‐western distribution in Europe and was resistant to only –13 °C. Erica vagans has a similar frost resistance to E. cinerea but has a restricted distribution in the British Isles, whilst the other rare heaths are more frost resistant than either E. cinerea or E. vagans. The frost resistance of all species increased with shorter daylengths and decreased with higher temperatures, suggesting that global warming might result in increased sensitivity to frost in these species. Main conclusions Whilst the distributions of the common heath species are in accord with their frost resistance, those of the rarer heaths are not. Consequently, factors other than susceptibility to cold winters must account for the restriction of rare species of Erica to the southern and western fringes of Britain and Ireland.