The Distribution and Composition of Hybrid Populations of Erica Ciliaris L. and Erica Tetralix L. in Dorset

Abstract

Erica ciliaris L., the 'Dorset Heath', referred to in earlier Floras as the Fringe-leaved Heath, Ciliated Heath and Four Burrow Heath, has a Lusitanian distribution being found in north-west Morocco, the coastal regions of Portugal, Spain and France, the south of England and the west of Ireland (Hansen 1950). The first British record of E. ciliaris is generally attributed to the Rev. J. S. Tozer who found it near Truro, Cornwall in 1828 (Lindley 1829), although the plant was probably known before that date. A hybrid between E. ciliaris and E. tetralix L., later found growing near Truro, was named E. x watsonii Benth. At present E. ciliaris is less abundant in Cornwall than in south-eastern Dorset where it was first recorded by J. Woods in 1848, while the first Dorset record of E. x watsonii is dated 1891 (see Mansel-Pleydell 1895). Within the British Isles E. ciliaris occurs in the west of Ireland (Webb 1966; McClintock 1968), Cornwall, Devon (at one site on Dartmoor) and on the heathlands and valley mires of south-eastern Dorset. A distribution map of E. ciliaris was given by Good (1948) following his survey of the Dorset flora between the years 1932 and 1937, and he commented: 'The particular interest in this plant is that its local geographical limits follow no obviously recognisable edaphic or climatic boundary and are therefore presumably set by some unusually subtle factor or combination of factors. The rigidity of the limitation is further emphasised by the fact that although the species does from time to time occur as scattered individuals on the neighbouring heathlands, it never spreads there.' A map showing sites for E. ciliaris in Dorset was given by Moore (1962), who used the plant as an indicator species in his evaluation of the conservation interests of Dorset heathland sites (see Fig. 1 for a map of S.E. Dorset). Although the hybrid E. x watsonii is recorded in several County Floras, the distribution and relative composition of hybrid populations of E. ciliaris and E. tetralix have been analysed only by Gay (1957, 1960) using a modification of Anderson's Hybrid Index technique (Anderson 1949). The present paper provides a more detailed account of the current distribution of E. ciliaris in Dorset, characterizing hybrid populations and suggesting ecological implications of the observed patterns of distribution.