Leaf variation in the genus Ulmus

Abstract

R.H. Richens made an extensive study of the variation of the genus Ulmus, based on measurements of leaves taken from individual trees. His collection began in East Anglia and the English counties then designated as Bedfordshire, Cambridgeshire and Huntingdonshire, but later extended over the whole of England and Wales. Subsequently, the collection was extended to France and the Channel Islands, to Northern Spain, and to the former Yugoslavia. The analyses of these data have been published in a series of papers, and the measurements and the working papers describing the locations from which samples were obtained have now been archived for future study. The analysis of these unique data has shown that the elm population is extremely variable, and that very few discontinuities occur within that variation. The great difficulty that taxonomists have encountered with the genus Ulmus results from the complexity of the botanical variation, and not, as so often in taxonomy, from artificial problems created by taxonomic methodology. It is apparent that much parallel variation has occurred and that many recognizable variants are polyphyletic. Many of the variants are also quite narrowly localized, but the elm population consists mainly of four principal taxa: U. glabra, U. procera, U. minor, and the U. minor x U. glabra hybrids. The U. minor x U. glabra hybrids show the most extensive variation, and this variation shows so little discontinuity that any taxonomic splitting of U. minor or the hybrid assemblages is probably imprudent. The limits of the variation of the four major taxa are wide and convergence between them has occurred.