Putative natural hybrids (Ulmus × hollandica) between the Smooth-leaved (Ulmus minor ssp. minor) and the Wych elm (Ulmus glabra) were observed in mixed forests along the rivers and rivulets around the central part of Lithuania. Eleven populations of elms (Ulmus L.) were studied to determine 1) the critical groups of phenotypes indicative for their taxonomic identity, 2) the variability of taxa from contact zones. Three characteristics for the trees and twenty one for their leaf, including nineteen applied by WinFOLIA 2004a program, were used to describe each sample of the fifty eight and to ascertain the degree of affinity between samples. As a result of this approach, four taxa could be distinguished at the contact zones within elms. Three elm species and hybrids, Ulmus glabra, Ulmus minor, Ulmus laevis, and Ulmus × hollandica, were fairly satisfactorily distinguishable, but there is much overlap between the Wych elm and the Field elm in their characters that can be explained only by the equivalence of their statistically discriminant phenotypic distinctness. On average, the putative natural hybrids between the Smooth-leaved and the Wych elm account for less than two percent of all individuals within group U. minor – U. × hollandica – U. glabra. The possibility of introgressive hybridisation is shown by the outstanding variability of morphological characteristics of the taxa from the contact zones and by the presence of extreme or novel properties of hybrids.

Keywords: Ulmus minor ssp. minor, Ulmus glabra, Ulmus minor, Ulmus × hollandica, Ulmus laevis, natural hybridisation.