Taxonomic relationships and population differentiation of the south-western Eurasian Zelkova species inferred in leaf morphology

The relict tree species Zelkova abelicea and Z. sicula (Ulmaceae) occur in Crete and Sicily, respectively. Zelkova abelicea grows in approximately 40 localities in the mountains of Crete, while Z. sicula has been found in only two stands in Sicily. We compared 25 morphological characters of the leaves of both species and used statistical methods (Tukey’stest, discrimination analysis, principal component analysis, agglomeration) to reveal the differences between these two species, their relation to Z. carpinifolia (the third Zelkova south-west Eurasian species), and between the leaves from different shoot types. Our study represents the first comparative biometric analysis of the three Zelkova species localized in the western edge of the current geographic range of the genus. We found that the species differed from each other both, in terms of leaf form characters and level of leaf variation. Zelkova carpinifolia was clearly different from Z. abelicea, while the two populations of Z. sicula, SIB and SIA, were more similar to Z. carpinifolia and Z. abelicea, respectively. The latter finding supports the results of recent molecular studies suggestingt he hybrid origin of Z. sicula. Zelkova abelicea, Z. sicula and Z. carpinifolia differ each other in the leaf characteristics and level of phenotypic variation.