Inference of taxonomic relationships between Rhododendron ferrugineum and R. myrtifolium (Ericaceae) from leaf and fruit morphologies

Rhododendron myrtifolium and R. ferrugineum are small, shrubby plant species that are morphologically similar to each other. They both grow on siliceous substrates in the subalpine belt of European mountains. In the present study, leaves and capsules from 11 populations of R. myrtifolium from the Carpathians were compared with those from 13 populations of R. ferrugineum from the Alps, Pyrenees and Sudetes. Fourteen leaf characteristics and 12 capsule characteristics of were analysed using ANOVA, discrimination analysis, principal component analysis and cluster analysis with agglomeration and the K-means method. All of the tested characteristics differed significantly between species, with the highest level of discrimination from the measured leaf traits. Less discriminatory were the proportions of the measured leaf features. According to the analyses, the populations of the two species were clearly separated, but individuals were partially intermixed. The similarities of the species may indicate a late divergence event, probably no earlier than the Pliocene. During the glacial cycles of the Pleistocene, populations of the two species may have mixed again. This may explain the presence of single individuals with R. myrtifolium morphology in some alpine R. ferrugineum populations and vice versa. The two species can be treated as another example of Carpathian-Alpine vicariance.