Taxonomic differentiation of Salix retusa agg. (Salicaceae) based on leaf characteristics
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The complex of Salix retusa includes S. retusa s.s., S serpyllifolia and S. kitaibeliana, small, prostrate willows occurring in the subalpine and alpine vegetation belts of the mountains of Central Europe: the Pyrenees, Alps, Apennines, Dynaric Alps, Carpathians and Rila. The systematic positions of these taxa are not fully resolved and are still disputed. The aim of the present study was to biometrically verify differences in leaf characteristics between these taxa. Material was collected from 47 populations, each represented by 25–52 individuals (33 on average). The study was based on 14 leaf morphological characters measured from scans using Win Folia software. The principal component analysis (PCA), Ward’s agglomeration method, the K-means cluster analysis (K-MCA) and Kruskal-Wallis tests were applied to verify the relationships between taxa and their populations. Differences between average leaf characteristics of S. retusa s.s., S. serpyllifolia and S. kitaibeliana were detected. The Pyrenean populations of S. retusa s.s. appeared more similar to S. serpyllifolia. Within the Alpine populations of S. serpyllifolia, several individuals resembling S. retusa s.s. were detected, and vice versa, within populations of S. retusa s.s., and there were also individuals similar to S. serpyllifolia. The controversial S. kitaibeliana was found to be typical of the Tatra Mountains. The results support close taxonomic relations, but also the separate status of S. retusa, S. serpyllifolia and S. kitaibeliana. The differences between them are mainly of a quantitative nature.