Challenge accepted: Evolutionary lineages versus taxonomic classification of North American shrub willows (Salix).

Premise: The huge diversity of Salix subgenus Chamaetia/Vetrix clade in North America and the lack of phylogenetic resolution within this clade has presented a difficult but fascinating challenge for taxonomists to resolve. Here we tested the existing taxonomic classification with molecular tools.

Methods: In this study, 132 samples representing 46 species from 22 described sections of shrub willows from the United States and Canada were analyzed and combined with 67 samples from Eurasia.