The dark morph of Sphagnum fuscum (Schimp.) H. Klinggr. in Europe is conspecific with the North American S. beothuk

Peat mosses (Sphagnum) are known to be difficult to identify, especially in the field, because of extensive morphological variation that can blur distinctions among closely related species. The extent to which phenotypic variation reflects genetic differentiation versus environmentally induced plasticity is, however, poorly understood. The common and widespread Sphagnum fuscum is relatively easy to recognize, but two different colour morphs, dark and pale brown, have been observed along the western coast of Europe for some time. In this study, we aimed to test whether the two colour morphs are genetically differentiated, and assess their phylogenetic relationship to the recently described S. beothuk from eastern North America. We used a combination of morphological characters and genetic markers (microsatellites and cpDNA sequences). Our results indicate that the dark colour morph of S. fuscum in Europe is conspecific with S. beothuk from eastern North America. On the other hand, the pale morph of S. fuscum and S. beothuk differ in morphology, are separated genetically, and have different habitat preferences and distribution ranges. The known occurrences of S. beothuk are restricted to the oceanic regions along the North Atlantic coast. It is one of few peat mosses restricted to the amphi-Atlantic region. We show that by combining morphology, genetic data, distributional information, and habitat knowledge, it is possible to evaluate taxonomic uncertainties in Sphagnum. We also epitypify the name S. fuscum.