Morphometric comparison of British Pseudorchis albida with Icelandic P. straminea (Orchidaceae: Orchidinae)

Although the majority of taxonomic studies of European orchids treat Pseudorchis as a monotypic genus, some observers have argued that the apparently circumboreal segregate P. straminea should also be treated as a full species. Here, we compare detailed in vivo measurements of 55 plants from nine populations of P. albida in Britain with ten plants from three populations of P. straminea in Iceland, We apply morphometric statistics to data for 31 morphometric characters, culled from an original set of 36. We also review current (limited) DNA-based evidence of their evolutionary divergence. Of the 31 variable morphometric characters, 14 help to distinguish the two species, including labellum dimensions, spur length, flower colour, bract and leaf lengths, leaf colour and apical hooding. Molecular divergence (notably in ITS) approximates the lowest level needed for acceptance of species-level distinction, but thus far, the molecular information is typological and does not encompass seemingly intermediate populations of ‘P. tricuspis’ occurring in the mountains of central Europe. Pseudorchis has the potential to become a valuable model system for the study of speciation and subsequent inter-/post-glacial migration. Despite persistent rumours, we found no evidence that P. straminea (or ‘P. tricuspis’) occurs in the British Isles; nonetheless, this species should continue to be sought in suitable habitats in northern Scotland. Morphological variation within P. albida is comparatively low, though the most exposed of our study populations exhibits (presumably ecophenotypic) dwarfing.