The genus Usnea is probably phylogenetically very old, judging from the fact that many of the species, including all East Fennoscandian Usneae, have very wide global distribution and have adapted to different habitats and climates. Usnea substerilis, for instance, has a slightly different morphological variation in East Fennoscandia compared to the British Columbia material, which may partly be due to climatic factors. The species diversity is distinctly higher in British Columbia than in East Fennoscandia, since most of the Usnea species occurring in these areas have more or less a southern distribution and favour a hygric maritime climate.
In addition to the considerable morphological variation most of the Usnea species are moderately to very variable in their chemistry, and some species have locally distributed chemotypes. However, without any significant morphologically distinguishable characters they cannot be regarded as separate taxa.
There still exist, however, some taxonomical problems which should be solved before the whole Usnea floras of East Fennoscandia and Pacific North America are totally clarified. The most difficult and urgent problem, concerning also the classification of threatened species, is the taxonomy and systematics of the pendent species U. barbata s. lat., U. chaetophora and U. filipendula s. lat. Usnea barbata s. lat. and U. filipendula s. lat. may contain many close species and at present we do not know whether these two species groups belong to different aggregates although they have a similar chemistry. Furthermore, U. chaetophora may represent a modification of U. barbata s. str. Among the shrubby species, the taxonomic status of U. substerilis and the British Columbia material of U. nidulans s. lat. and U. rigida s. lat. should be cleared up in further studies. The former species could be regarded as a subspecies of U. lapponica and the two latter may be distinct, undescribed taxa. Methods of molecular biology, together with morphological and chemical investigations, might give considerable help in taxonomical studies on the genus Usnea.