The genus Usnea Adans. (Parmeliaceae, lichenized Ascomycetes) is regarded as one of the taxonomically most difficult genera of macrolichens. Most of the species are widespread and highly variable in morphology. Many species are also very variable in chemistry, and may include several chemotypes. Numerous chemotypes which have been earlier regarded as distinct species have not recently been accorded any taxonomic rank since they are morphologically indistinguishable. For instance, Asahina (1956) divided Usnea longissima Ach. into many ”chemical” taxa, but according to Brodo (1984), they should not be recognized due to the absence of consistent morphological and geographic criteria to separate them.
For a long time Usnea used to be placed in the family Usneaceae. Studies on the apothecial ontogeny and ascus apical structures have indicated, however, that the family cannot be recognized as distinct from Parmeliaceae. Usnea was divided into six subgenera by Motyka (1936–1938): Protousnea Motyka, Neuropogon (Nees & Flot.) Motyka, Lethariella Motyka, Chlorea (Nyl.) Motyka, Eumitria (Stirt.) Motyka and Euusnea Jatta. In particular the systematic status of Chlorea and Lethariella is not clear and some taxonomists do not include them in the genus Usnea (e.g., Obermayer 1997). Only the subgenus Euusnea (i.e. now subgenus Usnea), which contains the distinct majority of the species, has been found in East Fennoscandia, i.e. Finland and the adjacent areas in Russia, and in British Columbia.
Motyka’s (1936–1938, 1947) world monograph of the genus contains 451 species and numerous infraspecific entities, but many of the taxa are now commonly regarded as environmental modifications (see Clerc 1997) or their delimitation needs revision. According to Räsänen (1951), the Finnish Usnea flora contains 34 species (plus the eight varieties mentioned), while Hakulinen (1963) reduced the amount to 25 species and seven varieties. My present opinion is that there are about only 12 species in Finland, but the number is not exact since all the taxonomic problems have not yet been solved.
In Europe and North America the genus Usnea has been under thorough revision in recent times (e.g., Myllys 1994, Clerc 1997, Clerc & Herrera-Campos 1997), but much work is still required. Randlane’s (1992) survey of the Estonian Usneae mainly followed Motyka’s species concepts. Bystrek (1994) has continued the Motyka tradition and has described many new taxa, but both P. Clerc (pers. comm.) and I are convinced that most of his views are not acceptable. Carlin & Swahn (1977) studied the Usnea flora of Sweden, but many of their opinions about the taxonomy are not commonly accepted. Taxonomic studies on Usnea have also been performed, e.g., in East Africa (Swinscow & Krog 1974, 1975, 1976, 1978, 1979) and Australia (Stevens 1999). Walker (1995) published a world-wide revision of the subgenus Neuropogon.