Usnea glabrescens (Vain.) Vain.

Nomenclatural data
Medd. Soc. Fauna Fl. Fenn. 48: 173 (1925).—Usnea barbata (L.) F. H. Wigg. var. glabrescens Nyl. ex Vain., Meddel. Soc. Fauna Fl. Fenn. 2: 46 (1878).—Usnea betulina Motyka, Lich. Gen. Usnea Stud. Monogr., Pars Syst. 1: 297 (1936).—Usnea compacta Motyka, Lich. Gen. Usnea Stud. Monogr., Pars Syst. 1: 294 (1936).—Usnea distincta Motyka, Lich. Gen. Usnea Stud. Monogr., Pars Syst. 1: 298 (1936), nom. illeg.—U. extensa Vain., Ann. Acad. Sci. Fenn., Ser. A4, 27(6): 68 (1928).
For a detailed description, see Halonen et al. (1998, 1999). Thallus originally shrubby, later may become subpendent; branching mainly isotomic-dichotomous; base distinctly blackened, often with transverse annular cracks; fibrils present at basal parts and sparse to absent at apical parts; isidiomorphs rarely present only on young soralia; papillae present on main branches and absent on secondary branches; young soralia punctiform, later becoming larger but usually staying distinctly rounded and discrete. Cortex rather thick; medulla variable.
Medullary chemistry
Chemistry is variable, with several chemotypes reported: (1) chemotype with norstictic and salazinic acids (K+ red, Pd+ yellow to orange) as main compounds plus other compounds of the stictic acid complex and protocetraric acid as accessory substances is the most common in Fennoscandia; (2) with norstictic acid as main substance (K+ red, Pd+ orange) plus stictic acid complex as accessories is widely known in Fennoscandia (Halonen et al. 1999) and Great Britain (James 2003); (3) chemotype with psoromic acid (K–, Pd+ yellow) and accessory substances is rare in Great Britain (Clerc 1992, James 2003); (4) chemotype without medullary substances (K–, Pd–) has been reported as rare in East Fennoscandia (Halonen et al. 1999).
In Estonia chemotype (1) is common (n=20) and chemotype (2) is rare (n=3) (Tõrra & Randlane 2007).
Distinction from closely related taxa see under U. fulvoreagens.
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Ecology and distribution
Europe (map legend)
Corticolous. Reported in Europe: Austria, Bulgaria, Czech Republic, Denmark, Estonia, Finland, Germany, Great Britain, Hungary, Ireland, Italy, Latvia, Lithuania, Norway, Poland, Romania, Russia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Ukraine.

72% of examined specimens grow on coniferous trees (Picea and Pinus) and 21% on Betula; other phorophytes are of less importance. Very frequent in Estonia (80 specimens), reported mainly on the northern coast and in south-eastern region of Estonia (Tõrra & Randlane 2007).