Usnea  hirta (L.) F.H. Wigg.

Nomenclatural data
Prim. Fl. Hols.: 91 (1780)—Lichen hirtus L., Spec. Plant. 2: 1155 (1753).—Usnea foveata Vain.—Usnea glaucescens Vain., Meddel. Soc. Fauna Fl. Fenn. 48: 172 (1925).—Usnea pulvinata Räsänen, Ann. Acad. Sci. Fenn. Ser. A 34(4): 20 (1931), nom. illeg.—Usnea romanica Cretzoiu—Usnea variolosa Motyka, Lich. Gen. Usnea Stud. Monogr., Pars Syst. 1: 104 (1936).
Morphology
For a detailed description, see Halonen & Puolasmaa (1995) and Halonen et al. (1998). Thallus shrubby, richly branched; branching mainly anisotomic-dichotomous; main branches slightly deformed and foveolate, also constrictions may occur; base pale; fibrils abundant; isidiomorphs numerous, scattered or in clusters, more abundant at the terminal parts; papillae absent; soralia punctiform and develop on the scars where isidiomorphs have broken off. Cortex thin; medulla thick and usually loose.
Medullary chemistry
(1) chemotype with fatty acids (of the murolic acid complex) (K–, Pd–) is widely distributed both in Europe and North America, presence of norstictic acid (K+ red, Pd+ orange) as an accessory substance has been reported in Great Britain and East Fennoscandia (Purvis et al. 1992, Halonen & Puolasmaa 1995); (2) chemotype without any medullary substances is known from North America (Halonen et al. 1998).
In Estonia both chemotypes are known (n=8 and n=6, respectively) (Tõrra & Randlane 2007).
Remarks
Shrubby thallus and presence of numerous isidiomorphs are similar to U. subfloridana. Slightly deformed main branches and lack of any papillae are the diagnostic features of U. hirta. Medullary chemistry – presence of fatty acids as main compounds – is of help to verify the identification.
Clerc (2007) declared recently that U. hirta lacks isidiomorphs, and young fibrils can be mistakenly treated as isidiomorphs.
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Ecology and distribution
Europe (map legend)
Corticolous and lignicolous, prefers acid bark (Fos & Clerc 2000). Reported in Europe: Albania, Austria, Belgium, Bulgaria, Czech Republic, Denmark, Estonia, Finland, Germany, Great Britain, Hungary, Island, Italy, Latvia, Lithuania, Luxembourg, Norway, Poland, Romania, Russia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Ukraine.

Estonia
Mainly grows on conifers (on Pinus 36% of examined specimens, on Picea 34%, on other 10%) and lignum (9%); deciduous trees are seldomly inhabited by this species. Very frequent in Estonia, found in all regions both in forests, open landscapes (wooded meadows, bogs, alvars, seashore areas) and in areas of human activities (Tõrra & Randlane 2007).