Usnea flavocardia Räsänen

Nomenclatural data
Revista Universitaria (Santiago) 21: 139 (1936).—Usnea quercina Bystrek & Gorzynska, Ann. Univ. Mariae Curie Sklodowska, Sect. 3, Biol. 17: 184 (1985).—Usnea wirthii P. Clerc, Saussurea 15: 33–36 (1984).
For a detailed description, see Clerc (1984b, 2007) and Purvis et al. (1992). Thallus shrubby, erect, to 5 cm long; base pale to almost black; branches rather stout, may be constricted at the base, distinctly segmented by conspicuous annular cracks (bordered with white rings of medullary tissue on the main branches); fibrils sparse or numerous; papillae present, low, inconspicuous; soralia numerous, especially towards the apical parts of the thallus, convex, plane or slightly excavate, soredia rather coarse; isidiomorphs few (on young soralia) or absent (on mature soralia). Cortex thin, often covered by scattered red spots; medulla thick, loose (around the axis) and more compact close to the cortex; inner part  of the medulla and axis are pale lemon to primrose yellow while outer part of the medulla is white.
Medullary chemistry
Chemotype (1) with psoromic acid (K+ yellow, Pd+ yellow) as a main substance and salazinic acid and unidentified fatty acid as accessories is known from Europe; chemotype (2) with norstictic and stictic acids (K+ yellow to red, Pd+ orange) is reported from North America (Halonen et al. 1998); chemotype (3) with salazinic (K+ yellow to red, Pd+ orange), norstictic and galbinic acids as main substances was detected from the type material from Chile (Clerc 2004b); chemotype (4) without medullary substances (K–, Pd–) has been reported from North America, Sonora (Clerc 2007).
Psoromic acid in chemotype (1) is concentrated in soralia, therefore the colour test with Pd should be done rather on soralia than on medulla.
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Ecology and distribution
Europe (map legend)
Corticolous on deciduous trees (e.g. Quercus, Salix, Sorbus etc.), rarely on mossy boulders (Clerc 2004). Reported in Europe: France, Great Britain, Ireland, Portugal, Spain.