Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2007. Lichen Flora of the Greater Sonoran Desert Region. Vol 3.
Thallus: erect-shrubby to subpendant, 2-15 cm long branching: anisotomic-dichotomous, divergent basal part: concolorous with branches branches: tapered, lateral branches not narrowed at attachment points segments: terete, cylindrical papillae: nearly absent to numerous (young fibrils?) tubercles: absent fibercles: few to numerous fibrils: numerous, short (< 3mm) and thick, giving a spinulous aspect to the branches isidiomorphs: absent pseudocyphellae: absent cortex: thick (9-12%), shiny with a red pigment sometimes spotty and sparse at the surface medulla: compact, thin to moderately thick, sometimes ±slightly orange pigmented axis: thick, fistulose in the thickest branches Apothecia: usually present, sometimes nearly absent, subterminal to terminal Spot tests: medulla K+ yellow turning red, C-, KC-, P + orangish yellow Secondary metabolites: medulla with norstictic (major) and salazinic (major) acids and ±protocetraric acid (minor). Substrate and ecology: on bark of Quercus at 2300 m in mixed pine-oak forest World distribution: East Africa, southern North America and South America (?) Sonoran distribution: Sierra Madre Occidental of Sinaloa. Notes: Usnea erinacea is easily distinguished from all other species of the area by its numerous apothecia, the absence of soralia, its red pigmented cortex, and its chemistry.