Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2007. Lichen Flora of the Greater Sonoran Desert Region. Vol 3.
Thallus: shrubby, usually short, 3 to 8 cm long, ±stiff branching: anisotomic-dichotomous, divergent basal part: concolorous to the branches or paler branches: main branches: irregular, distinctly segmented, ±swollen; lateral branches: not narrowed at point of attachment segments: terete to strongly ridged, cylindrical to ±sausagelike; transversal furrows/foveoles: numerous on main branches papillae: absent although young fibrils may be mistaken for such tubercles: absent fibercles: nearly absent to numerous, looking like pseudocyphellae fibrils: short, spinulous, numerous and crowded (80-100 fibrils/0.5 cm on main branches) soralia and isidiomorphs: absent cortex: thin (2-6%), dull to distinctly shiny, never cracked medulla: usually two layered: compact close to the cortex and lax around the axis, not pigmented axis: thin to wide Apothecia: terminal to subterminal, 2 to 17 mm in diam. spores: 7.5-9 x 3-5 µm Spot tests: medulla K-, C-, KC-, P- Secondary metabolites: usnic acid, ±fatty acids (minor), ±unknown (minor). Substrate and ecology: mainly on bark of Pinus spp. and Pseudotsuga menziesii in coniferous forests or pine-oak vegetation in the mountains between 1800 and 3100 m and exceptionally coastal on shrubs World distribution: southwestern U.S.A., Mexico, and South America Sonoran distribution: mountains of Arizona, Sierra Madre Occidental of Chihuahua, exceptionally along coastal California (one specimen found on San Nicolas Island at 150 m). Notes: The lack of soralia and the numerous apothecia in U. parvula separate it from the very similar U. hirta. Usnea parvula could be considered as the primary species of U. hirta. In Usnea intermedia, the short, numerous and crowded spinulous fibrils are absent, the basal part is usually black and salazinic acid is present in the medulla. Usnea cirrosa has lateral branches that are distinctly constricted at ramification points and norstictic acid in the medulla. A few specimens from Mexico (Chihuahua, Sinaloa, and Sonora) agree morphologically and anatomically well with U. parvula. However, they produce salazinic acid, ±norstictic or only constictic acid in the medulla. More field work and more specimens are needed to understand these chemical variants.