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, usually 3 to 7 cm long, rarely subpendant and up to 14 cm long, ±stiff branching: distinctly anisotomic-dichotomous, divergent basal part: paler than or concolorous with the branches, sometimes slightly brownish pigmented branches: cylindrical; lateral branches: not narrowed at attachment points, foveoles or transversal furrows sometimes present segments: terete and ±cylindrical papillae and tubercles: absent fibercles: present on main branches fibrils: short (1-2 mm) and ±spinulous, sometimes numerous and irregularly distributed on main branches soralia: punctiform with irregular outline, smaller than half the diameter of the branch, arising from fibercles (main branches) and then slightly stipitate or initially from the cortex and then even, plane to slightly convex, often fusing together, looking like large soralia and covering densely the whole branches isidiomorphs: numerous on young and mature soralia cortex: ±thick (6-11%), hard and shiny medulla: ±thin, dense to compact, with pink to red-wine pigment axis: thin to thick, sometimes pink pigmented Apothecia: not seen Spot tests: K± yellow, C+ orangish yellow, KC+ organish yellow P± orange Secondary metabolites: Eumitrin A2 (major), ±fatty acids of the murolic acid complex (major), ±norstictic acid (major), ±diverses unknowns (minor). Substrate and ecology: on bark, rarely on rock, in the chaparral and other coastal shrub communities, on Quercus spp. and diverse shrubs, between 0 and 500 m World distribution: southwestern Europe, eastern and western North America, eastern Asia Sonoran distribution: Channel Islands (mainly Santa Catalina) of southern California and Baja California. Notes: This species may morphologically and chemically somewhat ressemble U. hirta, but the presence of the wine red pigment in the medulla leaves no doubt about its identity. Moreover, U. mutabilis has a thicker cortex (6-11%) than U. hirta s. str. (3-7%). Finally, the medulla in U. hirta is lax; whereas, it is dense to compact in U. mutabilis.