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, 3-15 cm long branching: anisotomic-dichotomous, divergent basal part: darker than branches or of the same color with conspicuous annular cracks branches: tapering; lateral branches: not narrowed at attachment point segments: terete and cylindrical papillae: absent to numerous and indistinct on main branches tubercles: absent to numerous on main and secondary branches, usually absent on the terminal branches, short and verrucous, sometimes eroded at top fibercles: absent to numerous, irregularly distributed on main and secondary branches fibrils: nearly absent to numerous, short and spinulous to long and slender, irregularly distributed on the whole thallus soralia: punctiform, irregular in outline, even or slightly stipitate, plane to erumpent, developed initially from the cortex (on the whole thallus) or on top of tubercles (on main branches), or on fibercles (main and secondary branches), nearly absent to numerous, sometimes fusing together and entirely covering the terminal branches isidiomorphs: always present, generally numerous and conspicuous on secondary and terminal branches pseudocyphellae: absent cortex: moderately thick to thick (9-13%), shiny to vitreous, hard, frequently transversally or longitudinally cracked, with a red pigment medulla: thin to moderately thick, dense to compact, not pigmented axis: moderately thick to thick, not pigmented Apothecia: not seen Spot tests: K+ yellow slowly turning orange, C-, KC-, P+ deep orange Secondary metabolites: stictic (major), ±norstictic (minor), ±constictic (minor), ±cryptostictic (minor), ±menegazziaic (minor ) acids. Substrate and ecology: on bark of Quercus spp., Pinus spp., and diverse shrubs, rarely on rock, in oak, pine-oak forests and in chaparral or in coastal scrub communities between 0 and 500 m; or in the southern mountains between 1500 and 2500 m World distribution: suboceanic, oceanic temperate and tropical regions: Europe, eastern Asia, Australasia, Africa, North and South America, and Macaronesia Sonoran distribution: coastal areas of southern California (including the Channel Islands), Baja California, and Baja California Sur, and Sinaloa. Notes: Usnea rubicunda is a very polymorphic species characterized by its red pigment in the cortex and the numerous minute soralia with conspicuous isidiomorphs.