Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2007. Lichen Flora of the Greater Sonoran Desert Region. Vol 3.
Thallus: shrubby, compact, 2-5 cm long branching: isotomic- to anisotomic-dichotomous, divergent basal part: usually distinctly jet black branches: tapered, lateral branches not narrowed at attachment points segments: terete, cylindrical papillae: few to numerous, verrucous, irregularly distributed tubercles and fibercles: generally absent fibrils: usually slender and ±long (2-3 mm long when mature), few to numerous especially on main branches, ±regularly distributed soralia: deeply excavate, tearing off the cortex around the soralia, as large as the branches when mature, often reaching the central axis and encircling the branches, ±widely spaced and usually not confluent, arising initially on the cortex isidiomorphs: always absent cortex: dull to shiny, thick (7-13%) medulla: in average thin, dense axis: thick Apothecia: not seen Spot tests: medulla K+ dull yellow turning reddish orange, C-, KC-, P+ deep yellow, or K+ yellow slowly turning pale orange, C-, KC-, P+ deep orange, or K-, KC-, C-, P- Secondary metabolites: norstictic acid ±zeorin or stictic acid group ±zeorin or only usnic acid. Substrate and ecology: mainly on bark of Quercus spp. in low forests on exposed ridges or on shrubs in the chaparral between 200 and 500 m World distribution: Europe, western and eastern North America Sonoran distribution: coastal, southern California, including the Channel Islands. Notes: Besides Usnea lapponica, U. fulvoreagens is the only species in the area, that has lateral branches not narrowed at attachment points and deeply excavate soralia without isidiomorphs. Usnea fulvoreagens has an isotomicdichotomous branching pattern, tapered branches, a thinner medulla, a complex chemistry and belongs to the "Usnea florida group" whereas U. lapponica has an anisotomicdichotomous branching, cylindrical to irregular branches, a thicker medulla, mostly salazinic acid, a different distribution pattern and belongs to the "U. rigida (= U. intermedia).group". For differences from U. substerilis and U. diplotypus, see the discussion under U. substerilis.