Nash, T.H., Ryan, B.D., Gries, C., Bungartz, F., (eds.) 2007. Lichen Flora of the Greater Sonoran Desert Region. Vol 3.
Thallus: shrubby, forming rather dense erect tufts, 2 to 12 cm long branching: anisotomic-dichotomous basal part: concolorous with the branches to distinctly blackened branches: irregular, inconspicuously segmented; lateral branches: not narrowed at point of attachment segments: terete and ±cylindrical, sometimes slightly swollen; transversal furrows: often present papillae: usually numerous especially on main branches, thick and verrucous tubercles and fibercles: generally absent fibrils: unevenly distributed soralia: slightly tuberculate to slightly excavate but remaining superficial, as large as the branches when mature, sometimes ±encircling the end of branches, ±circular, ±widely spaced and usually not confluent, arising initially on cortex; soredia: generally granulose isidiomorphs: rare, only on young soralia, never on mature ones Apothecia: not seen cortex: thin to moderately thick (4-7%), dull to slightly shiny medulla: moderately thick, dense to compact axis: moderately thick Spot tests: medulla K± yellow turning dark red, C-, KC-, P± orange; K-, C-, KC-, P- Secondary metabolites: usnic, salazinic (major), ±barbatic (minor), ±4-O-demethylbarbatic (minor), ±caperatic (minor), ±lobaric (minor) acids or usnic acid alone. Substrate and ecology: on bark (mainly Quercus spp., Pseudotsuga spp., Pinus spp., Abies spp., Picea spp.) mainly in montane forests between 1900 and 3300 m, rarely coastal in Quercus woodland between 600 and 800 m World distribution: circumpolar, mainly in continental areas Sonoran distribution: mountains of Arizona, southern California and Baja California. Notes: Usnea substerilis is closely related to U. lapponica and is frequently mixed with this species, which has deeply excavate soralia often reaching the central axis and torn off the cortex around the soralia and never produce isidiomorphs. Usnea diplotypus can also be confused with U. substerilis, but the former species has punctiform soralia and usually abundant isidiomorphs on young and mature soralia. However, in U. diplotypus, the punctiform soralia often fuse together and then appear like large soralia making it difficult to separate both species, especially when U. diplotypus has few isidiomorphs. For differences from U. ammannii, see under that species.