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.
For a detailed description see James (1979), James et al. (1992), Ohmura (2001), Clerc (2007) and Herrera-Campos (2016).
Short Description.Usnea rubicunda and U. erinacea are the only Galapagos species of Usnea with reddish orange pigmentation in their cortex and not in their medulla. Usnea rubicunda is further characterized by terete branches, not constricted at their attachment point, and minute soralia that are typically very abundant, always well delimited and rarely enlarge or fuse toward the branch apices. The species has a thick and hard cortex, shiny in section, and a thin and compact medulla, with an is presumably over-collected, it must nevertheless be considered by far the most common Usnea in the Galapagos. Weber (1986: 487) called “reddish” Galapagos specimens Usnea rubiginea, a name that Elix & McCarthy (1998) generally assumed to refer to U. rubicunda. Truong et al. (2011) also re-examined several of these specimens, showing that other species of Usnea with a reddish orange cortical to subcortical pigmentation were also present, though generally less common. The species occurs in virtually all vegetation zones, but it is most frequently found in the transition zone. It typically occurs in sunny, wind- and rain-exposed habitats, almost always on bark (including cacti), with only four specimens on rock, and one on wood. A/M-ratio typically > 1.5.
The characteristic red-orange cortical pigmentation is often very pronounced, and then only apical branches may be less strongly pigmented. These heavily pigmented specimens are very conspicuous, but weakly to almost unpigmented specimens can also be observed. These almost “colorless” specimens may have only few, faint pigment spots, sometimes visible only in longitudinal branch sections or close to the base of the thallus. Despite such an apparent “lack of pigmentation” these specimens can nevertheless safely be identified as U. rubicunda; their terete branches have a thick and shiny cortex, bearing numerous minute soralia. Usnea rubicunda is highly polymorphic. Molecular studies by Truong et al. (2013a) suggest that, with its fertile counterpart U. erinacea, this taxon forms a large, still poorly resolved species complex. The highly variable characters, presently used to characterize this taxon, are: (1) thalli that are very variable in size – from small and erect to large and then almost pendulous, (2) a trunk with variable pigmentation – blackened or concolorous with the branches, (3) a surface with papillae, fibrils and isidiomorphs – either abundant or very sparse, and (4) soralia highly variable in their morphology. For differentiation with U. rubricornuta and U. subrubicunda P.Clerc, which only occur on the continent, see Truong et al. (2011).
In Galapagos its fertile counterpart U. erinacea is the only other taxon with distinctly cortical pigmentation; in all other “reddish” species the pigmentation is confined to the medulla (e.g., U. dorogawensis, U. grandisora, U. poliothrix, U. subcornuta, and U. subdasaea). These sorediate species, although they lack a pigmented cortex, nevertheless often appear “reddish”, because the subcortical pigment from their medulla “shines through” to the surface. Thus, at first glance, they could be confused with the sorediate, reddish U. rubicunda.
Chemistry. In Galapagos two chemotypes have been observed: (1) stictic, ±norstictic acid (traces) [K+ yellow slowly turning orange, P+ deep orange], and (2) salazinic, norstictic, ±connorstictic (traces), ±lobaric acid (traces) [K+ yellow turning red, P+ orange-yellow]. In the Andes two additional chemotypes have been reported (Truong et al. 2011), (3) one with norstictic and connorstictic (K+ yellow turning red-orange, P+ deep yellow), but no stictic acid, and (4) another one with unidentified tri-terpenoids (typically P–, K–), rarely with traces of norstictic (K+ yellow turning red-orange, P+ deep yellow).
Ecology and distribution.Usnea rubicunda has an oceanic to sub-oceanic worldwide distribution, from the tropics into temperate regions (Clerc 2007). Although this conspicuously pigmented species is presumably over-collected, it must nevertheless be considered by far the most common Usnea in the Galapagos. Weber (1986: 487) called “reddish” Galapagos specimens Usnea rubiginea, a name that Elix & McCarthy (1998) generally assumed to refer to U. rubicunda. Truong et al. (2011) also re-examined several of these specimens, showing that other species of Usnea with a reddish orange cortical to subcortical pigmentation were also present, though generally less common. The species occurs in virtually all vegetation zones, but it is most frequently found in the transition zone. It typically occurs in sunny, wind- and rain-exposed habitats, almost always on bark (including cacti), with only four specimens on rock, and one on wood.