For a detailed description see Truong et al. (2011; as U. poliotrix).
Short Description.Usnea poliothrix is a very characteristic species with its orange subcortical pigmentation (visible in section), with very irregular branches that are often ridged, with abundant foveoles, and an absence of fibrils, instead having numerous isidiofibrils emerging from minute soralia. Together with a very thin cortex that easily breaks and the presence of lobaric acid in its medulla, this combination of characters readily distinguish this species from any other Usnea.
Occasionally, old herbarium specimens (as in the holotype) may lack isidiofibrils but nevertheless display all other diagnostic characteristics of the species; particularly the ridged branches, which have minute scars, presumably where the isidiofibrils eroded. Specimens with faint subcortical pigmentation nevertheless can be very difficult to identify correctly. Although they do share all other characters typical of U. poliothrix, some specimens appear to be almost entirely colorless. The presence of lobaric acid has been confirmed by TLC in all of these “colorless” specimens and thus appears to be a reliable character to identify these specimens. TLC plates should be carefully checked as lobaric acid spots on the plates are not very conspicuous.
Chemistry. Medulla with salazinic, galbinic, norstictic, and lobaric acid [P+ yellow orange, K+ yellow turning deep red, C–, KC–].
Ecology and distribution. North America (Clerc 2016), South America and South Africa (Motyka 1936 –38; Vareschi 1973). Truong et al. (2011) mention that the species appears to be very rare on the continent, while it is surprisingly common in the Galapagos, suggesting that the insular populations may be relicts of a species once more common also on the continent. Most Galapagos specimens were collected in the transition zone, few in the dry zone and only very few in the humid zone. Most collections are from open, exposed habitat, on bark of trees or shrubs (mostly native: Bursera, Zanthoxylum, Cordia, Hippomane, one specimen on stems and fronds of Pteridium), rarely on decorticated wood, and very rarely on exposed, sunny rock outcrops where the humidity from fog is presumably high.