A detailed description has been provided by Clerc (2006).
Short Description.Usnea subflammea is characterized by cylindrical branches with terete segments, abundantly covered by eroded tubercles. Its branches are regularly segmented with thin annulations, which are more abundant on the trunk and basal branches. The soralia of U. subflammea arise at the top of eroded tubercles, they are initially minute and slightly stipitate, but at maturity become circular and capitate, although they remain well delimited by a typical thin cortical margin. Short isidiomorphs often arise abundantly from the soralia. The cortex is opaque (i.e., mat in section), moderately thick to thick, often with transverse cracks that are particularly conspicuous in section. The species has a compact medulla, often with a faint pink-orange periaxial pigmentation.
In Galapagos U. aff. columbiana is similar, it has branches that are equally thick, an opaque cortex, and abundant tubercles. Unlike U. subflammea, however, U. aff. columbiana has a distinctly blackened trunk, the pigmentation extending well beyond the first ramification.
Chemistry. Medulla with stictic acid [P+ orange, K+ yellow, C–, KC–]. Truong et al. (2013b) reported two chemotypes from South America, one with the same chemistry as the type (stictic acid, K+yellow), and another one with galbinic acid (K+ yellow turning red). The medulla of both Galapagos specimens reacts K+ yellow, not turning red and thus belongs to the typical chemotype.
Ecology and distribution. Clerc (2006) originally described U. subflammea from the Azores; Truong et al. (2013b) later discovered it also in Central and South America (Bolivia, Brazil, Colombia, Costa Rica, Ecuador, Peru, Venezuela). The two specimens recently discovered among historic material collected by Weber (cited below) represent here the first report of this species from the Galapagos. No collections were made during our recent inventory. Usnea subflammea, like U. angulata, could therefore presumed to be locally extinct. Usnea angulata is, however, much more easily recognized by its conspicuous morphology and we specifically searched its known collection site on several occasions, trying to rediscover this species. This contrasts with U. subflammea, where the historic material was only recently discovered, and its collection sites need to be intensively surveyed again.