Usneaaff. columbianaMotyka ex Räsänen Revista Univ. (Santiago) 21: 138 (1936).
Type: Chile. Valdivia: Cordillera Pelada C. de Corral., 200 – 400 m alt., 21-Mar-1930, Looser s.n. [lectotype in H (9500229), specimen annotated as lectotype on 5-Oct-1990 by Tavares, I., annotation not previously published]. %C/M/A: 15/55/12. Chemistry: usnic and salazinic acid.
Description (based on Galapagos specimens).Thallus erect-shrubby to subpendulous, rarely almost pendulous; ramifications ±isotomic-dichotomous; trunk often multiple, with several well-visible trunks but also small hapters on basal branches, pigmentation blackish (rarely brownish), irregularly extending above first ramification, with cortex surface of basal branches looking necrotic or burnt, with thin annular cracks along trunk and basal branches; branches cylindrical to irregular in diameter, not inflated; lateralbranches not constricted at ramification; maculae absent; pseudocyphellae absent; papillae absent (see tubercles); tubercles usually abundantly covering the branches, hemispherical to almost verrucose, rarely only locally distributed; fibrils slender, scattered to irregularly distributed; fibercles absent; soralia developing from tubercles on secondary branches and on the cortex of terminal branches, slightly stipitate, not enlarging; isidiomorphs few to abundant, usually short within soralia on terminal branches, sometimes developing into isidiofibrils on secondary branches; cortex opaque (mat in section, appearing almost milky-white), moderately thick, (9 –)10 –12(–13 %); medulla compact and thin, (6 –)8.5 –12.5(–14 %); axis white, thick (48.5 –)52– 61.5(– 64.5 %), with an A/M-ratio > 4.5; apothecia and pycnidia not observed.
Chemistry. Medulla of most Galapagos specimens with norstictic acid [P+ yellow, K+ yellow turning orange-red (crystals), C–,KC–]; rarely medulla lacking secondary metabolites [P–, K–, C–, KC–].
Distribution and ecology. New to Galapagos; according to Truong et al. (2013b) a common species throughout South America, reported from Chile (type specimen), Southern Brazil, (Gerlach et al. 2107), Argentina and Uruguay (Rodríguez et al. 2011), and Mexico (Herrera-Campos 2016). In Galapagos, both phenotypical varieties occur throughout the humid zone; two specimens of the typical phenotype have also been found in the high altitude dry zone of Cerro Azul. The atypical phenotype has so far only been collected at disturbed sites, in farmland areas, along open pastures, on fenceposts or introduced guava trees (Psidium guajava). The typical phenotype occurs in these areas too, where it has been found among remnants of native Zanthoxylum woodland.
Notes. The species is characterized by a thallus with regularly cylindrical, non-inflated branches, an opaque cortex, a thallus surface that is abundantly tuberculate, a trunk with blackish-brown pigmentation that typically extends above the first ramification, basal branches with a necrotic cortex, not enlarging soralia, and by a thick axis (A/M-ratio > 3.5). A full description is also provided by Herrera-Campos (2016).
The identity of specimens in Galapagos, here referred to U. aff. columbiana, is not entirely resolved. Material analyzed by HTLC either contains both usnic and norstictic acid or, more rarely, usnic acid only. Most specimens thus react K+ orange-red (crystals). On the continent this chemotype is present also, but Usnea columbiana s.str. (including the type) is instead characterized by salazinic, not norstictic acid [K+ yellow turning red (no crystals)]. The Galapagos specimens display a high degree of phenotypical variation, particularly with regard of their abundance of fibrils and isidiomorphs on the thallus branches. Few atypical specimens (cited separately) exhibit thin and filamentous terminal branches, contrasting with thick basal branches (±anisotomic-dichotomous ramifications). These specimens are also much less tuberculate than usually observed, the tubercles mostly confined to the secondary branches (tubercles sparse or even absent from the basal branches). This contrast with the more typical phenotype, where basal and terminal branches do not distinctly differ and the thalli are generally abundantly tuberculate throughout. Chemically the two phenotypes cannot be distinguished among the Galapagos specimens.
Similar chemical and morphological plasticity has been observed by the second author in continental South America. It is therefore quite possible that the specimens included here within the variation of U. columbiana would better be treated as several different taxa. Comparative studies of material from a much broader range across South America (including the Galapagos) are necessary, ideally in combination with molecular analysis. The prolonged isolation of the Galapagos populations from the mainland may explain part of this variation.