Usnea sphacelata

Usnea sphacelata is a species of saxicolous (rock-dwelling), fruticose lichen in the large family Parmeliaceae. It is found in both polar regions of Earth, as well as in southern and northern South America and in New Zealand.

Taxonomy
The lichen was formally described as a new species in 1823 by Scottish botanist Robert Brown. The type specimen was collected from Melville Island in the Canadian Arctic Archipelago.

A 2007 study using phylogenetic and morphological analysis of a large number of Antarctic Usnea specimens suggests that Usnea sphacelata comprises at least two genetically distinct groups with no clear differences in morphology.

Description
Usnea sphacelata has a range of distinctive morphological characteristics. Its is relatively thin, accounting for about 20–40% of its structure. Soralia, which are numerous, are typically restricted to the ultimate. Minute, which are often pigmented, can be observed on its surface. are usually absent from this species. Additionally, its pigmentation is noteworthy; the upper portion is conspicuously variegated, or it can be continuously pigmented towards the tips. This set of features helps to distinguish Usnea sphacelata from other sympatric Antarctic Usnea species, including U. antarctica, U. aurantiaco-atra, U. subantarctica, and U. trachycarpa.

Distribution and habitat
The lichen has a bipolar distribution; that is, it occurs in both the Antarctic and the Arctic. It is also found in southern South America, the northern Andean regions of South America, and the South Island of New Zealand. In the Windmill Islands area of continental Antarctica, Usnea sphacelata tends to grow in drier sun-exposed areas where snow does not tend to persist. Typical lichen associates at this location include Buellia frigida, Buellia soredians, Pseudephebe minuscula, and Umbilicaria decussata. In some regions, the crustose Buellia frigida is often the only species that can become established on smooth, ice-polished rock. Once its thallus is about 2 cm or more in diameter, Pseudephebe minuscula or Usnea sphacelata often start growing near the centre of the thallus.