Usnea is a genus of fruticose lichens in the large family Parmeliaceae. The genus, which currently contains roughly 130 species, was established by Michel Adanson in 1763. Species in the genus grow like leafless mini-shrubs or tassels anchored on bark or twigs. Members of the genus are commonly called old man's beard, beard lichen, or beard moss. Usnea lichens are characterized by their shrubby growth form, elastic branches with a central cord, and distinctive soralia that produce vegetative propagules. They vary in colour from pale green to yellow-green, grey-green, reddish, or variegated, and range in size from a few millimetres in polluted areas to over three metres long in species like Usnea longissima.
Members of the genus are similar to those of the genus Alectoria. A distinguishing test is that the branches of Usnea are somewhat elastic, but the branches of Alectoria snap cleanly off.
Ecology
thumb|right|Usnea ceratina
Usnea lichens reproduce via vegetative means through fragmentation, asexual means through soredia, or sexual means through ascogonium and spermatogonium. The growth rate of lichens in nature is slow, but can be increased in laboratory conditions.
Like other lichens, Usnea often grows on sick or dying trees due to the pre-existing loss of canopy leaves, allowing for greater photosynthesis by the lichen's algae; this leads some gardeners to mistakenly blame the lichen for the tree's leaf loss and illness.
A study in Maritime Antarctica found that Usnea communities are predominantly distributed on wind-exposed convex terrain, making their presence an effective proxy for identifying areas with minimal snow cover during winter. High-resolution remote sensing imagery can accurately map these communities, offering insights for modeling permafrost distribution and ecological processes influenced by snow cover.
Usnea is very sensitive to air pollution, especially sulfur dioxide. This sensitivity has made air pollution-sensitive species such as Usnea hirta useful as air-quality bioindicators.
Uses
By humans
Traditional medicines
According to Paul Bergner, Author of Medical Herbalism, "the usnic acid in Usnea is effective against gram-positive bacteria such as Streptococcus and Staphylococcus, making Usnea a valuable addition to herbal formulas for sore throats and skin infections. It is also effective against a bacterium that commonly causes pneumonia."
Bolivian traditional healers called the Kallawaya use Kaka sunka in decoction to cure lung problems. The lichen is macerated in alcohol and rubbed onto the body of those suffering from "nervous fragility". The National Toxicology Program evaluated the issue, undertaking research involved feeding male and female rats and mice ground Usnea lichens containing usnic acid for three months at various concentrations. Rats suffered severe toxicity, with significant liver damage observed at various concentrations, while mice experienced liver toxicity, ovarian atrophy, and changes in reproductive cycles at higher doses. Additionally, both species showed weight loss at elevated exposure levels, and mice exhibited potential genetic damage after two weeks at high concentrations. A safe exposure level was established at 60 parts per million, below which no adverse effects were observed.
Dyes
Usnea species have been used to create yellow, orange, green, blue, and purple dyes for textiles. This wide variety of possible colors can be achieved due to variations in chemical composition depending on the species, locality, and race of a particular specimen. Specifically, Usnea can contain thamnolic, squamatic, barbatic, salazinic, and alectorialic acids, all of which can affect dye color.
Firestarters
When dry, Usnea lichens are flammable and can be used as a fire starter.
Food
Some Usnea species have been used as food sources during times of scarcity. For example, people in Bosnia and Herzegovina ate Usnea barbata during the Bosnian War, particularly in the winter, when other plant material was not readily available. They ground it into powdery "flour" to make bread or ate it as mush.
By other organisms
The northern parula, a species of New World warbler which breeds in North America, uses Usnea lichens in the construction of its nest in some parts of its range. Where these lichens have declined due to air pollution, the bird has also vanished as a breeding species.