Usnea

Usnea is a genus of mostly pale grayish-green fruticose lichens that grow like leafless mini-shrubs or tassels anchored on bark or twigs. The genus is in the family Parmeliaceae. It grows all over the world. Members of the genus are commonly called old man's beard, beard lichen, or beard moss.

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.

Systematics
The genus Usnea was circumscribed by Michel Adanson in 1763. He used the name designated by Johann Jacob Dillenius, whose earlier published description did not meet the rules of valid publication as established by the International Code of Nomenclature for algae, fungi, and plants. However, he did not specify a type specimen; the species Usnea florida, moved to the genus by Friedrich Heinrich Wiggers in 1780, has been designated as the lectotype. Since the establishment of the genus, hundreds of Usnea species have been described. A three-volume series by Józef Motyka published in 1936 and 1947 listed 451 species. By 2006, the genus contained more than 600 species, which made it one of the largest genera within the family Parmeliaceae. However, many former species are now regarded as morphological varieties and adaptations to local circumstances. The number of recognized species in Finland has decreased for this reason, for example, dropping from 34 in 1951 to 25 in 1963 and only 12 in 2000. In addition, some former Usnea species have been moved to other genera; for instance, Usnea longissima was renamed Dolichousnea longissima in 2004. By 2022, the overall number of species assigned to the genus had dropped to 355.

The name Usnea is probably derived from the Arabic word Ushnah, meaning moss or lichen, though it may also mean "rope-like". Based on a fossil Usnea found in Baltic amber, the genus is known to date back to at least the late Eocene, about 34 million years ago.

Description
Usnea lichens are fruticose. Structurally they are shrubby, often with many branches, and can be erect or. Some trailing species can grow to considerable size; strands of Usnea longissima, for example, may exceed 3 meters in length. Colours vary depending on the species, from straw-coloured, yellow-green or pale green through green or greyish-green to reddish or variegated red and green. Unlike other similar-looking fruticose lichens, species in this genus have an elastic chord or axis running through the middle of the thallus that can be revealed by gently pulling a filament apart from either end.

Usnea looks very similar to the plant Spanish moss, so much so that the latter's Latin name is derived from it (Tillandsia usneoides, the 'Usnea-like Tillandsia').

Distribution and habitat
Usnea lichens are widely distributed in both the northern and southern hemisphere, in both temperate and tropical regions.

They appear in areas with low levels of air pollution. They can often be found on the ground in areas with where trees or branches have recently been cut, such as orchards (after pruning) and active logging areas.

Ecology
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.

Usnea is very sensitive to air pollution, especially sulfur dioxide. Under poor growing conditions, such as areas high in pollution, they may grow no larger than a few millimetres, if they survive at all. Where the air is unpolluted, they can grow to 10–20 cm long. It can sometimes be used as a bioindicator, because it tends to only grow in those regions where the air is clean, and of high quality.

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".

Some believe that Usnea, in high concentrations, could possess some toxicity. 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.

Many indigenous peoples of Central and South America, including the Tarahumura and Mapuche people, have a history of dyeing with Usnea, generally to obtain orange and brown hues. The Tarahumura use them to dye wool blankets in brown and russet hues, and the Mapuche have used Usnea florida to obtain orange. There is also anthropological evidence that Usnea cocca sonca was historically used for dyeing in Peru, and a lichen called cuaxapaxtle was used near Mexico City.

Cosmetics
Usnea barbata has been used in cosmetic production for its antimicrobial and antifungal properties as a preservative and deodorant.

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.

Species

 * Usnea acromelana Stirt. (1898)
 * Usnea alboverrucata G.N.Stevens (1999)
 * Usnea amblyoclada (Müll.Arg.) Zahlbr. (1930)
 * Usnea angulata Ach. (1814)
 * Usnea antarctica Du Rietz (1926)
 * Usnea aranea Truong & P.Clerc (2016)
 * Usnea articulata (L.) Hoffm. (1796)
 * Usnea aurantiaciparvula A. Gerlach & P.Clerc (2017)
 * Usnea austrocampestris Øvstedal (2012) – Falkland Islands
 * Usnea bismolliuscula Zahlbr. (1930)
 * Usnea boomiana P.Clerc (2015)
 * Usnea brattiae P.Clerc (2007)
 * Usnea cavernosa Tuck. (1850)
 * Usnea cedrosiana P.Clerc (2007)
 * Usnea ceratina Ach. (1810)
 * Usnea chaetophora Stirt. (1883)
 * Usnea cirrosa Motyka (1937)
 * Usnea clerciana Truong (2016)
 * Usnea confusa Asahina (1956)
 * Usnea cornuta Körb. (1859)
 * Usnea crenulata Truong & P.Clerc (2013)
 * Usnea crocata Truong & P.Clerc (2011)
 * Usnea cylindrica P.Clerc (2011)
 * Usnea diplotypus Vain. (1925)
 * Usnea effusa G.N.Stevens (1999)
 * Usnea elata Motyka (1936)
 * Usnea elixii G.N.Stevens (1991)
 * Usnea esperantiana P.Clerc (1992)
 * Usnea exigua J.M. Rodr. & P.Clerc (2011)
 * Usnea filipendula Stirt. (1881)
 * Usnea firmula (Stirt.) Motyka (1936)
 * Usnea flammea Stirt. (1881)
 * Usnea flavocardia Räsänen (1936)
 * Usnea flavorubescens Truong & P.Clerc (2012)
 * Usnea fleigiae A.Gerlach & P.Clerc (2017)
 * Usnea florida (L.) Weber ex F.H.Wigg. (1780)
 * Usnea floriformis C.W.Dodge (1948)
 * Usnea foveata Vain. (1928)
 * Usnea fragilescens Hav. ex Lynge (1921)
 * Usnea fulvoreagens (Räsänen) Räsänen (1935)
 * Usnea galapagona Truong & P.Clerc (2011)
 * Usnea geissleriana P.Clerc (2006)
 * Usnea glabrata (Ach.) Vain. (1915)
 * Usnea glabrescens (Nyl. ex Vain.) Vain. (1925)
 * Usnea glauca Motyka (1930)
 * Usnea grandisora Truong & P.Clerc (2011)
 * Usnea grandispora A.Gerlach & P.Clerc (2017)
 * Usnea himantodes Stirt. (1883)
 * Usnea hirta (L.) Weber ex F.H.Wigg. (1780)
 * Usnea inermis Motyka (1936)
 * Usnea intermedia (A.Massal.) Jatta (1909)
 * Usnea kalbiana A.Gerlach & P.Clerc (2017)
 * Usnea krogiana P.Clerc (2006)
 * Usnea lambii (Imshaug) Wirtz & Lumbsch (2011)
 * Usnea lapponica Vain. (1925)
 * Usnea leana Bungartz, Truong & Herrera-Camp. (2018)
 * Usnea lutii J.M.Rodr. & P.Clerc (2011)
 * Usnea macaronesica P.Clerc (2006)
 * Usnea maculata Stirt. (1882)
 * Usnea marivelensis (Vain.) Motyka (1937)
 * Usnea mayrhoferi Herrera-Camp., Bungartz, Truong & P.Clerc (2018)
 * Usnea mekista (Stirt.) G.Awasthi (1985)
 * Usnea messutiae Wirtz & Lumbsch (2011)
 * Usnea molliuscula Stirt. (1883)
 * Usnea myrmaiacaina P.Clerc (2007)
 * Usnea neuropogonoides Motyka (1936)
 * Usnea nidifica Taylor (1847)
 * Usnea nidulifera Motyka (1937)
 * Usnea oncodeoides G.N.Stevens (1999)
 * Usnea oncodes Stirt. (1881)
 * Usnea oreophila A.Gerlach & P.Clerc (2019)
 * Usnea pacificana Halonen (2000)
 * Usnea pallidocarpa Wirtz & Lumbsch (2011)
 * Usnea parafloridana K.Mark, S.Will-Wolf & T.Randlane (2016)
 * Usnea patriciana Bungartz, Herrera-Camp. & P.Clerc (2018)
 * Usnea pendulina Motyka (1936)
 * Usnea perplexans Stirt. (1881)
 * Usnea poliothrix Kremp. (1874)
 * Usnea praetervisa (Asahina) P.Clerc (2004)
 * Usnea propagulifera C.W. Dodge (1948)
 * Usnea pulvinata Fr. (1846)
 * Usnea pycnoclada Vain. (1909)
 * Usnea pygmoidea (Asahina) Y. Ohmura (2011)
 * Usnea quasirigida Lendemer & I.I.Tav. (2003)
 * Usnea ramulosissima G.N.Stevens & R.W.Rogers (1979)
 * Usnea roseola Vain. (1921)
 * Usnea rubicunda Stirt. (1881)
 * Usnea rubricornuta Truong & P.Clerc (2011)
 * Usnea rubriglabrata Truong & P.Clerc (2016)
 * Usnea rubrotincta Stirt. (1881)
 * Usnea sanguinea Swinscow & Krog (1979)
 * Usnea saxidilatata J.M.Rodr. & P.Clerc (2011)
 * Usnea scabrata Nyl. (1875)
 * Usnea scabrida Taylor (1844)
 * Usnea silesiaca Motyka (1930)
 * Usnea sphacelata R.Br. (1823)
 * Usnea subalpina G.N.Stevens (1999)
 * Usnea subaranea Truong & P.Clerc (2016)
 * Usnea subcapillaris (D.J.Galloway) F.J. Walker (1985)
 * Usnea subcomplecta Truong, P.Clerc & Herrera-Camp. (2018)
 * Usnea subcornuta Stirt. (1881)
 * Usnea subdasaea Truong & P.Clerc (2011)
 * Usnea subeciliata (Motyka) Swinscow & Krog (1979)
 * Usnea subflammea P.Clerc (2006)
 * Usnea subflaveola Truong & P.Clerc (2013)
 * Usnea subfloridana Stirt. (1882)
 * Usnea subglabrata Truong & P.Clerc (2015)
 * Usnea subparvula A.Gerlach & P.Clerc (2017)
 * Usnea subrubicunda P.Clerc (2011)
 * Usnea subscabrosa Nyl. ex Motyka (1937)
 * Usnea tamborensis (Hepp) Motyka (1938)
 * Usnea taylorii Hook.f. & Taylor (1844)
 * Usnea torulosa (Müll.Arg.) Zahlbr. (1930)
 * Usnea trachycarpa (Stirt.) Müll.Arg. (1889)
 * Usnea ushuaiensis (I.M.Lamb) Wirtz, Printzen & Lumbsch (2008)
 * Usnea viktoriana P.Clerc & Otte (2018)
 * Usnea vrieseana Mont. & Bosch (1856)
 * Usnea wasmuthii Räsänen (1931)
 * Usnea xanthopoga Nyl. (1876)