Usnea is a genus of mostly pale grayish-green fruticose lichens that grow like leafless mini-shrubs or tassels anchored on bark or twigs.[2]: 203 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.[2]: 203
Members of the genus are similar to those of the genus Alectoria.[2]: 148 A distinguishing test is that the branches of Usnea are somewhat elastic, but the branches of Alectoria snap cleanly off.[2]: 148
The genus Usnea was circumscribed by Michel Adanson in 1763.[3] 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.[4] 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.[5] By 2006, the genus contained more than 600 species, which made it one of the largest genera within the family Parmeliaceae.[6] 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.[7] In addition, some former Usnea species have been moved to other genera; for instance, Usnea longissima was renamed Dolichousnea longissima in 2004.[8] By 2022, the overall number of species assigned to the genus had dropped to 355.[9]
The name Usnea is probably derived from the Arabic word Ushnah, meaning moss or lichen, though it may also mean "rope-like".[10][11] 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.[12]
Usnea lichens are fruticose. Structurally they are shrubby, often with many branches, and can be erect or pendent.[11] 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.[13] 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.[14]
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').
Usnea lichens are widely distributed in both the northern and southern hemisphere, in both temperate and tropical regions.[13]
Usnea lichens reproduce via vegetative means through fragmentation, asexual means through soredia, or sexual means through ascogonium and spermatogonium.[15] The growth rate of lichens in nature is slow, but the growth rate has been sped up in laboratory conditions where Usnea is being cultured.[16]
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.[17]
Usnea is very sensitive to air pollution, especially sulfur dioxide.[18] 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,[19] and of high quality.
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"[20]
There is reason to believe that Usnea, in high concentrations, could possess some toxicity.[21] The National Toxicology Program is currently evaluating the issue.[22]
Usnea species have been used to create orange, yellow, green, blue, and purple dyes for textiles.[23][24]
Usnea barbata has been used in cosmetic production for its antimicrobial and antifungal properties as a preservative and deodorant.[25]
When dry, Usnea lichens are flammable and can be used as a fire starter.[26]
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.[27] They ground it into powdery "flour" to make bread or ate it as mush.[28]
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.[29]
Usnea is a genus of mostly pale grayish-green fruticose lichens that grow like leafless mini-shrubs or tassels anchored on bark or twigs.: 203 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.: 203
Members of the genus are similar to those of the genus Alectoria.: 148 A distinguishing test is that the branches of Usnea are somewhat elastic, but the branches of Alectoria snap cleanly off.: 148
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