Usnea is a genus of mostly pale grayish-green fruticose lichens that grow like leafless mini-shrubs or tassels anchored on bark or twigs.^[1]: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, or beard lichen.^[1]:203

Like other lichens it is a symbiosis of two or three fungi and an alga.^[2] In Usnea, the fungus belongs to the division Ascomycota, while the alga is a member of the division Chlorophyta.

Members of the genus are similar to those of the genus Alectoria.^[1]:148 A distinguishing test is that the branches of Usnea are somewhat elastic, but the branches of Alectoria snap cleanly off.^[1]:148

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.^[10]

Usnea is very sensitive to air pollution, especially sulfur dioxide.^[11] 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,^[12] and of high quality.

(From Wikipedia, June 2021)