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, or beard lichen.: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
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.
(From Wikipedia, June 2021)