The western bushy beard lichen, Usnea arizonica Motyka is a striking yellow-green foliose lichen that is easily recognizable by its long branches (3-10 cm long) adorned by short, thin, perpendicular spines. The thallus is also frequently decorated at the branch tips by large (3-10mm), conspicuous, saucer-shaped apothecia that bear spines on the margins. The branches of the thallus are not hollow, instead filled with white, cottony tissue.
This lichen can grow quite large, 30-40 cm in diameter, and is commonly found on trees in southwestern USA and near the coast of northern California. Although it can be confused with other superficially similar species (U. rigida, U, strigose, U. cirrosa, and U. subfusca), it can be distinguished via chemical spot tests. The medulla of U. arizonica turns yellow in the presence of p-phenyldiamine (PD). When exposed to potassium hydroxide (KOH), this species initially turns yellow, eventually fading to red. Although these chemical reactions develop slowly at times, they reliably distinguish U. arizonica from similar species in the genus.
Arizona State University Lichen Herbarium http://nhc.asu.edu/lherbarium/lichen_info/tlc.php
Brodo IM, Sharnoff SD, Charnoff S. Lichens of North America. New Haven, CT: Yale University Press; 2001.