XVI International Botanical Congess
Usnea is a cosmopolitan genus whose species grow mainly in well-lit, humid, cold or temperate sites. Mostly epiphytic on conifers and on deciduous trees, some may also grow on dead wood or rocks, few species are totally saxicolous. Following some categories proposed for Mexican bryophytes (Delgadillo 1993), according to their known geographic distribution, several elements can be recognized among the Mexican species of Usnea: widely distributed elements, Mexican elements, Mesoamerican elements, and more narrowly-distributed endemic elements. A small number of species grows in the Sonoran Desert and adjacent regions in Baja California, Sonora, Sinaloa, and Chihuahua. Among these species are the endemics to this region U. ammannii P. Clerc & Herrera-Campos and U. nashii P. Clerc & Herrera-Campos, and others as U. cirrosa Mot., U. dasaea Stirt., U. halei P. Clerc, U. hirta (L.) F. H. Wigg., U. mutabilis Stirt., U. rubicunda Stirt., and U. trachista Mot.