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 my 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 Chihuahjua. Among these species are the endemics to this region U. ammanni P. Clerc & Clerc & Herrera -Campos and U. nashii P. Clerc & Herrera-Campos, and others as U. cirrosa Mot., U. dasaea Stirt., U. hale P. Clerc, U. hirta (L.) F.H. Wig., U. mutabilis Stirred., U. rubicund Stirred., and U. trachista Mot.