1.4. Ecology

All East Fennoscandian and British Columbia Usnea are primarily epiphytes, but most species may also occasionally grow on decorticated wood or more rarely on rocks. Primarily saxicolous Usnea species occur, for instance, in arctic and antarctic areas (subgenus Neuropogon), in the southern parts of North America (Clerc & Herrera-Campos 1997) and in neotropical mountains. Most Usnea species are more or less hygrophilous and photophilous, and therefore are found most abundantly in moist and open sites.

On the whole, Usnea species are highly sensitive to atmospheric pollution (e.g., Hawksworth & Rose 1970) and have become sparse or absent in urban and industrial areas. Usnea hirta (L.) F.H. Wigg., however, is relatively tolerant to pollutants (Kauppi & Halonen 1992), and occurs to some extent even in urban areas, while U. longissima has drastically declined in Europe partly due to air pollution (Esseen et al. 1981).