3.4. Ecology

Usnea species present in East Fennoscandia and British Columbia are primarily epiphytic (I–V), but may occasionally occur also on decorticated wood (especially Usnea hirta) (I, III, IV). Most of the East Fennoscandian shrubby species have been collected also from rocks (III), but only one saxicolous Usnea specimen (U. subfloridana) was found in the British Columbia material (IV). In East Fennoscandia, Betula spp., Picea abies and Alnus spp. are the most common substrates (III), and in British Columbia most species are equally common on conifers and deciduous trees (IV). U. rigida s. lat. represents a species which displays a distinct phorophyte preference, since it has been found solely from deciduous trees in British Columbia (IV), while U. hirta grows mainly on acid barks and lignum, also in East Fennoscandia (I, III, IV). The preference is partly due to bark properties, especially acidity, but climatic conditions have probably much more significance. For instance, Pinus is the most common tree species in dry habitats which are most suitable for U. hirta (I, III).

Species occurring in the moist old-growth forests of East Fennoscandia are best represented by U. glabrescens and U. longissima. The latter has been collected from Picea abies and Betula (one site) in East Fennoscandia (II), and mainly from Picea sitchensis, Tsuga heterophylla and Malus fusca in British Columbia (IV). U. longissima is probably not distinctly host specific since in Norway it does not favour spruces as phorophytes, but it favours old, humid spruce forests (Tønsberg et al. 1996). U. hirta has a divergent ecology being the most xerophytic of the Usnea species studied (I, III, IV). It grows most frequently in open pine forests and is commonly present even in semi-arid forests in British Columbia.