World distribution: The known European distribution of Usnea florida before 1950 is mapped by Clerc (1984), and includes Austria, Belgium, the former Czechoslovakia, France, Germany, Hungary, Norway, Poland, Portugal, Rumania, Spain, Sweden, Switzerland, Turkey, the former Yugoslavia, and Russia eastward to Caucasus. It is not known from Finland, and does not enter the Mediterranean region. In the British Isles, the species is southwestern (Seaward & Hitch 1982), and the northern limit of distribution is reached in southern Norway and Sweden (Almborn 1948). It is considered to be extinct from Denmark (Alstrup & Søchting 1989). The European distribution is comparable to that of Sticta sylvatica, as well as that of Bryoria bicolor, and Clerc (1984) classified the distribution as suboceanic. Collections from Central Europe are concentrated in the altitudinal interval 700-1500 m (Clerc 1984)
Ecology: Results. Reported substrates were Fagus sylvatica (9), Quercus spp. (9), Alnus glutinosa (3), Picea abies (2), Sorbus aucuparia (2), Pinus sylvestris (1), Betula sp. (1), Acer platanoides (1), Populus tremula (1), and Tilia cordata (1). The species grew on Quercus spp. in all known intact localities, but in two localities one and two additional substrate tree species were recorded. The species was not found on Fagus sylvatica, a previously common substrate. In 3 localities the species was only found on one single tree. In the largest population visited (3258), U. florida occurred on 10 trees. Visited localities with intact populations were open forests; 3258 was even southerly exposed. In two localities, the species inhabited branches 10 m above the ground, in one of these U. florida had survived even on the single old Quercus that was left in a young Picea abies plantation. Discussion. Habitat and substrate preferences of U. florida seem invariant over Europe (Clerc 1984). The species is considered to be photophilous, and especially frequent along forest borders, on roadside trees, and in forest clearings (Clerc 1984). The species is listed among the main indicators of ecological continuity in England (Rose 1992)
Threats: Results. Recorded threats are forestry (6), air pollution (5), and construction (1). Discussion. The species seems to be dependent on deciduous trees, especially Quercus spp. and Fagus sylvatica, but lack of suitable trees and habitats can hardly explain why the species has become so rare. Preferring the upper canopy, U. florida is especially exposed to airborne pollution, and its decline (see status below) may be due to increased air pollution. Usnea florida is rapidly declining over most of Europe because of air pollution and changes from mixed forests to Picea monocultures of (Seaward & Hitch 1982, Clerc 1984). The species is among the most sensitive lichens to SO2 (Hawksworth & Rose 1970, Marti 1985), and is listed among the species which have become very rare or extinct in polluted areas of England (Hawksworth et al. 1973)
Status: Seven old localities were visited. The species was found in 2, of uncertain status in 2, and assumed to be extinct from 3. Three new localities were reported. Highest abundance was recorded in locality 3258, which is protected as a nature reserve. Almborn (1948), based on notes from early lichenologists, suggested that the present Scandinavian distribution only represented fragments of an earlier distribution covering the whole area in which Fagus sylvatica occurs. The species is now sparse or absent in parts of Vestfold where it was frequently collected in the beginning of this century. The largest population visited by us consisted of only 10 trees, and we believe that the species is still declining
Specimens in other herbaria, litterature, etc.