World distribution: The species has a strongly disjunct distribution. It occurs in the boreal region of both the Atlantic and Pacific coasts of North America (Jørgensen 1978, Goward et al. 1994), south-west Greenland (Alstrup 1986), northern Japan and the Pacific coast of Russia (surroundings of Vladivostok), and Europe (Jørgensen 1978). In Europe, it is restricted to Norway and Sweden. There are only four localities in Sweden (Jämtland, Lycksele Lappmark, and Pite Lappmark; Aronsson et al. 1995, Jørgensen 1988)
Ecology: Results. The substrate was indicated in all 32 localities. The main substrate is small Picea abies twigs (30). In two localities (983, 984) the substrate was moss covered rock. Notably, it has not been recorded on deciduous trees in Norway. The habitat is most often spruce forest in small brook ravines or in north-facing slopes close to rivers on marine sediments. The saxicolous habitats were shaded rock walls facing north, i.e. very different from the other recorded habitats. Noteworthy associates include Gyalideopsis piceicola, Lichinodium ahlneri, Lobaria pulmonaria, L. scrobiculata, Nephroma spp., Parmeliella parvula, Pseudocyphellaria crocata, and Ramalina thrausta. Discussion. According to Jørgensen (1978) Pannaria ahlneri is mainly a corticolous species which in Europe is confined to Picea abies. It is extremely hygrophilous, being dependent on ravines and north-facing slopes that provide shelter from strong insolation and desiccating winds. Only where the macroclimate is very humid it is less habitat selective, occurring in forests also outside ravines. Saxicolous specimens, mostly on schistose rock, are all from the marginal parts of its distribution area. In North America it is recorded also on deciduous trees. Pannaria ahlneri is probably a weak competitor on the main branches of Picea abies, being easily overgrown by large foliose lichens. The thinnest branchlets, where competition is expected to be weaker, are probably too dry. Accordingly it occupies a narrow ecological niche
Threats: Results. Recorded threats are forestry by clearcut felling (15), development, including road construction (5), pollution from nearby roads (3), agricultural expansion (2), and overgrowth (1). In the Dølaelva forest reserve (848), logging in surrounding areas is a serious threat making the locality more susceptible to stormfelling and desiccating winds. Discussion. In most localities the P. ahlneri populations have reached a critically low number. Logging is the main reason for the decline of the species. It is not known if vital populations can be maintained. This will depend on how the remaining known localities, and eventual new localities, and the whole forest landscape, are managed. The species is in strong danger of extinction from Europe
Status: All localities discovered by Ahlner from 1938 to 1954 (23) along with 4 localities discovered in the seventies and eighties have been investigated. In only three of these (848, 1436, 3555) the species was still present, in 23 it was regarded as extinct, and one was uncertain. With 5 recently discovered localities, the species is now known to exist in 8 localities, mostly with very small populations. More than ten specimens are known from localities 1704 and 3552 only. Two localities are within forest reserves (848, 1436)
Notes: Two collections listed by Jørgensen (1978) could not be found in the indicated herbarium, and are not included in this treatment.