The small, isolated, and widely-disjunct subpopulations of this epiphytic cyanolichen are restricted to areas of long ecological continuity in northern hemisphere, temperate, highly humid/hyperhumid, oceanic/montane areas of western Europe and wastern North America. This species is reported from 10 northern hemisphere countries. Over 40% of records come from the countries where the species has already been regionally assessed as threatened: listed as Near Threatened in Britain (Woods and Coppins, 2012) and Endangered in Norway (Bjelland and Ihlen, 2013). The subpopulation in Norway has been documented to have declined by 45% between 1995-2020. Climate change and poor habitat quality is forecast to cause losses in the UK, one of its population strongholds. Current losses and the climate change threat combine with declines forecast from the impacts of forestry in Canada and France, and a lack of new habitat. As the species also occurs on trees with a less acidic bark, it is susceptible to the decline of the European Ash (Fraxinus excelsior), which is under threat from an invasive fungus, Hymenoscyphus fraxineus. Future population reductions of 30-70% are estimated based on projecting current rates of decline forward over the next 90 years, with the lower estimate estimated based on direct losses to host trees from forestry, development and management changes and the higher figure suspected by adding the effects based on climate change forecasts. Threats are exacerbated by its low occurrence numbers and widely disjunct subpopulations. For these reasons, this species is listed as Endangered under criteria A3ce+4ace.
Leptogium hibernicum is known from limited and widely disjunct locations in eastern North America (southern Appalachians, USA, and Nova Scotia, Canada) and oceanic/montane western Europe i.e. Norway, western Ireland, western Scotland, Spain, Portugal, France, Greece (Muggia et al. 2018). A recent discovery in the Caucasus Mountains near the Black Sea has been documented by Urbanavichus et al. (2020).
Leptogium hibernicum has been recorded from 10 countries. The status of the global population is suspected to be unstable and declining and, in at least 40% of the countries, habitats where it occurs are under threat from human activity.
Most of the known records come from its apparent strongholds in Norway, the British Isles and western France, where, although it is most frequent, it has been assessed as under threat: Endangered in Norway, Near Threatened in Ireland and Scotland and and considered rare and threatened in France (Roux et al. 2020; D. Masson pers. comm. 2021), despite 45 records from the Pyrenees Atlantique Departement in the south-west of France (D. Masson pers. comm. 2021). In Norway, the cessation of practices such as pollarding and coppicing, which provide the species’ apparent preferred microhabitat there, has caused a 45% decline in population size (Bjelland and Ihlen 2013; T. Bjelland pers. comm. 2021). Other regional population declines are difficult to estimate because site specific data over the last 50 years are not readily available.
In most of the other countries, 15 or fewer locations have been reported, with only a single location each known for Greece and the Caucasus (Russia). Collections made in Spain and Portugal in the 1980’s are not from the same sites as specimens or observations made since 2000, with one or two exceptions, but there are no reports of revisits to the earlier sites to assess population trends. In North America, only Canada’s (Nova Scotia) population is known to have experienced a decline.
Future population reductions of 30-70% are estimated based on projecting current rates of decline forward over the next 90 years (three generations), with the lower estimate estimated based on direct losses to host trees from forestry, development and management changes and the higher figure suspected by adding the effects based on climate change forecasts.
Population Trend: decreasing
Leptogium hibernicum is a leafy epiphytic cyanolichen of temperate humid to hyperhumid oceanic/montane occurrence usually inhabiting stands of mature to old deciduous trees with neutral to basic, and frequently moss-covered, bark. In Nova Scotia, Canada, it also occurs on cedar (Thuja occidentali). In the UK, it was identified based on expert opinion as belonging to a guild of lichen epiphytes that are restricted to woodlands of long ecological continuity (Coppins and Coppins 2002), meaning that the appropriate microhabitat needs to have existed over long time periods (multiple centuries) to secure viable populations.
This species requires continual high humidity and moderate temperatures. The known locations are either within 25 kms of the coast or at elevations of more than 400 metres and are usually somewhat sheltered by topography or existing forest stands. In northern Europe (Ireland, Scotland and Norway) it has only been recorded on western coasts. Leptogium hibernicum spreads by means of relatively large and, therefore, heavy vegetative propagules, which are suspected to be predominantly effective over short distances (principally up to tens of metres; see review in Ellis 2012); no known fertile specimens exist (Jorgensen 2007, Smith et al. 2009), which likely limits its dispersal range. Its known occurrences consist of small subpopulations, widely distributed, and strongly disjunct, suggesting that its habitat requirements are seldom met.
Anthropogenic habitat loss and changes in forest management are the primary threats to this species. In eastern Canada (Nova Scotia), four known locations in the south-western part of the province are under pressure from clearcutting and biomass harvesting which can not only alter the required moisture regime, but can also reduce the available habitat for new juveniles. In Norway it occurs mostly on pollarded trees, a traditional woodland practice no longer in common use and some of the older occupied trees have fallen, limiting re-establishment. Forestry practices such as clearcutting and biomass harvesting in two countries (Canada, France), and natural senescence of mature to old trees in as yet undisturbed forests in all countries, will also limit or reduce current populations and restrict the establishment of new thalli. In some coastal areas of western Europe, particularly Portugal, housing and urban developments have and will continue to encroach on the species’ coastal locations.
In addition, climate change can alter moisture regimes to cause prolonged periods of drought and resultant risk of wildfires. Drought and wildfires which have affected France, Portugal and Spain in recent years are expected to continue, and changes in flammability of habitats is also expected due to invasion of non-native eucalypts in Spain. Small numbers of individuals are susceptible to destruction from stochastic events such as fire, windfall, and drought.
Considering that the species occurs on trees with a less acidic bark, it is susceptible to decline of the European Ash (Fraxinus excelsior), an important host tree in northern Europe that is now under threat from an invasive fungus, Hymenoscyphus fraxineus.