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Physconia subpallida Essl.

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Scientific name
Physconia subpallida
Common names
IUCN Specialist Group
Assessment status
Assessment date
IUCN Red List Category
IUCN Red List Criteria
B2ab(i,ii,iii,iv,v); C2a(i)
McMullin, T., Allen, J. & Lendemer, J.
Stone, D.

Assessment Notes

The content on this page is fetched from The IUCN Red List of Threatened Species: https://www.iucnredlist.org/species/80703034/80703037


Physconia subpallida occurs in severely fragmented locations throughout eastern North America. Its Extent of Occurrence, Area of Occupancy, and extant and quality of its habitat have declined substantially over the past century due to widespread logging, air pollution, and development. Now, there are fewer than 2,500 individuals left (total population size estimated at 400 mature individuals) after substantial declines, and most subpopulations are very small, consisting of 1-5 individuals, with the largest subpopulation consisting of fewer that 250 mature individuals. The habitat for this species is now very limited throughout its range as it appears to require old-growth deciduous forests, which are rare and fragmented throughout its core range in south-eastern Ontario (Henry and Quinby 2010, McMullin 2015, McMullin et al. 2016, Allen and McMullin 2019). The total Area of Occupancy [AOO] is estimated to be very small, with the known AOO only 260 km2, up to a potential maximum of 324 km2. Therefore, this species is Endangered, B2ab(i,ii,iii,iv,v); C2a(i).

Geographic range

Physconia subpallida is endemic to the Appalachian Mountains, Great Lakes region, and Ozark Highlands of eastern North America (Esslinger 1994, McMullin 2015, McMullin et al. 2016, Allen and McMullin 2019, CNALH 2020). It has a wide-pread but scattered distribution and is rarely encountered (McMullin 2015, CNALH 2020).

Population and Trends

In 2009, 27 sites were known throughout its range, but only four were thought to be extant, two in Canada and two in the U.S.A. (COSEWIC 2009). Since 2009, the number of sites know has increased to c. 80, with c. 50 thought to be extant  (McMullin 2015, McMullin et al. 2016, Allen and McMullin 2019, CNALH 2020). Despite intensive lichenological exploration throughout north-eastern North America, Physconia subpallida is rarely reported. Fewer than 400 individuals are known (McMullin 2015, McMullin et al. 2016, Allen and McMullin 2019, CNALH 2020).

Demographic studies are needed to assess and monitor the sizes of remaining populations. Study of an extant population in Ontario has shown it to be stable since 2004, however many of the locations are known only from historical occurrences. Studies of herbarium vouchers have shown that the size of individuals and frequency of fruiting bodies has decreased over time. As such, we classify the population trends as decreasing. The only known region where this species is somewhat frequent is a c. 200 km2 area in south-eastern Ontario.

Population Trend: decreasing

Habitat and Ecology

The species typically colonises old deciduous dominated forests with low stem density and a sparsely vegetated understorey (McMullin 2015, McMullin et al. 2016). Most Canadian sites are in close proximity to water bodies, which helps maintain high and sustained relative humidity (COSEWIC 2009, McMullin 2015). Thalli at these sites usually occur in sheltered parts of the forest, where wind and light are limited on tree boles between 0.5 and 3 meters from ground level (McMullin 2015). Most extant individuals are on mature Ostrya virginiana with flaking bark, but it is also on Fraxinus, Populus, and Quercus (COSEWIC 2009, McMullin 2015, McMullin et al. 2016). Historical collections of Physconia subpallida are also on Juglans nigra, Ulmus, wood, and rock (COSEWIC 2009).


The widespread decline of Physconia subpallida is likely due to the large-scale loss of habitat from development, agriculture, and forestry, as well as air pollution. These pressures continue today, particularly from air pollution created by on-going development near extant sites, while the species is also impacted by harvesting. For example, the site with the highest number of known individuals (71), Calabogie Peaks (Lewis 2011), was harvested in 2019 and a considerable number of individuals were lost. Individuals on private land are particularly vulnerable to harvesting.

Conservation Actions

Allen and McMullin (2019) developed predictive models for suitable habitat for Physconia subpallida in south-eastern Ontario that resulted in the discovery of 37 new individuals. The development of models for suitable habitat in other parts of is range, followed by surveys, are recommended. All extant sites should be monitored for evidence of decline, and demographic studies are needed to assess and monitor the sizes of remaining populations. Education about this species, including identification training, should be provided. In regions where this species in known to occur, harvesting of trees from suitable habitat should not be carried out. Physconia subpallida should also be legally protected in the United States, where it is least abundant, and a recovery strategy that is similar to the one implemented in Ontario and Canada should be developed (Environment Canada 2015, Lewis 2011). Urgent action is necessary for the preservation of this species.

Use and Trade

Specimen collection is a potential threat to this species.

Source and Citation

McMullin, T., Allen, J. & Lendemer, J. 2020. Physconia subpallida. The IUCN Red List of Threatened Species 2020: e.T80703034A80703037. https://dx.doi.org/10.2305/IUCN.UK.2020-3.RLTS.T80703034A80703037.en .Accessed on 4 February 2024

Country occurrence