This species was described in 2013 from scattered sites in the southeastern United States as Tylophoron americanum. Since that time, additional populations have been located in the region, and several northern disjunctions have been found. No occurrences outside of eastern North America have been found and the species has been treated in detail as part of a taxonomic revision (Frisch et al. 2015) wherein it was transferred to the genus Sporodophoron. The species is easily recognized by its blue-gray crustose thallus, Trentepohlia photobiont, and white sporodochia.
Sporodophoron americanum is endemic to eastern North America. The majority of known occurrences are located in the southern Appalachian Mountains, especially eastern Tennessee (Blount, Monroe, Sevier Counties; Great Smoky Mountains and Unicoi Mountains). Scattered sites are also known from the Ozarks and Piedmont in the southeastern United States. Several disjunct sites have been located in Minnesota, and one has been found in Ontario, Canada.
The total and presumed extant EOO is 1532335 km2, and AOO is 164 km2.
The population was likely naturally fragmented historically, occurring on a small range of host tree species in mature age classes and only at sites in mature forest stands or in very limited areas of spatially restricted rock outcrops. Based on observations of the extant occurrences, the species is not locally abundant, occurring as 1-5 clustered functional individuals per site and often with colonies that are very limited in size relative to the total area of the host tree or rock outcrop. The current population size is estimated at 350-700 individuals based on a conservative estimate of 5-10 functional individuals per site, and the known occurrence at 70 sites. We suspect that the population declined historically (during the last 3 generations; 90 years, based on a 30 year generation time) due to extensive of logging, habitat loss, and land use change throughout its range (Martinuzzi et al. 2015, Yarnell 1998). These activities have led the present extant population to become increasingly fragmented, as the species is restricted to mature forest stands in suitable habitat and these areas have become very limited in extent and are no longer contiguous (e.g., Ervin 2016). We suspect that the already fragmented and reduced population is currently decreasing due to numerous ongoing and projected trends in anthropogenic and climate change impacts (Cartwright & Wolfe 2016; Keyser et al. 2013, Klepzig et al. 2014).
ASSESSMENT: Endangered B2a,b(iii)
This species is assessed as Endangered under criterion B2 based on the current AOO (164 km2), the severely fragmented population, and the continuing declines in quality of habitat observed and projected across range of the southern subpopulation.
Population Trend: Decreasing
This species occurs primarily on the bark of hardwood trees, especially chestnut oak (Quercus montana), in the grooves formed on the boles of mature individuals. It also rarely occurs on sheltered non-calcareous rock overhangs in high humidity habitats, usually directly associated with talus slopes or bodies of water. All of the northern disjunct occurrences are from rock overhangs associated with bodies of water.
There are two primary threats to this species, habitat fragmentation and loss (historical and ongoing) and impacts from air pollution, climate change and natural disasters (historical, ongoing and projected). The species occurs primarily on existing public lands, some of which are large in overall area and some of which are protected from resource extraction or habitat alteration. However it occurs in isolated locations where suitable habitat exists within large areas that are not suitable (i.e., remnant mature forest stands or massive rock outcrops with high humidity are spatially restricted within a matrix of younger forests, forests without appropriate tree hosts, drier habitats as well as more generally within a highly fragmented matrix anthropogenic land uses). These naturally dispersed locations were degraded and fragmented historically (last 90 years) due to extensive logging, building of roads, alteration of riparian corridors by dams, air pollution, agriculture and urbanization (Anderson et al. 2013, Keyser et al. 2014). All of the above are still impacting the southern subpopulation, although threats vary depending on the individual location. Within the last 30-40 years, fragmentation has continued as the region has undergone rapid population growth (Keyser et al. 2014, Klepzig et al. 2014). Available data indicate that the species is highly localized where it occurs, the habitat it has occurred in has become fragmented in the past and is increasingly fragmented in present. Further the region is currently experiencing climate change impacts (increased fire frequency and severity, intense storms that damage forest stands, droughts, temperature changes) and extensive alteration of forest communities due to invasive species (Klepzig et al. 2014). Although the small number of sites in the northern subpopulation are less threatened by the above forces, the Minnesota sites occur along rivers and could be extirpated by a major flooding event.
Many areas where the species is known are within existing public lands, however locations outside of National Parks and federally designated wilderness could be subjected to resource extraction or further fragmentation in the future. Increased education about the species and its threatened status is needed. Inclusion in local and national conservation policy is needed.
The distribution and ecology of the species are well known, however comprehensive location level demographic data and population estimates are needed. Targeted efforts to locate additional populations in suitable habitat are needed. A monitoring and recovery plan needs to be developed.