Assessment Synopsis – Cetradonia linearis is a fruticose species that grows on rock outcrops and boulders and is narrowly endemic to the southern Appalachians. This species is threatened by habitat degradation due to invasive tree pests, climate change and resource extraction. Continued protection under the Endangered Species Act is required to ensure that this species does not decline.
Distinguishing Traits – The Rock Gnome Lichen looks like patches of small green fingers (squamules) growing out from rock outcrops and boulders. It is often fertile and bears black apothecia at the ends of the podetia. The podetia are solid, and occasionally branched.
Explanation of Chosen Red List Category and Criteria – This species warrants listing as Vulnerable under criterion C1. There are ~4,000 mature individuals documented throughout the range of Cetradonia linearis. The number of mature individuals was calculated from element occurrence records held by national forests, natural heritage programs, and national parks from throughout the range of the species. A mature individual was considered a distinct colony that is producing apothecia.
A generation time for this species is estimated to be 33 years, so three generations is a total of 99 years. A minimum 10% decline is projected for this species within the next three generations due to a combination of 1) Abies fraseri mortality caused by the Balsam Wooly Adelgid and climate change (Farjon 2013a), 2) Tsuga canadensis mortality caused by the Hemlock Woolly Adelgid (Farjon 2013b), and 3) climate change impacting cloud immersion regimes for high-elevation subpopulations (Cullata and Horton 2014).
There are 77 locations in North Carolina, seven in Tennessee and one location each in Georgia, South Carolina and Virginia (USFWS 2013). In total, there are ~4,000 mature individuals documented throughout the range of Cetradonia linearis. The number of mature individuals was calculated from element occurrence records held by national forests, natural heritage programs and national parks from throughout the range of the species. A mature individual was considered a distinct colony that is producing apothecia. When the number of individuals was not explicitly documented, it was calculated from the total reported cover, where a mature individual was presumed to be 10 cm in diameter. Thus, if a site was documented to have 1.0 m2 cover, it was estimated to consist of 100 individuals. Sites on high-elevation rock outcrops with extensive coverage were calculated differently as their average colony size is often about 1.0 m2 ranging up to many square meters for a single colony, thus 1.0 m2 was used as the colony size for large, high-elevation locations (e.g., Devil’s Courthouse and Whiteside Mountain). Sites in streams in Great Smoky Mountains were estimated to consist of 100 individuals based on the experience of the author, since no detailed observation data was available.
Population Trend: decreasing
As many occurrences of Cetradonia linearis occur in spruce-fir forests, impacts to this highly threatened ecosystem also pose a threat to this species (White et al. 2010). Currently, the greatest concern in these ecosystems is the Balsam Wooly Adelgid, which is decimating Fraser’s Fir throughout its range (Rose and Nicholas 2008, Rollins et al. 2010, White et al. 2010, Farjon 2013a). Lower elevation subpopulations are threatened by losses of hemlocks (Tsuga sp.) due to the Hemlock Woolly Adelgid (Krapfl et al. 2011). There have been recorded declines abundance and health of C. linearis in forests that are heavily impacted by the Balsam Wooly Adelgid (North Carolina Element Occurrence Report 2015).
The high-elevations in this region are characterized by daily, extensive cloud immersion; however, changes in these humidity regimes and cloud immersion have recently been documented (Culatta and Horton 2014). These changes will likely impact Cetradonia linearis subpopulations negatively as this species is not tolerant of heat and desiccation.If this species were no longer protected under the Endangered Species Act, subpopulations could be threatened by resource extraction, such as logging and mining, and road building and maintenance. However, as long as its legal protection status continues subpopulations on public lands will continue to be protected.