Arthrorhaphis citrinella (Ach.) Poelt
Bacidia flavovirescens, Arthrorhaphis flavovirescens, Bacidia citrinella
Basis for Listing
Arthrorhaphis citrinella (golden-dot lichen) is a species that occurs in the boreal forest and arctic, primarily in the western mountains and New England, with a small number of sites in the Great Lakes region. In the Great Lakes area, this species is apparently rare, having only been collected in Michigan and Minnesota. In Minnesota, this species is restricted to a handful of isolated populations. Arthrorhaphis citronella was listed as a threatened species in Minnesota in 2013.
Arthrorhaphis citrinella is a small crustose (crust-like) or squamulose (having small scales) lichen species. The thallus (lichen body) consists of small clusters of bright yellow squamules that can break down into granular clusters of soredia (powdery vegetative reproductive structures). Apothecia (disk-shaped fruiting bodies) are small and black but are typically absent.
In Minnesota, there are only a few species that superficially resemble A. citrinella. Arthrorhaphis citrinella can be separated from other species by the following characteristics: it has a yellow thallus that does not react to potassium hydroxide (KOH) in a chemical spot test (a chemical test used in lichenology); the thallus fluoresces bright orange when exposed to ultraviolet light, and it grows on soil or mosses in boreal forests instead of on bark or rock, where most similar species grow (Wetmore 2005; Lendemer 2010).
Throughout its North American range, A. citrinella occurs on noncalcareous soil (Brodo et al. 2001) in arctic and boreal regions. In Minnesota, this species is only known from a few locations. At one site, A. citrinella occurs on the top of a prominent rock feature in close proximity to Lake Superior (North Shore Highlands). This population was observed growing on soil under Pinus banksiana (jack pine) in a jack pine (bedrock) woodland. The site is exposed to the elements and likely persists at this location due to the climate modifying effect of Lake Superior. An addidtional site occurs in a northern mesic mixed forest on a small rocky point along the shore of Lake Superior. In this location, the species grows along the edge of the vegetated zone directly upslope from the Lake Superior rocky shore. A separate series of collections were made from thin soil along a road cut. In this location, A. citrinella occurs on soil stabilized by mosses and other lichens as well as on adjacent rock features exposed by road construction. Although this site does not represent a native plant community, it suggests that A. citrinella prefers sites with mild disturbance and little competition.
Biology / Life History
Arthrorhaphis citrinella reproduces asexually through the dispersal of soredia. Soredia are reproductive structures that contain both fungal and algal partners and can be dispersed over short distances. This species also reproduces sexually through the distribution of spores by wind, rain, and animals.
Conservation / Management
Arthrorhaphis citrinella has likely always been rare in Minnesota. This arctic/boreal species appears to require a specific suite of environmental conditions, which in our area are localized around Lake Superior. The handful of populations known in the state are located on public lands in areas where regular foot traffic may jeopardize the habitat of this terrestrial lichen species; therefore, the long-term survival of A. citrinella in Minnesota is uncertain.
Best Time to Search
Searches for A. citrinella can be conducted year-round, whenever they are not covered by snow or ice.