Stereocaulon pileatum Ach.
Pixie Foam Lichen
Basis for Listing
Stereocaulon pileatum (pixie foam lichen) is widespread in the northern regions of North America and Europe, with disjunct occurrences at higher elevations elsewhere (Brodo et al. 2001). In the Great Lakes region, the species appears to be fairly rare, having only been collected from a handful of sites in Michigan, Ontario, and Minnesota (CNALH 2017). This species was unknown in Minnesota until a population was discovered in Cook County in 1951. Despite an abundance of potentially suitable habitat in the state, this species has only been observed a handful of times (Laurentian Mixed Forest Province). Because of its apparent rarity in Minnesota and the Great Lakes region, S. pileatum was listed as a species of special concern 2013.
Stereocaulon pileatum is an inconspicuous small white fruticose (shrub-like) lichen, with a crustose thallus. The thallus (lichen body) is granular, being composed of many distinct granules. The stalks (pseudopodetia) are short, typically less than 5mm (0.2 in.) tall and have soredia near the top.
There are approximately a dozen species of Stereocaulon (snow lichens) in Minnesota. Of these species, S. pileatum is distinct because of its small size and the presence of soredia near the top of the stalks. Pilophorus cereolus (nail lichen), an extremely similar species, has been reported from Minnesota, though these reports have not been verified and may, in fact, represent misidentifications of S. pileatum. The two species can be separated based on the tendency of the primary thallus of P. cereolus to be areolate, while that of S. pileatum is granular; additionally, the soredia of S. pileatum react pink to a KC spot test, while those of P. cereolus do not react to KC (Brodo et al. 2016).
Throughout its range, S. pileatum is a saxicolous (rock dwelling) species. With only a few collections of S. pileatum having been made in Minnesota, it is difficult to draw conclusions about the species’ preferred habitat. It appears that S. pileatum favors larger rock communities such as cliff and talus systems as well as rock outcrops; however, at least one specimen was documented from boulders in a fire-dependent forest. Additionally, all known populations of S. pileatum in Minnesota are in close proximity to bodies of water, where humidity levels are high.
Biology / Life History
Most populations of S. pileatum are sterile; the species primarily 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. Additionally, S. pileatum can reproduce sexually through the dispersal of spores. Spores are spread by wind, water, or potentially various animals and must land in suitable habitats. Once a spore has landed in a suitable area, it must find a specific algal partner in order for a thallus to form.
Conservation / Management
Likely the biggest threat to S. pileatum in Minnesota is anthropogenic disturbance. The rock communities in which this species occurs are stable systems. Mining, trampling, and air pollution are the most likely causes for decline in the populations of S. pileatum and other saxicolous lichen species.
Best Time to Search
Searches for S. pileatum can be conducted year-round, whenever lichens are not covered by snow or ice.
Conservation Efforts in Minnesota
In an attempt to better understand the distribution of S. pileatum in Minnesota, scientists are searching for this species as well as other rare lichens.