Species of lichen
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Placidium arboreum, commonly known as the tree stipplescale, is a species of corticolous (bark-dwelling), squamulose (scaley) lichen in the family Verrucariaceae. "Placidium arboreum" is primarily found in the southeastern United States, but it also has occurrences in the western and northeastern United States, Mexico, the West Indies, Argentina, and Ontario, Canada. In its habitat, it typically grows at the base of hardwood trees, particularly oak species, and can occasionally be found on other tree genera or even over mosses on limestone. Its preferred ;dfn id="" ;substrate ;/dfn ; is the bark of oak trees, and the lichen usually establishes itself in bark crevices.
Originally described as "Endocarpon arboreum" in 1831, "Placidium arboreum" underwent various taxonomic changes before acquiring its current generic placement in 2004. Some authors had referred to it as "Endocarpon tuckermanii", but the name with the species epithet "tuckermanii" is now considered synonymous.
Taxonomy.
The lichen was first mentioned in the scientific literature as "Endocarpon arboreum" in an 1831 publication of Swedish botanist and mycologist Elias Fries, who attributed authorship to German-American botanist and mycologist Lewis David de Schweinitz. Fries wrote of the lichen: "In addition there is a variety, "Endocarpon arboreum" of Schweinitz from North America, which by its description should be referred here but from its whole structure seems to be a poorly developed "Sticta"". Fries does not seem to have accepted it as a valid species. Further, because no description or ;dfn id="" ;diagnosis ;/dfn ; of the taxon was provided (i.e., as a "nomen nudum", or "naked name"), the name was not validly published according to nomenclatural rules. American botanist Ezra Michener published the taxon validly when he included a brief description of the species in American botanist William Darlington's 1853 work "Flora Cestrica". American lichenologist Bruce Fink proposed a transfer to the genus "Dermatocarpon" in 1910. The taxon acquired its current generic placement when American lichenologist James Lendemer transferred it to "Placidium" in 2004. Molecular phylogenetics reveals that "Placidium arboreum" is sister to a clade of "Placidium" species characterized by cylindrical asci in their early developmental stages.
According to Lendemer, some later authors have used the specific epithet "tuckermanii" for this species ("Endocarpon tuckermanii" Ravenel ex Mont. was validly published by Camille Montagne in 1856). This was because in 1956, American lichenologist Mason Hale designated the name "Endocarpon arboreum" as a "nomen nudum", which opened the way to use the epithet "tuckermanii"; he was apparently not aware of Michener's publication. Since Michener's description of the species was published earlier than Montagne's, it has priority, and those names (and later recombinations) with epithet "tuckermanii" are relegated to synonymy.
Lichens in the genus "Placidium" are generally known in the vernacular as "stipplescale lichens"; the specific common name used for this species is "tree stipplescale". "Placidium arboreum" belongs to the ;dfn id="" ;catapyrenioid ;/dfn ; group within the Verrucariaceae, characterized by over 80 lichens that are squamulose (scaley), possess ;dfn id="" ;simple ;/dfn ; ascospores (devoid of septa), and do not contain algae in the hymenium.
Description.
It has a squamulose (scaly) thallus comprising individual rounded lobes typically measuring wide, although ;dfn id="" ;squamule ;/dfn ; widths of up to are also reported. These are relatively large, more or less overlapping squamules. In a field guide to lichens of Great Smoky Mountains National Park, the authors highlight the distinctiveness of the species due to its uniquely large squamules. This characteristic makes it stand out, and in this region, it is rarely mistaken for other species. The lichen is normally shades of brown and grey, but becomes bright green when wet. The medulla is white.
Dark brown, dot-like perithecia are scattered on the thallus surface. The underside of the thallus is paler in colour, with tufts of ;dfn id="" ;rhizohyphae ;/dfn ; that attach to the ;dfn id="" ;substrata ;/dfn ;. All of the standard chemical spot tests are negative, and it does not have any known lichen products. The ascospores made by the lichen are ellipsoid in shape, with dimensions of 10–15 by 4–6 μm. The ;dfn id="" ;photobiont ;/dfn ; partner of the lichen is ;dfn id="" ;chlorococcoid ;/dfn ; – a green alga from genus "Chlorococcum".
Similar species.
"Catapyrenium cinereum" is another squamulose lichen that bears a resemblance to "Placidium arboreum". Several distinctions can be observed in "Catapyrenium cinereum" – its brown to black rhizohyphae, the formation of a thick, dark ;dfn id="" ;hypothallus ;/dfn ;, a typically densely ;dfn id="" ;pruinose ;/dfn ; upper surface, and ;dfn id="" ;clavate ;/dfn ; (club-shaped) asci. Although some species from the genus "Clavascidium" may appear similar, they are characterized by specific rhizines and/or clavate asci.
Habitat and distribution.
"Placidium arboreum" is widely distributed in the southeastern United States, with a few scattered occurrences in the western and northeastern United States. It is rare in California, with some reports from locations in the Coast Range. The North American range of the lichen extends south to Mexico. The lichen has also been found in the West Indies and has been recorded in Tucumán, Argentina, where it was growing in a rainforest at an elevation of . In 2017, it was recorded for the first time in Canada after it was found in a few locations in Ontario; it is considered an uncommon species in this province.
The lichen typically grows at the base of hardwood trees, and the bark of oak is a preferred substratum. "Placidium arboreum" has been recorded on several oak species: "Quercus alba" (white oak) is a predominant associate in the northern part of its range, "Q. stellata" (post oak) and "Q. muehlenbergii" (chinkapin) occur further south, while "Q. virginiana" (southern live oak), "Q. arizonica" (Arizona white oak), and "Q. douglasii" (blue oak) are associated with the lichen in the south and southwest parts of its range. Tree genera that associate with "Placidium arboreum" less commonly include "Ulmus" (elm), "Fraxinus" (ash), "Carya" (hickory), "Platanus", "Liquidambar" (sweetgum), "Acer" (maple), and "Salix" (willow). The lichen generally establishes itself in the crevices of the bark. In rare instances, it has been recorded growing over mosses on limestone.
References.
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