Caloplaca parvula    Wetmore

A Species of Lichen 

MN Status:
Federal Status:


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Minnesota range map
Map Interpretation
North American range map
Map Interpretation

  Basis for Listing

Caloplaca parvula was unknown to science when this species was first collected in Minnesota in 1978 near Kettle Falls in St. Louis County. Only 2 additional specimens have been found in the state since then, both in St. Louis County. When Dr. Clifford M. Wetmore (1994) wrote a monograph of this group and borrowed specimens from all of the major lichen herbaria in the United States and Canada, one additional specimen was found that had been collected in Mackinac County, Michigan. The study of all related species in North America and Europe have proven C. parvula to be previously undescribed and not easily confused with any other known species. This species' rarity and habitat threats from changing land use practices where C. parvula grows necessitated its designation as a state endangered species in 1996.


A crustose lichen, C. parvula has a thin, greenish to dark grey thallus (lichen body), with black apothecia (disk-like fruiting bodies). Apothecia are small, 0.1-0.2 mm (0.004-0.008 in.) in diameter. Apothecia margins are thin to disappearing. Soredia (powdery vegetative reproductive structures) and isidia (finger-like projections) are absent from the thallus. When subjected to chemical spot-tests, the thallus of C. parvula does not react to potassium hydroxide. Spores are clear with 2 chambers and very small (10-12.5 µm x 4-5.5 µm). The isthmus between spore chambers is very narrow; no more than 1.5 µm wide, often little more than a septum (Wetmore 1981, 1994).


Caloplaca parvula occurs in old-age, deciduous-tree swamps, specifically Fraxinus nigra (black ash) bogs, near open water. These swamps typically have 30-36 cm (12-14 in.) diameter trees, but this lichen species is not found on these trees. Instead, the preferred substrate is 8-10 cm (3-4 in.) young regrowth, near the tree base, within 30 cm (1 ft.) of the ground. The forest canopy is fairly open, allowing lots of sunlight. Humidity in these habitats is high. Groundcover is minimal, typically composed of just a few Carex spp. (sedges). Caloplaca parvula does not occur in sites with abundant shrubs and groundcover. It is found in sites with some amount of standing water, but has never been found in a site without standing water. Water is in depressions in the swamp. The young trees or regrowth on which this species is found may themselves be in standing water. When searching for C. parvula, there is no value in checking branches or the canopy of trees as this species has never been found more than 30 cm (1 ft.) from the ground. Other lichen species often found in the same wet forest habitat include Lobaria quercizans (smooth lungwort) and L. pulmonaria (lungwort). Acer rubrum (red maple) is not typically found in these sites; however one specimen in Michigan was found on Acer rubrum (U.S. Forest Service 1999).

  Biology / Life History

Caloplaca parvula reproduces sexually, with the fungal spores having to land in a favorable environment and then associate with algal spores to be successful. This species has no asexual propagules. Spores can be distributed by air, wind, or water. Spores could possibly be carried by birds or insects, although this dispersal method is only speculative.

  Conservation / Management

Caloplaca parvula occurs in wet forests near lakes. Manipulation of lake levels could present a threat if it altered the hydrological regime of the habitat, possibly making this species disappear. Logging is not currently thought to be a threat, but natural loss of Fraxinus nigra bog habitat, such as from beaver (Castor canadensis) activity, severe windstorms, or old age, could negatively affect C. parvula populations. To protect this lichen, wetland forests need to be conserved. Transportation projects could degrade wetlands supporting C. parvula, especially if roads are built through Fraxinus nigra habitats. Wetland mitigation will not be effective at maintaining this species' viability, because C. parvula requires old wetlands (U.S. Forest Service 1999).

Searches for C. parvula may be conducted year-round when they are not covered by snow or ice.

  Conservation Efforts in Minnesota

Scientists are searching for C. parvula and other rare lichens in an attempt to identify and possibly preserve local populations.

  References and Additional Information

U.S. Forest Service. 1999. Population viability assessment in forest plan revision. Statement of purpose and reason. Draft species data records: Caloplaca parvula. United States Forest Service, Region 9.

Wetmore, C. M. 1981 (revised 2005). Keys to the Lichens of Minnesota. Department of Plant Biology, University of Minnesota, St. Paul, Minnesota. 92 pp.

Wetmore, C. M. 1994. The lichen genus Caloplaca in North and Central America with brown or black apothecia. Mycologica 86(6): 813-838.

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