Montana Field Guide

GROWTH FORM: Foliose

THALLUS: Medium to large, stratified, and with an upper and lower cortex. Lobes closely appressed, elongate, narrow, and mostly 0.3-0.5 mm wide. Upper surface pale yellowish green, dull. Medulla white. Lower surface ivory to gray, black, or purplish and velvety (pruinose). Rhizines short, single, and scattered; sparse. Sources: Goward et al. 1994; McCune and Geiser 2023

PHOTOBIONT: Green-algal. Layer appears brighter green. Sources: Goward et al. 1994

REPRODUCTIVE TYPES: Sorediate. Apothecia. Sources: Goward et al. 1994; McCune and Geiser 2023

CHEMICAL REACTIONS: Cortex K+ pale yellow, KC+ yellow, UV+. Medulla UV+ white, C+ slowly yellow, KC+ reddish

TAXONOMY & NOMENCLATURE
The large and diverse Parmeliaceae family has been revised three times by Mason Hale (1929-1990), premier lichenologist with the Smithsonian Institution in Washington, DC (Wikipedia 2025a). Within this family, Hale became a leading expert on the genus Parmelia, which once included lichens of great morphological differences (Wikipedia 2025b). In 1974, Hale established the genus Xanthoparmelia (Vain.) Hale to separate out the "obligate saxicolous, epicorticate, usnic-acid containing lichen species" that were formerly classified under Parmelia (Hale 1986). In 1986 Hale established the genus Arctoparmelia Hale to separate from Xanthoparmelia the species that have a velvety, ivory-white to purplish lower surface, contain alectoronic acid, and are restricted to arctic-boreal regions (Hale 1986).

Arctoparmelia species are also called the Rockfrog Lichens, in reference to their green upper surface and strict association with rocks (Goward et al. 1994). The authors suggested Dissolving Rockfrog as the common name for Arctoparmelia subcentrifuga, which is assumed to refer to its soft and eroding cortex.

Arctoparmelia and Xanthoparmelia species are superficially similar, but differ in their chemistry, lower surface, and lobe width (McCune and Geiser 2023).

Arctoparmelia
* Upper Surface: Often dull.
* Lower Surface: Often velvety from apparent pruinosity
* Chemistry: Cortex KC+ yellow
* Lobes: Closely appressed to substrate. Elongate and narrower, often averaging 0.5 mm wide or less.

Xanthoparmelia
* Upper Surface: Often shiny or with a sheen.
* Lower Surface: Somewhat shiny; not velvety.
* Chemistry: Cortex K-
* Lobes: Tightly appressed to semi-erect. Elongate and wider, 0.5-5.0 mm.

IDENTIFYING ARCTOPARMELIA SPECIES

Arctoparmelia subcentrifuga - Subcentric Ring Lichen, SOC
* Upper Surface: Soft cortex that appears to erode.
* Lobes: Often turned downwards at tips.
* Reproduction: Soredia present and diffuse on the upper surface.

Arctoparmelia centrifuga - Concentric Ring Lichen, SOC
* Upper Surface: Firm cortex
* Lobes: Rarely turned downwards at tips.
* Reproduction: Lacking soredia

Arctoparmelia incurva - Subcentric Ring Lichen, NOT DOCUMENTED
* Upper Surface: Firm cortex
* Reproduction: Soredia present in discrete, roundish soralia.
* Lower Surface: Dark.
* Distribution: It could be found in Montana as the nearest occurrence is in British Columbia, Canada.

Arctoparmelia subcentrifuga grows on acidic or siliceous rock in the arctic-alpine to boreal zones (Goward et al. 1994) In Montana, lichens grow on somewhat base-rich rock in rather sheltered areas in the montane zone (Goward et al. 1994; McCune and Geiser 2023). Specifically, they are found in cold, rocky sites, often where there is cold air-drainage at the base of north-facing coarse talus slopes (McCune and Geiser 2023). Known also as “slide-rock coolers” by early Anglo-American settlers, winter moisture penetrates deeply into the talus where it freezes and gradually thaws in the summer (McCune et al 2014). Air that contacts the buried ice and cold rock seeps out along the base of the talus. Recreationalists recognize these areas by the cool air they feel on hot summer days. These habitats are important refugia for northern species.

In a study of the Jonas Rockslide in Alberta, Canada, Arctoparmelia subcentrifuga was found to grow on steep rock surfaces (John 1989). The study found that steep rockfaces tend to receive less solar radiation than shallow rockface slopes, especially if north-facing. Steep rockfaces are drier because snow is not retained, less precipitation is received, and runoff is faster.

CHEMICAL COMPOUNDS
Lichens produce primary and secondary chemical compounds that provide a variety of essential and specialized roles (McCune and Geiser 2023). Readers are encouraged to consult Nash (1996) and McCune and Geiser (2023) for more information.

Compounds found in Arctoparmelia subcentrifuga include: atranorin and usnic acids in the cortex and alectoronic acid in the medulla (Goward et al. 1994).

TALUS SLOPE-LICHEN COMMUNITY
Rock loving lichens (saxicolous) can exhibit species-specific microhabitat requirements and be sensitive to the microclimate conditions (John 1989). This is partially because of their small size, intimate relationship with their substrate, and a relative lack of regulatory mechanisms to control their temperature and water loss. Rocks absorb little moisture and exhibit large fluctuations in temperature on a daily and seasonal basis. The spatial pattern of saxicolous lichens was studied at the Jonas Rockslide in Jasper National Park, Alberta (John 1989). Of the 106 saxicolous lichen species found on the rockslide, lichen coverage was 87%. Growth forms were composed of: 47% crustose, 15% foliose, 9% umbilicate, 7% fruticose, and 6% filamentous. In this system, mosses had less than 1% cover. In general, the high light environment coincided with a darker colored thallus and apothecia and a greater degree of pruinosity when individuals of a given species were compared to those in less exposed habitats. They also found that the erosive power of wind coincided with eroded and depauperate thalli when growing in high light talus slopes, particularly for foliose species. Although some species are generalists, many lichen species tended to associate with specific microhabitats, such as, crevices and overhangs; exposed edges; rock face centers with low slopes; steep rock surfaces; sheltered rock surfaces; periodically flooded rock; and newly exposed rock surfaces. Arctoparmelia subcentrifuga was found to grow on steep rock surfaces – see Habitat for more details.

Arctoparmelia subcentrifuga propagates through vegetative (asexual) diapsores: soredia. Sexual reproduction of the fungus occurs by spores produced from apothecia.

SYMBIOTIC PROPAGULES
Soredia propagate the whole lichen, asexually, without re-lichenization (Budel and Scheidegger in Nash [ed.] 1996). For Arctoparmelia subcentrifuga soredia are scattered on the upper surface.

GENERATIVE REPRODUCTION
Apothecia are cup- or disk-shaped fruiting bodies on the thallus in which spores, sexual reproduction of the fungus, develop (Budel and Scheidegger in Nash [ed.] 1996). For species of Arctoparmelia, apothecia are characterized as having brown discs located on the upper surface of the thallus and spores that are simple, ellipsoid, and colorless; 8 per ascus (Goward et al. 1994).

Climate change may be a threat to the habitat of Arctoparmelia subcentrifuga.