Dirinaria frostii is a highly distinctive foliose lichen of sheltered rock habitats that occurs in deciduous forests. It is distributed in mountainous areas of eastern and central North America and the Sonoran Desert region of southeastern North America and Central America. The population is estimated to consist of 9,500 functional individuals and suspected to have declined 43% in the last three generations with documented declines in EOO and AOO during this same period. A future decline of 10% is estimated based on the high likelihood of extirpation of sites in the southwestern portion of the range due to changing fire regimes with larger and more intense fires. For these reasons the species is assessed as Vulnerable under criteria A2c and C1.

This species was first described as Squamaria frostii over a century ago and belongs to the family Caliciaceae. It has been continuously recognized as distinct since it was described and is not likely to be confused with any other foliose lichens in its range. Previous reports may be found under the names Lecanora frostii (Tuck.) Tuck., Physcia frostii (Tuck.) Zahlbr., and Pyxine frostii (Tuck.) Tuck.

This is likely a LC species proposed to be treated in an upcoming workshop.

Dirinaria frostii has a disjunct range, with occurrences found widely along the Appalachian Mountains, Ozark Highlands and other mountainous areas of central North America. The distribution extends into the Sonoran Desert Region of southern Arizona, and the northern Sierra Madre Occidentale across multiple states of Mexico (Baja California Sur, Chihuahua, and Sonora).

Dirinaria frostii is presumed extant at 181 sites based on fieldwork since 1990, with an AOO of 348 km2 and EOO of 1,936,000 km2. Based on field study of the species in the Central Appalachian and Southern Appalachian mountains of eastern North America, the majority of occurrences consist of 10 or fewer functional individuals, which suggest a population size of 1,810 or fewer functional individuals. However given the lack of detailed demographic data and exhaustive targeted search efforts for individuals at every site throughout the entire range of the species, we conservatively estimate the population size to be 9,050 functional individuals based on a higher maximum of 50 functional individuals per occurrence. This accounts for the potential discovery of additional undetected functional individuals at sites that have not been intensively studied to date.

Based on a decline from 318 sites documented since 1934, to 181 remaining presumed extant after 1990, the species has declined in EOO by 48% and AOO by 40% over the last three generations. This decline corresponds to a suspected loss from 137 sites, the majority of which are concentrated in the northeastern United States. Considering that the species is a slow growing foliose lichen of sheltered rock habitats, the number of individuals documented at recently surveyed sites is suspected to reflect the historical abundance of the species. Using the same population size estimate above for the extant population, we suspect the historical population to have comprised a minimum of 3,180 functional individuals and a maximum of 15,900 functional individuals, accounting for a range of 10-50 functional individuals per site. This suggests that there has been a 43% reduction in the number of functional individuals during the last three generations.

In addition to the past declines in EOO, AOO and number of functional individuals, we estimate a decline of at least 10% of the population due to the likely extirpation of the species from extant sites in the Sonoran Desert within the next 100 years due to increased fire extent and intensity.

This species occurs in deciduous forests on cool, shaded, vertical, non-calcareous cliffs and ledges.  It is rarely found on bark where it grows at the bases of trees adjacent to rocks that also host the species.

Dirinaria frostii, like all cliff-face lichens, may be at risk from recreational climbing. A review of the impact of climbing on all cliff organisms found ambiguous effects on lichens (Holzschuh, 2016), but an experimental study focused specifically on lichens found a decline in lichen cover of 4.2% - 9.5% due to climbing (Schweizer et al., 2021). Changing fire regimes are a general threat to lichens including this species (Miller et al., 2018).  The Sonoran Desert in particular is experiencing a shift in fire regime due to climate change and invasive species, resulting in larger and more severe fires (Aslan et al., 2021). Increased fire extent and intensity are a risk for Dirinaria frostii in the Sonoran Desert. Loss of Dirinaria frostii in the southwest United States and adjacent northern Mexico would decrease the number of extant sites the species occurs at by at least 10%. Climate change throughout the range of the species is also a threat as increased heat and changing forest composition is resulting in temperature extremes and shifting habitats that are likely to impact the cool, humid, protected microhabitats the species occupies.

No direct conservation actions are known at present. Some occurrences may have benefited indirectly from protections or conservation actions, mostly associated with the encompassing protected land in US National Forest Wilderness Areas and in US National Parks. Dirinaria frostii inhabits both public and protected land. Given its habitat, it is less vulnerable to typical anthropogenic stressors like urbanization, but more vulnerable to specialized human activities like recreational climbing. Some specific sites may need additional protection.  An increase in public awareness and education about the species, particularly among rock climbers, is needed.

Further research that will aid in the conservation of this species includes population assessments and monitoring, life history research, population genetics studies, and ecological studies that incorporate threats to the species.

The species is collected by scientists for use in research and biodiversity documentation.