Notes: Locality: Diablo Wash, Ajo Mountains, Organ Pipe Cactus National Monument, Pima Co., Arizona, USA; 32° 0’36.63"N, 112°43’53.46"W, el. 695 m

Substrate and habitat: Growing on rough talus boulders in cactus and thorn scrub desert on the lower slopes of the Ajo Mountains.

Identification: The lichens in this observation belong to Wetmore’s Caloplaca Group G (see below): On rock; thallus yellow, orange, or cream." Caloplaca luteominia is somehow on the Pima County list, and occurs in Wetmore’s group G, but according to the range maps in the Sonoran Flora and on CNALH, that species is found only in California and Baja California. Caloplaca flavovirescens has a distinctive yellowish green thallus and orange apothecia, which does not fit this observation. Caloplaca squamosa and Caloplaca subsolita are both common throughout southern Arizona, and may even be found on the same rocks, according to Wetmore. He characterizes them, in part, as follows: “The typical C. subsoluta has a thinner, flatter, areolate to subsquamulose thallus. Typical C. squamosa is thicker, more convex, and usually distinctly squamulose. … The apothecia of C. subsoluta typically have no obvious thalline margin while C. squamosa always does.” That seems to support assigning the present observation to Caloplaca squamosa.

Caloplaca of Pima County, Arizona

The CNALH checklist of the lichens of Pima County, Arizona lists 17 species in the genus Caloplaca. Wetmore arranged the species of Caloplaca of the Sonoran Desert into 10 categories in his treatment of the genus in the “Lichen Flora of the Greater Sonoran Desert Region, Vol. 3” (2007, pp. 179-220). The Pima County species of Caloplaca are assigned to Wetmore’s categories as follows:

A. Thallus fruticose

none

B. Thallus and apothecia without anthraquinones

none

C. Lobed thallus on rock, without soredia or isidia

none

D. On rock, thallus gray or brown, K-

Caloplaca epithallina
Caloplaca sonorae
Caloplaca luteominia var. luteominia
Caloplaca pellodella

E. Thallus isidiate or sorediate, on bark or rock

Caloplaca pinicola
Caloplaca microphyllina
Caloplaca persimilis
Caloplaca citrina
Caloplaca demissa

F. On rock; thallus not obvious; disks orange to red-brown

Caloplaca arenaria
Caloplaca luteominia var. luteominia

G. On rock; thallus yellow, orange or cream

Caloplaca flavovirescens
Caloplaca luteominia var. luteominia
Caloplaca squamosa
Caloplaca subsoluta

H. On soil, mosses, or plant remains

Caloplaca luteominia var. luteominia

I. On bark; thallus gray or orange

Caloplaca durietzii
Caloplaca cerina
Caloplaca holocarpa

J. Parasitic species

Caloplaca wetmorei
Caloplaca epithallina