Cladonia brevis

Primary thallus persistent. Squamules small, 1-3 mm long, shallowly lobed, glaucous green to a rich brown (dry), olive-brown (wet) and ± cracked on the upper surface, white below with a smooth arachnoid surface, not sorediate. Podetia, appear rare in Britain, to 8 mm in length though sometimes undeveloped, clavate, unbranched or branched near the apex, flattened, the surface corticate and areolate. Apothecia brown, at the tips of podetia. Pycnidia large, black, constricted at the base, on the primary thallus. Thallus Pd+ yellow, K–, KC–, C–, UV– (psoromic and 2’-O-demethylpsoromic acids).

Cladonia brevis has a scattered distribution in Europe and is more common in North America, though populations lacking podetia are probably overlooked. It is related to the predominantly northern species C. macrophylla which has a similar chemistry but much larger squamulose podetia. In the absence of podetia it can be confused with C. pulvinata and C. symphycarpa which react Pd+ yellow. However, C. symphycarpa grows on basic soils, is much larger and is often K+ yellow becoming red or K+ yellow. The squamules of C. pulvinata are slightly larger, more lobed, with a strongly tomentose underside and frequently have pruinose tips. For Cladonia brevis, the  large black pycnidia constricted at the base are highly distinctive (but can be rare or absent) and are absent on other Pd+ yellow Cladonia species with small squamules. When glaucous green, the small rounded squamules with smooth white undersides are very similar to those of Cladonia peziziformis, but this is Pd+ orange-red.