A apparently rare lichen typically forming sterile squamule mats of small mid-brown shallowly lobed squamules with white smooth undersides with an arachnoid surface. Distinctive features are the Pd+ yellow reaction of the medulla and the large black pycnidia on the squamules that are constricted at the base. Seems to be very rare in open low productivity habitats on old mine waste and in grazed heathland.
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, not seen in Britain to date, 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 and are absent for other Pd+ yellow Cladonia species with small squamules.
Over small stones on soil of an old lead mine in Wales and on acid humus over gravel on a little used tack in short grazed heathland in the New Forest. Associated with Cladonia firma, Cladonia strepsilis and Pycnothelia papillaria at the New Forest site. Almost certainly overlooked elsewhere and should be looked for in stable but very open short vegetation on acid soils.
In 2023 known from two sites, one in Carmarthenshire, Wales and the other in the New Forest, Hampshire, very likely to occur elsewhere.
Not assessed, potentially a Red List species but currently data deficient. Likely to be dependant on very open but stable vegetation, so vulnerable to loss to vegetation over growth with reductions in grazing pressure and dependant on infrequent disturbance of low productivity habitats.