Description: Primary thallus crustose, ephemeral and rarely seen, consisting of subglobular, 0.1-1.5 mm wide warts. Podetia densely anisotomically (i.e. main branches thicker) mainly tri- or tetrachotomously branched, shrub-like, hollow inside, esquamulose, ecorticate, the surface compact or finely arachnoid, 4-10 cm tall, loosely attached, dying from the base, pale yellowish white, with open axils, the necrotic parts yellowish white; main axis distinct, 0.7-1.5(-2) mm thick; terminal branchlets unilaterally deflexed, curved, often browned at tips. Apothecia rare, terminal, dark brown, convex. Asci 8-spored, clavate, thickened at apex, with a K/I+ blue tholus and a K/I+ strongly blue outer gelatinous sheath, Cladonia-type. Ascospores 1-celled, hyaline, ellipsoid. Pycnidia dark, semi-immersed on the tips of podetia; pycnidial jelly colourless. Conidia cylindrical. Photobiont chlorococcoid. Spot tests: K-, C-, KC+ yellow, P+ orange-red (rarely P+ deep yellow or rarely P-), UV-. Chemistry: the most common chemotype contains usnic and isousnic acids (major) and substances of the fumarprotocetraric complex; other, much rarer chemotypes have no fumarprotocetraric acid (P-), or additional psoromic acid (P+ yellow).
Note: a circumpolar, boreal-subarctic-subalpine lichen, one of the most abundant elements of lichen-rich tundra-like vegetation on mineral soil in exposed habitats, sometimes also found on lignum. In the Nordic Lichen Flora, Ahti & Stenroos (2013) included subsp. squarrosa, probably the most widespread entity of the C. arbuscula complex in the Alps, into subsp. arbuscula, regarding the psoromic acid strain as a taxonomically unimportant chemotype. I follow them here, also because the species was often cited from Italy without specifiyng the chemistry. Subsp. squarrosa has been reported from Friuli (Tretiach & Hafellner 2000), Veneto (Thor & Nascimbene 2007), Trentino-Alto Adige (Bilovitz & al. 2014b), Piemonte (see Nimis 1993: 224), Valle d’Aosta (Matteucci & Vanacore Falco 2015) and Sardegna. (see Nimis 1993: 224).
Growth form: Fruticose
Substrata: soil, terricolous mosses, and plant debris
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly asexual, by thallus fragmentation