Description: Thallus foliose, heteromerous, dorsiventral, very closely attached, forming orbicular, regular rosettes to 3 cm diam, several thalli often coalescing to cover much wider surfaces. Lobes radiating, contiguous, flat, 0.5-0.8(-1.5) mm wide, sublinear, irregularly dichotomously branched. Upper surface whitish to grey, often pale brown at margins and lobe tips, dull, smooth, with powdery, white to blue-grey soredia arranged in well-delimited, circular, laminal soralia which sometimes coalesce into a continuously sorediate crust. Lower surface gradually darkening from a narrow brown zone at margins to dark brown or black in the center, with sparse, usually simple rhizines. Upper cortex paraplectenchymatous; medulla white; lower cortex of hyphae running parallel to the surface. Apothecia very rare, lecanorine, 1-2 mm across, with a brown disc and a sorediate thalline margin. Asci 8-spored, clavate, Lecanora-type. Ascospores 1-celled, hyaline, narrowly ellipsoid, 8-15 x 2-4 µm. Pycnidia rare. Conidia filiform, falcate, 16-30 x c. 1 µm. Photobiont chlorococcoid. Spot tests: cortex K+ yellow, C-. KC-, P-; medulla P-, K-, KC-, C-, UV+ white. Chemistry: cortex with atranorin; medulla with divaricatic acid.
Note: a mainly boreal-montane, circumpolar lichen found on basal parts of trunks, especially of conifers, with a long snow-lie; ecology and distribution resemble those of P. ambigua, but this lichen is slightly less photo- and more hygrophytic.
Growth form: Foliose, narrow lobed
Photobiont: green algae other than Trentepohlia
Reproductive strategy: mainly asexual, by soredia, or soredia-like structures (e.g. blastidia)