Parmeliopsis ambigua (Wulfen) Nyl.

    Thallus foliose, rosette-forming, to 3-5 cm diam., tightly appressed, lobes to 0.5-1 mm wide, usually with tips indented and divided, upper surface smooth, pale grey-yellowish, pale grey when dry, pale yellow-greenish, greenish when moist, external lobe margin pale brown, numerous small soralia towards the lobe margin, isolate, contrasting with large subglobose soredia towards the thallus center, lower surface brown-blackish with rhizines, simple, concolourous, paler at tips. Apothecia very rare, 1-2 mm diam., disc flat, light-brown, exciple concolourous to the thallus, wavy. Ascospores simple, slightly kidney-shaped, colourless, 7-12 x 3-4 µm. Photobiont: Chlorococcoid. Cortex: C-, K+/- faint yellow, KC+/- yellow, Pd-; medulla : C-, K-, KC-, Pd-, UV+ white. On acid bark of trees (coniferous notably), seldom on wood (and in that case together with Hypocenomyces scalaris) or on rocks.

Beware of similar species:

_ Imshaugia aleurites is rather similar and shares the same habitat, but is covered with isidia and Pd+ orange.

_ Hypogymnia farinacea of which the thallus and soralia are wrinkled,  and medulla KC+ red.

_ On rocks, not to be confused with Xanthoparmelia mougeotii that is Pd+ orange.

N.B. Parmeliopsis hyperhopta, stricto sensu, is a very close species, but more grey and more often fertile.