Thallus: foliose, orbicular, often forming rosettes, 1-3 cm diam. or irregularly expanded to 6 (-10) cm, often contiguous with adjacent thalli closely adnate and appressed lobes: generally radiating and contiguous (to partly overlapping centrally, or ± discrete at margins, only occasionally much reduced, flat to ± concave, 0.5-0.8 (-1.5) mm wide, short sublinear; irregularly dichotomously branched; margins: sparingly indented; tips: usually closely appressed upper surface: pale greenish yellow to yellow (becoming straw-colored in herbarium), rarely gray-green or with dead parts becoming black, often brownish at margins and lobe tips, dull, smooth, continuous or weakly maculate, flat and roughened near periphery, becoming rugose towards thallus center soredia: powdery, yellowish green to green, paler or concolorous with thallus, in ± discrete, well-delimited soralia; soralia: laminal (often on ridges) or subterminal (pedicillate, at the tips of short lobes), orbicular, eroded, flat or occasionally ± convex and forming mounds, rarely capitate, sometimes ± contiguous to form a continuously sorediate crust in older (central) parts of thallus upper cortex: 10-20 µm thick medulla: white, loose, sometimes poorly developed, 10-20 µm thick, of 2-4 µm thick hyphae; algal layer: continuous lower cortex: well developed, composed of hyphae ± parallel to the surface lower surface: gradually darkening from narrow brown zone at the margins to dark brown or black towards center (only occasionally uniform light brown to brown), shiny; rhizines: moderately abundant, to the margins and often extending beyond the lobe edges, concolorous with lower surface, simple (to sparsely branched) Apothecia: rare, occasional on lignicolous specimens, sessile to shortly pedicillate, up to 1-2 mm diam.; thalline margin: yellow or brownish, entire or becoming sorediate; disc: chestnut brown or red-brown, dull, at first concave, becoming flat or convex, epruinose; ephymenium: 8-12 µm pale brown or yellowish, smooth, with a covering of amorphous material; hymenium: hyaline, 50-60 µm high ascospores: ellipsoid or one end more pointed, 6.5-13.0 x 1.5-5 µm Pycnidia: rare conidia: falcate, 12-18 (-22) x 0.5-1 µm Spot tests: upper cortex K- or + faintly yellow, C-. KC+ yellow, P-, UV-; medulla P-, K-, KC-, C-, UV+ white Secondary metabolites: cortex with usnic acid (major) and sometimes atranorin (minor or trace); medulla with divaricatic acid (major) and nordivaricatic acid (minor). Substrate and ecology: common on bark of conifers, old wood, stumps and logs, rare on broad-leaved trees, sandstone and mosses over rocks; often on or near tree bases, below the snowpack, only occasionally on twigs or small branches. In open to shady forests or woodlands, occurring with P. hyperopta, but more tolerant of dry conditions World distribution: circumpolar, arctic to mid-boreal and higher elevations in the south; North America, Europe, northern Asia and Australasia Sonoran distribution: common in Arizona in spruce-fir zone, 1740-3535 m, occasional in Baja California (2800 m, on Abies) and in mountains north of southern California. Notes: In western North America this species is easily recognized in the field by the very narrow, closely appressed greenish yellow lobes, usually with abundant roundish soralia. Xanthoparmelia mougeotii is superficially similar but is collected only on rocks and has a generally shiny upper surface throughout and a K+ yellow, P+ orange medulla (stictic acid). Related species in eastern North America are Parmeliopsis subambigua Gyeln., which has diffuse laminal soralia arising from pustules and is restricted to the southeastern coastal plain, and P. capitata R. C. Harris, which has mostly capitate soralia on the tips of upturned narrow lobes, and a consistently pale lower surface. The illustration in Hale (1979) labeled as P. ambigua is actually of P. capitata (Hinds 1999).