page_title: Physcia caesia
section_title: Distribution and habitat
hierarchical_section_title: Physcia caesia / Distribution and habitat
caption_attribution_description: English: Physcia caesia, a lichen species, growing on rock on St. Paul Island, Alaska.
context_page_description: Physcia caesia, known colloquially as blue-gray rosette lichen and powder-back lichen, is a species of foliose lichenized fungus. First described by Georg Franz Hoffmann in 1784, it is common across much of Europe, North America and New Zealand, and more patchily distributed in South America, Asia, Australia and Antarctica. There are 2 subspecies, P. c. caesia and P. c. ventosa, as well as a number of distinct forms and varieties. Molecular studies suggest that the species as currently defined may be polyphyletic. It is typically pale gray shading to darker gray in the center, and grows in a small rosette, usually some 2–3 cm across at maturity. It only rarely has apothecia, instead reproducing most often vegetatively via soredia, which are piled in round blue-gray mounds across the thallus's upper surface. It grows most often on rock—principally calcareous, but also basaltic and siliceous—and also occurs on bone, bark and soil. It is nitrophilic and is particularly common on substrates where birds perch.
context_section_description: Physcia caesia is widespread across much of the world, found in Arctic, boreal and temperate vegetation zones. Widely distributed across Europe, it is one of the most common species of Physcia in Greenland, and is abundant throughout Britain. Though found across much of North America, it is absent from central and southeastern United States, parts of the Great Basin, and northern Alaska. In South America, it is found in Argentina, Chile, and Peru; common and widespread in temperate areas, it occurs only rarely and only at elevations above 3,700 m (12,100 ft) in tropical regions. It occurs patchily across Asia—in India, Bhutan, Nepal and Japan—as well as in East Africa. It is described as "cosmopolitan" in New Zealand, has been reported from a number of sites in southeastern Australia and Tasmania, and also occurs in Antarctica.
  It is common on calcareous substrates, growing on limestone (including tufa) and concrete. It also grows on basaltic and siliceous rock, as well as on bone. It is particularly common on rocks where birds perch and defecate. It is uncommon on bark; however, in the Colorado Rocky Mountains, it is one of the predominant lichens on juniper trees, growing primarily within 0.1 m (3.9 in) of the ground on the northern and eastern side of trunks. In Antarctica, it grows on mosses, soil and gravel, and may be among the species contributing to the formation of a soil crust on the continent. In California, it is found in montane and subalpine forests ranging from 1,000–8,000 ft (300–2,440 m) in elevation.
caption: Physcia caesia on rock on St. Paul Island, Alaska
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