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Physcia caesia, known colloquially as blue-gray rosette lichen and powder-back lichen, is a species of foliose lichen 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 (though some forms are considerably darker), and grows in a small rosette, usually some 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 nitrophily and is particularly common on substrates where birds perch.
Capable of growing at a multitude of angles on a variety of surfaces, Physcia caesia also tolerates a wide range of environmental extremes from the high temperatures of desert locations to the low temperatures of the Antarctic. It grows on both dry stone and that moistened by seepage, and can survive being irregularly submerged for extended periods underwater. Like many lichens that grow on rock, Physcia caesia is able to extract nutrients from the substrate upon which it grows, as well as from rainwater and atmospheric dust. It is threatened by habitat loss through development, as well as trampling or overgrowth of its location. A number of lichenicolous species are known .
The genus name Physcia means "inflated" or "sausage-like", referring to the form of the type species. The specific epithet caesia is a Latin word meaning "blue-gray". Physcia caesia is known colloquially as blue-gray rosette lichen and powder-back lichen.
The lichen's cortex and medulla react positively with potassium hydroxide (K), turning yellow. Cortex and medulla react positively with para-phenylenediamene (Pd) as well, also turning yellow. They produce no reaction with calcium or sodium hypochlorite (bleach – C), nor with KC (potassium hydroxide quickly followed by bleach). Among the substances produced by the lichen are atranorin and zeorin.
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 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 in elevation.
Like many lichens that grow on rocky substrates, Physcia caesia is able to accumulate a number of necessary nutrients—including nitrogen, phosphorus, potassium and iron—from the substrate upon which it grows, as well as from rainwater and atmospheric dust. Its hyphae can extend into substrate as much as . It is a and can tolerate higher levels of atmospheric ammonia than can many other species of lichen, because it can survive on substrates with a higher pH. It has been recorded growing on a lead surface (probably a weathered oxide). Physcia caesia is known to accumulate high concentrations of heavy metals, including chromium, zinc, copper and iron. It is slow-growing, with an increase in size of only per year for a colony observed in one study.
In Greenland, it often grows in association with other lichen species tolerant of enriched environments, including Umbilicaria arctica, Polycauliona candelaria, Rusavskia elegans, Physcia dubia, and Rhizoplaca melanophthalma. In Poland, it is part of bryophyte communities found on asbestos tile roofs, occurring with Syntrichia ruralis, Orthotrichum anomalum, Grimmia pulvinata, Schistidium apocarpum and others.
Foliose lichens such as Physcia caesia are intermediate in their response to air pollution — less sensitive than , but more sensitive than . Physcia caesia is found in cities, though at lower frequencies than more pollution-tolerant species. As with most lichens, Physcia caesia is impacted by habitat loss through development, as well as by the trampling or overgrowth of its location. However, the retention of old stone walls, buildings and bridges can help to support its populations, even in cities. Physcia caesia is attacked by lichenicolous fungi, including Polycoccum galligenum, Polycoccum pulvinatum, Zwackhiomyces physciicola and Bryostigma epiphyscium.
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