A large, foliose lichen that is yellow orange when exposed to strong sunlight and greenish grey in shade. The outer edges of the lobes widen and are often turned up. Apothecia are often abundant in the centre of the thallus, and have an orange disc surrounded by a paler margin.
In Scotland, Xanthoria parietina is one of the most common foliose lichens that can be found on nutrient-rich and nutrient-enriched bark and rock. It is particularly abundant on farmland trees and on trees and rocks along the coast. BLS distribution map
Xanthoria parietina can be distinguished from Xanthoria aureola by the formation of more or less neat rosettes consisting of lobes that broaden at the outer edges as opposed to separate, but overlapping strap-like lobes that do not broaden at the apex significantly. Both Xanthoria parietina and Xanthoria calcicola form neat patches, but Xanthoria parietina tends to have a brighter yellow-orange rather than deep orange, less wrinkled appearance with apothecia rather than coralloid isidia.
Xanthoria parietina is one of the foliose species that is most resistant to both sulphur dioxide and nitrogen pollution, which explains its abundance on farmland trees. It can, however, also be found in situations that are naturally nutrient-rich, such as trees and rocks in coastal areas.
scottishlichens.co.uk 