Melanohalea olivacea

Melanohalea olivacea, the spotted camouflage lichen or spotted brown shield, is a species of brown foliose lichen in the family Parmeliaceae. It forms rounded, olive-brown to dark brown patches (rosettes) that are typically 5–8 centimetres across on tree bark, particularly on birches in boreal forests and mountain woodlands. The lichen is distinguished by small pale spots called pseudocyphellae scattered across its upper surface and a black lower surface with dark rhizines (root-like attachment threads). Originally described by Carl Linnaeus in 1753 as Lichen olivaceus, the species was moved through several genera before DNA-based phylogenetic studies in 2004 placed it in the newly created genus Melanohalea. In that genus, it is recognized as distinct from similar brown species based on both genetic data and physical characteristics.

Melanohalea olivacea has an almost circumpolar distribution (around the high latitudes of the Northern Hemisphere) across the Northern Hemisphere's boreal zone, occurring throughout Scandinavia and other parts of northern Europe, as well as North America, and parts of Asia, though it becomes rare towards more southerly temperate regions. The species is known for forming a clear upper boundary on trunks, sometimes called the "olivacea line", which matches the maximum winter snow depth in subarctic birch forests, a pattern long used as a natural indicator of snow depth in Scandinavian mountains. Its height on the trunk is strongly controlled by how long snow covers the bark, and it is sensitive to prolonged ice encapsulation during mild winters. Changes in snow cover and winter icing have been discussed as factors that could affect its local abundance and vertical position on trunks. Although globally secure, the species is listed as critically endangered in Germany and regionally extinct in the Czech Republic, where threats include nutrient enrichment of habitats (eutrophication) and changes in forestry and agricultural land use.