Spotted Camouflage Lichen

Melanohalea olivacea is a brown, foliose lichen that forms rounded rosettes, commonly about 5–8 cm across and sometimes reaching around 10 cm in diameter. The thallus is typically closely attached to the surface it grows on (the substrate) and divided into fairly broad lobes (the leaf-like divisions of the thallus), usually 2–5 mm wide, which can appear wavy or folded, especially towards the centre of older rosettes. Its upper surface ranges from olive-green to dark brown and is often dull in the central parts, while the outer lobes may look slightly shinier. The species typically lacks isidia and soredia (two common asexual propagules in lichens). Small, pale pseudocyphellae usually appear as small dot-like (punctiform spots, mainly on the lobe surface, and are easiest to see near the margins. The lower surface is typically black, with a narrow brown rim near the margins that lacks rhizines, and it bears dense, dark rhizines that are mostly simple (occasionally sparsely branched). Apothecia are common in mature thalli and are usually concentrated towards the centre rather than extending across the outer lobes. They are generally a few millimetres wide, with a dark brown disc and a raised margin that tends to become strongly crenulate and often pseudocyphellate as it ages. …

Melanohalea olivacea is an epiphytic lichen (growing on plants, especially tree bark) that grows mainly on bark in cool, open, moderately humid habitats, particularly in boreal forests and montane birch woodlands. It is most often recorded from birch bark (Betula spp.), but also from other broadleaved trees and shrubs such as alder (Alnus) and rowan (Sorbus), and only occasionally on dead wood or rock. In Fennoscandia it is strongly associated with well-lit situations and can be especially abundant in mountain birch forests, where it tends to avoid the lower parts of trunks that remain under snow for long periods. In northern Norway, the species preferentially occupies the upper, snow-free parts of birch trunks, unlike some other epiphytic lichens that remain buried under snow for much of the winter. In subarctic birch forests, M. olivacea often forms a conspicuous lower limit on tree trunks, commonly known as the "olivacea line", which corresponds closely to the maximum depth of the winter snowpack. Below this level, prolonged snow cover and ice encapsulation reduce thallus vitality, whereas above it the species can dominate the bark surface and overgrow slower-growing epiphytes. The height of this boundary varies with habitat type and winter conditions and has long been used as a natural indicator of late-winter snow depth in Scandinavian mountain birch woodlands. …

The species is sensitive to prolonged ice encapsulation at mild subfreezing temperatures; increased winter icing events have been discussed as a potential risk to its vitality in boreal and subarctic regions. Across boreal birch forests, variation in epiphytic lichen composition has been shown to be influenced more strongly by factors on individual trees, especially height on the trunk, than by broader ecosystem-scale variables, a pattern that applies to M. olivacea and related species. Because epiphytic lichens respond to climatic gradients independently of ground-layer lichens, M. olivacea and similar canopy species are considered useful indicators of atmospheric and macroclimatic change in boreal forests.