Iceland Moss

Iceland Moss (Cetraria islandica) is a terricolous lichen of the family Parmeliaceae that grows in dense low mats and cushions on open ground rather than on trees. Despite the name it is neither a moss nor confined to Iceland — it is circumpolar, occurring across arctic tundra and on mountain heaths well south of the Arctic Circle.

The thallus form sits somewhere between foliose and fruticose: the lobes are flat straps, but they are erect and strongly inrolled so that they read as channels or half-tubes, and the colony stands up from the ground in a bristly tuft 3-10 cm high.

The genus name comes from a shield-like reference to the apothecia, but in the field the character that identifies Cetraria islandica is the fringe of short, stiff, dark projections (spinulose cilia) along the lobe edges, which give the margins a minutely toothed silhouette when viewed against the light.

Thallus and lobes

The thallus forms erect, loosely branched, tufted cushions typically 3-10 cm tall. Lobes are flattened straps 3-20 mm wide, irregularly forked, and characteristically curled or channelled inward so that each lobe looks like a trough or partly rolled tube.

Colour

Upper surface is chestnut-brown, olive-brown to dark reddish-brown, often paler and greener toward the lobe bases and considerably paler — greyish to whitish-brown — on the inner (lower) face. Colour deepens and the thallus turns leathery and pliable when wet, becoming stiff and crisp when dry.

Margins

The lobe margins carry short, stiff, dark cilia or spinules that project outward like minute thorns. This fringed edge, best seen with a hand lens against a bright background, is the key field character.

Surface detail and fruiting

Scattered white pseudocyphellae (small pale pores in the cortex) appear as fine flecks and short streaks on both faces. Apothecia are uncommon: flat to slightly concave dark brown discs, formed on the flattened tips of lobes with a slightly toothed margin.

Iceland Moss grows on the ground among mosses, dwarf shrubs and other terricolous lichens in open, exposed, acidic habitats — arctic and subarctic tundra, dwarf-shrub heath, moorland, blanket bog margins, open pine and birch woodland floors, and alpine fellfield above the treeline. It needs light and dislikes competition from tall vegetation, so it thrives where snow cover and thin soils suppress vascular plants.

The distribution is circumpolar boreal-arctic, extending through Iceland, Scandinavia, northern Russia, Siberia, Alaska, Canada and Greenland, and reaching south along mountain ranges such as the Alps, Carpathians, Scottish Highlands and the Rockies. It is perennial and persists through winter beneath snow, resuming growth as soon as it thaws and rehydrates.

Cetraria islandica is a self-supporting photosynthetic partnership of fungus and green alga that lives on the soil surface, drawing water and minerals from precipitation and thin humus rather than from a host. It is a slow but persistent coloniser of open ground and, in heavily lichen-dominated tundra, forms a major component of the ground layer alongside Cladonia species.

Ecologically these lichen mats stabilise thin soils, insulate the ground surface, retain moisture between rain events and slowly build organic matter as older basal tissue decays. Spread is mostly by fragmentation — dry lobes snap easily and are moved by wind, water and grazing animals — with sexual spores from the infrequent apothecia playing a smaller role.

Cetraria ericetorum is the nearest match and differs in having narrower, more deeply inrolled, almost tubular lobes, with pseudocyphellae confined mainly to the margins rather than scattered over the faces. Cetraria aculeata and related dark species are much more slender, round in cross-section and near-black, lacking the broad flattened channelled straps.

Cladonia species that grow in the same heath share the erect habit but are typically pale grey-green to whitish and build hollow stalks (podetia) rather than brown flattened lobes. Flavocetraria nivalis, another tundra companion, is a pale yellowish to straw colour with crisped, wrinkled, spineless lobe margins rather than the dark brown spiny-edged straps of Iceland Moss.