Reindeer Lichen

Reindeer Lichen is the classic mat-forming lichen of northern landscapes, building springy grey carpets that can cover hectares of open ground in boreal forest and tundra. Despite the name 'reindeer moss' it is not a moss at all but a fruticose lichen — a three-dimensional, shrub-like partnership of fungus and alga with no flat leafy thallus at all.

Cladonia rangiferina sits within the very large genus Cladonia, which includes the cup lichens and the red-tipped British soldiers. The reindeer lichens form a distinctive subgroup once separated as Cladina: their branches are hollow, lack a proper outer cortex, and have no squamules (small leafy scales) at the base by the time they are mature.

What makes it recognisable is the colour and the branching pattern. The whole cushion is a soft ash grey to almost white, without any green or yellow cast, and each stem forks repeatedly — usually into three or four branchlets at each node — with all the fine tips characteristically bent in the same direction, giving the mat a subtly combed appearance.

Growth form

The lichen forms rounded cushions or extensive continuous mats 3–10 cm deep, made up of many erect, intricately branched podetia (stalks). There is no basal rosette of leafy scales in mature material; the whole visible structure is branching stems. Mats are spongy and springy when moist and become brittle and crunchy when dry.

Branches

Podetia are 3–10 cm tall and 1–2 mm thick, hollow, and repeatedly forked. Branching is typically into three or four at each fork, with the branch angles fairly wide. The ultimate tips are fine, tapering and brown-tipped, and characteristically all nod or curve toward the same direction across a patch — a very useful field character.

Colour and surface

Colour is a distinctive pale ashy grey to greyish white, without the yellowish-green tone of some relatives. The surface is matt and slightly downy or felty rather than shiny, because there is no smooth outer cortex; the surface layer is loose and cottony. Potassium hydroxide typically produces a yellow reaction on the surface, and para-phenylenediamine a red one, reflecting the atranorin and fumarprotocetraric acid content.

Fruiting bodies

Apothecia are uncommon and inconspicuous: tiny dark brown dots at the very tips of branches. Most reproduction and spread is by fragmentation of the mat rather than by spores, so a patch may go for years without visible fruiting.

Reindeer Lichen occupies open, nutrient-poor, well-drained ground: lichen woodland under widely spaced pine and spruce, sandy heath, rocky barrens, dune systems, bogs and blanket peat margins, alpine fell-field and arctic tundra. It needs high light and does poorly under a closed canopy or in shade cast by vigorous vascular plants.

It is circumboreal, abundant across northern Europe, Scandinavia, Siberia, Alaska, Canada and the northern United States, with montane occurrences further south. It is perennial and evergreen, present in every season, and grows slowly — a few millimetres per year — so extensive mats represent decades of undisturbed accumulation. Recovery after fire or heavy trampling is correspondingly slow.

The lichen is a symbiosis between a fungal partner that constructs the hollow branching podetia and green algal cells of Trebouxia housed just beneath the loose surface layer. Having no roots, it absorbs water and dissolved nutrients directly from rain, mist and dew across its whole surface, which is why it thrives on nutrient-poor sand and rock yet suffers badly where airborne nitrogen is high.

It is a keystone element of northern ground-layer vegetation. Deep mats insulate the soil, moderate permafrost thaw, and shade out competing seedlings, while releasing organic acids that slowly weather mineral grains. The extensive lichen woodlands it builds define the winter ranges that support caribou and reindeer herds, and the mats also shelter insects, spiders and small vertebrates. Its slow growth and easy fragmentation mean that spread is mostly vegetative, with wind, water and animal traffic carrying broken branch fragments to new ground.

Other reindeer lichens are the main confusion. Cladonia arbuscula and Cladonia mitis are distinctly yellowish-green rather than ash grey and tend to branch more loosely; Cladonia portentosa (formerly Cladonia impexa) is also greenish and branches in a more tangled, evenly spreading way with tips pointing in many directions rather than nodding uniformly. Cladonia stellaris forms neat rounded head-like cushions with dense terminal branch clusters and a hollow centre to each head, and is pale yellowish-white.

Cladonia uncialis is greenish-yellow with shiny, smooth, sharply pointed branch tips and a firm cortex, unlike the matt, felty, cortex-free surface here. Cup-forming Cladonia species such as the pixie cups and trumpet lichens have obvious goblet-shaped podetia and persistent basal squamules, both absent from a mature reindeer lichen mat. Mosses growing alongside can superficially resemble the mats but have true leaves along their stems and turn deep green when wet, whereas this lichen stays grey.