Freckle Pelt Lichen

Freckle Pelt is among the easiest lichens to name on sight. Its wide, thin lobes lie over moss and forest litter, and after rain the whole thallus flushes a startling grass or apple green — a colour no other common ground lichen of the boreal forest matches. Dotted across that green surface are the freckles: small, dark brown to blackish warts called cephalodia.

The species belongs to Peltigera, the pelt lichens, but it is unusual in being a three-partner symbiosis. The main photobiont is a green alga, which gives the lichen its green colour, while nitrogen-fixing Nostoc cyanobacteria are packaged separately inside those wart-like cephalodia on the upper surface.

Thalli can be large, spreading across half a metre of mossy ground in old forest, and the combination of bright green colour, brown freckles, and a pale veined underside makes it a signature species of undisturbed northern woodland.

Thallus

Foliose, loosely attached and often forming large overlapping sheets. Lobes are broad and rounded, 2–5 cm wide, thin and slightly leathery, with wavy margins that may lift when dry.

Upper surface

Bright green to apple green when wet, fading to grey-green, pale bluish grey or almost tan when dry — the colour change is dramatic and reversible within minutes of wetting. The surface is smooth to faintly downy near the margins and carries scattered cephalodia: rounded to lumpy warts a millimetre or two across, dark brown to blackish, sitting proud of the surface like freckles.

Lower surface

Pale cream to whitish with a network of broad, felty, brown to dark brown veins and scattered short rhizines. In P. aphthosa the lower surface tends to be diffusely tomentose and the veins are broad and poorly delimited rather than sharply outlined.

Apothecia

Produced on upturned lobe tips, reddish brown to dark chestnut, saddle-shaped and rolled into vertical flaps. The medulla is white.

Freckle Pelt grows over mosses, humus, rotten wood and mossy boulders, almost always in cool, humid, shaded conditions. Its heartland is boreal and montane conifer forest — spruce, fir and pine woods with deep feather-moss carpets — as well as subalpine heath, tundra hummocks and shaded rock ledges.

The species is circumboreal, common across northern Europe, Siberia, Alaska, Canada and the mountains of the northern United States, extending south along ranges such as the Rockies, Cascades and Appalachians at higher elevations. It is perennial and can be found year-round, but is most obvious after rain or snowmelt when it is fully hydrated and green.

The green algal partner supplies carbohydrate to the fungus, while the cyanobacteria sealed inside the cephalodia fix atmospheric nitrogen. This makes Freckle Pelt a meaningful nitrogen source in old boreal forests, where nutrients are otherwise locked up in slow-decaying litter; nitrogen released as thalli decay feeds mosses and forest-floor plants.

It is a slow-growing, moisture-dependent species most abundant in long-undisturbed stands with continuous moss cover, and it is often reduced after logging or heavy trampling. Dispersal occurs by ascospores from the upturned apothecia and by broken lobe fragments, which can re-establish where they come to rest on damp moss.

Peltigera leucophlebia is the closest match, also green with cephalodia, but its lower surface has sharply defined, strongly contrasting dark veins against a white background, and the upper surface is usually shinier and more distinctly net-marked than in P. aphthosa. Peltigera britannica also carries cephalodia, but they are hollow and pop or flake off easily, sometimes leaving pits, and the lobes are thinner and more delicate.

Peltigera canina and P. membranacea share the pelt form and the veined underside but stay grey-brown even when soaked and never bear cephalodia on the upper surface. Lobaria pulmonaria can look green and lobed too, but it grows on tree trunks and mossy branches rather than the ground, and its surface is deeply ridged and pitted into a net of hollows.