Smooth Rock Tripe Identification Guide

First Impression and the Place You Find It

Smooth Rock Tripe is not a mushroom but a lichen, and it is one of the most conspicuous organisms on shaded rock in the eastern deciduous forest. The typical encounter is at the foot of a cliff, on a mossy boulder field, along a stream gorge, or on a north-facing outcrop below the canopy. What you see from a distance is a scatter of large, drab brown-grey flaps hanging on the rock face, looking almost like scraps of leather or peeling paint.

Come closer in dry weather and the impression is of something dead. The thallus is stiff, curled at the edges, dull grey-brown, and it crackles when touched. Come back after rain and the same colony has transformed: the flaps are limp, supple, olive-brown to greenish-brown, and lie flat against the wet stone. This dry-brittle, wet-pliable behaviour is completely normal for the species and is one of the more satisfying things to watch in the field.

Size is the immediate clue. Individual thalli commonly reach 5 to 20 cm across, and in undisturbed humid sites they can exceed 30 cm, making this the largest umbilicate lichen in eastern North America. If you are looking at a single leathery brown disc the size of a dinner plate on a shaded cliff, you are almost certainly there already.

Reading the Upper Surface

The thallus is foliose - leaf-like - and usually consists of a single broad lobe, sometimes with a few secondary lobes torn from the margin. The outline is roughly circular in young material and becomes irregular, ragged, split and tattered with age, especially at the edges where wind, ice and animals break pieces away.

The upper surface is the character in the name. It is smooth to gently undulating, matte to slightly shiny, without pustules, blisters, ridges or a network of raised lines. Colour ranges from a mid to dark brown through olive-brown to grey-brown, and it is not coated with a white frosted bloom. Run a thumb across it: it should feel like fine leather, not like a bubbled or corrugated surface.

Near the centre you can often see a slight puckering where the attachment sits beneath, but even there the surface stays essentially even. Old thalli may develop small perforations and thin, translucent worn patches.

The Attachment Point

Umbilicate is the technical term and the field test at the same time. Slide a fingernail under the edge of a thallus and lift gently. The whole flap peels away from the rock everywhere except at one point near the centre, where a short, stout, cord-like holdfast - the umbilicus - anchors it to the stone. Everything else is free.

That single central attachment separates the rock tripes from every other large foliose lichen you might meet. Peltigera, for example, is fixed along the whole of its lower surface by rhizines; crustose lichens are welded to the substrate over their entire extent and cannot be lifted at all.

Handle wet material if you can. A dry thallus shatters when you flex it, and a broken specimen loses the very characters you are trying to read.

The Lower Surface

Turn the thallus over. The underside of Smooth Rock Tripe is a deep coal black or blackish-brown, matte, and finely and evenly velvety or minutely fuzzy to the touch - a dense short pile rather than a set of distinct outgrowths. There are no coarse black rhizines, no branched holdfast fibres, no pale patches, and no netted pattern of ridges and pits.

That smooth black felted underside, paired with the smooth brown upper surface, is the combination that names the species. Check it on more than one thallus, because a single weathered individual can be misleading.

Fruiting Bodies and Fine Structure

The fungal partner produces apothecia, but on this species they are uncommon and easy to overlook. When present they appear as small black discs seated on the upper surface, often with concentric or contorted ridging within the disc rather than a simple flat face. Most colonies you encounter will be entirely sterile and reproduce instead by fragments broken from the ragged margins, which is one reason large thalli often occur in tight groups on the same rock face.

Under a hand lens the margin shows the thallus in section: a thin dark upper layer, a pale internal layer, and the black lower cortex. There is no soredia-covered edge and no powdery bloom.

Substrate, Season and Ecology

Substrate is a hard identification criterion here. Smooth Rock Tripe grows on non-calcareous rock - granite, gneiss, schist, quartzite, sandstone and similar acidic stone - and specifically on shaded, humid faces. Look on the underhangs and lower sections of cliffs, on the shaded sides of large glacial erratics, along ravines, on rock in the spray zone of waterfalls, and on outcrops within closed forest. Exposed, sunny, dry rock supports smaller and different lichens. It does not grow on soil, wood or bark.

There is no season in the fungal sense. Lichens are present all year and grow whenever they are wet and above freezing, which in this habitat means spring and autumn rains, fog, snowmelt and summer humidity. The best time to examine a colony is during or immediately after rain, when the thallus is pliable and its colour is at its most saturated.

Biologically it is a partnership between a fungus and a green algal partner housed in a layer beneath the upper surface. The lichen is poikilohydric: it has no means of regulating its own water content, so it dries out completely and suspends metabolism, then rehydrates and resumes photosynthesis within minutes of wetting. Growth is extremely slow, on the order of millimetres per year, so a large thallus represents decades of undisturbed conditions. This makes big colonies useful indicators of long-term habitat stability, and the group is well known for its sensitivity to air pollution.

Separating It from the Look-alikes

Umbilicaria americana is the closest confusion in the same forests. Its upper surface carries a distinct whitish pruina, giving it a frosted grey appearance rather than clean brown, and its lower surface is not simply felted: it bears conspicuous coarse black rhizines and cord-like outgrowths. Turn the thallus over and the difference is immediate.

Umbilicaria muehlenbergii is generally smaller and has an upper surface strongly sculptured with raised ridges radiating and branching from the centre, producing a netted or reticulate pattern quite unlike the smooth face here.

Lasallia papulosa shares the habitat and the single attachment but its upper surface is covered with rounded blisters or pustules, each of which corresponds to a pit on the underside, so the thallus looks and feels bubbled.

Umbilicaria deusta is much smaller, darker, closely appressed, and minutely warty or granular, forming dark crusty patches rather than large free flaps.

Dermatocarpon species are also umbilicate but turn a distinctly bright green when wet, appear pale grey when dry, and have a pale brown to tan underside rather than a black one; they also prefer periodically wet rock near water.

Peltigera species can be large and brown but grow on soil, moss and rotting wood, are attached along the whole underside by veins and rhizines, and have a pale, conspicuously veined lower surface.

Finally, dry weathered scraps of bark, flakes of peeling rock varnish and even old plastic on cliffs can imitate a dry thallus at a glance. Wet the object: a lichen softens, greens and flexes.

A Working Sequence

Shaded acidic rock in humid forest, large leathery brown flap, smooth upper surface without pustules or a frosted bloom, single central holdfast, coal-black finely felted underside with no rhizines, brittle dry and supple wet. Six checks and Smooth Rock Tripe is settled on the cliff face.