Smooth Rock Tripe

Smooth Rock Tripe is the largest of the North American rock tripes, forming floppy, hide-like sheets that can exceed 20 cm across on a single boulder. The whole thallus is fixed to the rock by one central cord — the umbilicus — so it hangs like a leather flap and can be lifted at the edges with a fingertip.

It belongs to Umbilicaria, the umbilicate lichen genus in the family Umbilicariaceae, all of which share this single-point attachment. What separates U. mammulata from its relatives is the smoothness of both surfaces: the upper side is even and matte, and the underside is uniformly black, velvety-smooth and free of the rhizines, cords or netted ridges seen in many congeners.

The lichen swings dramatically in appearance with moisture. Wet, it is supple, olive to chocolate brown and lies flat against the rock; dry, it curls inward, stiffens to a brittle grey-brown parchment and can rattle in wind, only to rehydrate within minutes of rainfall.

Thallus

A single broad lobe, roughly circular to irregularly kidney-shaped, commonly 5–20 cm wide and occasionally much larger, with wavy, sometimes torn or shallowly lobed margins. Thickness is that of soft leather. Large specimens may become perforated with age.

Upper surface

Smooth to slightly rumpled, matte or faintly satiny, ranging from mid-brown through olive-brown to greyish tan when dry. There are no soredia (powdery propagule patches) and no isidia (finger-like outgrowths), so the surface stays even.

Lower surface

Uniformly black to very dark brown, smooth and velvety, entirely lacking rhizines, hairs and the coarse netted or plated texture found in several other rock tripes. This clean black underside is the key field character.

Attachment and fruiting bodies

A single tough central umbilicus fixes the thallus to the rock, leaving the rest free. Apothecia are uncommon; when present they are small black discs on the upper surface with concentric ridges. The medulla is white and the thallus tears rather than snaps when hydrated.

Smooth Rock Tripe grows on non-calcareous rock — granite, gneiss, quartzite, sandstone and similar acidic stone. It favours shaded to partly shaded vertical faces, overhangs, cliff bands and large boulders inside humid deciduous or mixed forest, and often carpets whole rock walls in seepage-shaded ravines.

It is essentially an eastern North American species, abundant from the Appalachians and New England through the Great Lakes region and southeastern Canada, thinning out westwards. Because it is perennial it can be found in every month, appearing dark and pliable after rain and pale, curled and crisp in dry spells.

The fungal partner supplies structure and attachment, while a green algal photobiont in a thin layer beneath the upper cortex carries out photosynthesis. With only one narrow point of contact with the rock, the lichen depends almost entirely on rain, run-off and humid air for its water and mineral supply, which explains its preference for shaded, damp cliff microclimates.

Growth is slow and colonies are long-lived, with large sheets representing many decades on stable rock. Dense rock-tripe cover creates a sheltered microhabitat exploited by mites, springtails and other small invertebrates, and shed fragments contribute organic matter to crevice soils. Reproduction is largely by fragmentation of the brittle dry thallus, since fruiting bodies are produced only sparingly.

Umbilicaria muhlenbergii is similar in outline but has a strongly ridged or netted upper surface pattern radiating from the umbilicus, and it carries conspicuous black, concentrically grooved apothecia. Umbilicaria americana has a whitish-pruinose, frosted upper surface and a lower surface densely covered with black rhizines and a woolly texture, quite unlike the bare black underside of Smooth Rock Tripe.

Lasallia papulosa, the Common Toadskin Lichen, is also umbilicate but is far smaller and its surface is raised into blister-like pustules with matching pits below. Umbilicaria mammulata stays smooth on both faces. Peltigera species can look brown and leathery too, but they lie on soil and moss rather than bare rock, have many lobes rather than one, and show a veined lower surface with pale rhizines instead of a single central holdfast.