Map Lichen Identification Guide

The Setting You Will Be Standing In

Map Lichen is a fungus of bare, hard, acidic rock in the open. The typical setting is upland: a summit tor, a boulder field, a scree slope, a glacial moraine, a drystone wall built of local stone, a roadside rock cutting on a mountain pass, or a scattering of erratic boulders in rough grazing. The rock itself should be siliceous. Granite, gneiss, schist, quartzite, rhyolite and hard sandstone all suit it; limestone, mortar and other base-rich surfaces do not, and finding a bright yellow-green crust on limestone is grounds for looking elsewhere in the key.

It wants full light, clean air and rain. The most spectacular colonies grow where snowmelt or run-off keeps the surface intermittently wet but where water does not stand.

The First Impression

The first impression is of colour and pattern. A vivid, almost fluorescent yellow-green patch spreads flat across bare stone, cracked into a mosaic of tiny tiles and outlined in jet black. Where several colonies meet, the black boundaries between them look exactly like the drawn borders of countries on a map, which is where both the common name and the epithet come from.

Unlike a mushroom, there is nothing to pick up. The identification is done in place, with a x10 or x20 hand lens, ideally a penknife, and, for confirmation, a few drops of reagent.

Step One: Learn the Parts You Are Reading

A crustose lichen has no cap, gills or stipe, so the anatomy has to be learned separately. There are four things to find, in this order.

The thallus is the crust itself, the living body, fused so tightly to the rock that it cannot be peeled off without taking stone with it. The areoles are the small angular tiles into which the thallus is divided. The prothallus (or hypothallus) is the black fungal mat that lies between and around the areoles. The apothecia are the black discs in which spores are produced.

Step Two: The Thallus and Areoles

The crust is thin and tightly attached, and it is areolate, meaning it is broken into discrete small plates. The individual areoles are typically 0.2-1 mm across, angular to rounded, flat or very slightly convex, with a smooth to slightly shiny upper surface. They sit side by side like crazy paving, separated by thin dark lines.

Colour is bright yellow-green to chartreuse, a distinctly cold, acid yellow. It can dull towards ochre-yellow in shade, in polluted air or in very old colonies, and it looks greener and more translucent when soaking wet. The pigment sits in the upper cortex.

If you can sacrifice a fragment, scrape one areole off with a knife point and look at it edge on with a lens. You should see a thin coloured skin, a bright green band immediately below it where the algal partner lies, and a white layer beneath that. The green band is the photobiont, a single-celled green alga of the genus Trebouxia, and it is the reason the crust is a lichen rather than a simple crust fungus.

Step Three: The Prothallus

The black mat is the character that most reliably confirms the genus in the field. Look at the outermost edge of the colony, where it meets bare rock: there should be a distinct jet-black, matt border, typically 0.5-2 mm wide, running around the whole margin. Between the areoles the same black tissue shows as fine lines and networks.

A yellow crust without a black prothallus is not Rhizocarpon geographicum, and that single check eliminates most of the yellow crustose look-alikes at a stroke.

Step Four: The Apothecia

The spore-bearing structures are black, matt and 0.3-1.5 mm across, angular to irregular in outline, sunken between the areoles and roughly level with the crust surface or slightly below it. The margin is thin and the same black as the disc, with no rim of yellow thallus tissue around it; in lichen terms the apothecia are lecideine rather than lecanorine. They are often crowded towards the middle of a colony, and old colonies can be densely peppered with them.

Get the lens right down onto them. The distinction between an apothecium ringed in yellow crust and one ringed only in black is the main separator from the nearest relative.

Instead of a Spore Print: Reading the Spores

Lichens do not shed a visible spore mass, so there is no print to take. The equivalent step is a hand-cut section.

With a razor blade, shave a very thin vertical slice through an apothecium, drop it into water on a slide, and look at it under a microscope. You should see a dark brown-black upper layer (the epithecium), a colourless layer of packed spore sacs below it (the hymenium), and a very dark base (the hypothecium). Within the hymenium, the asci contain a small number of large ascospores. Those spores are the point: they are muriform, divided by both cross-walls and longitudinal walls into many cells like the brickwork of a wall, colourless when young and turning brown as they mature, often surrounded by a gelatinous halo, and large, roughly 20-40 by 10-20 microns.

Muriform browning spores, black lecideine apothecia, a yellow areolate crust and a black prothallus on siliceous rock is a complete identification to the Rhizocarpon geographicum group.

Step Five: Chemical Spot Tests

Three small bottles finish the job. On the yellow surface, potassium hydroxide (K) produces no colour change, and bleach (C) produces none either. The informative test is on the white inner layer: scrape a few areoles away to expose the medulla and apply iodine, and it turns blue. That amyloid medulla, combined with a K-negative yellow cortex, is characteristic.

Season, Growth and Ecology

There is no fruiting season. The thallus is perennial and evergreen and can be examined in any month, including under snow. What matters instead is growth rate, and it is famously slow: after an initial faster phase, colonies expand by a fraction of a millimetre in radius per year, and large thalli on stable rock surfaces may be centuries or even millennia old.

That slowness underpins lichenometry, in which the diameter of the largest thalli on a rock surface is used to estimate how long ago that surface was exposed. The technique has been applied to glacial moraines, rockfalls, landslides, scree and old walls, using local growth curves calibrated against surfaces of known age.

Ecologically it is a pioneer. It needs no soil, drawing water and mineral nutrients from rain, mist and windblown dust, while the Trebouxia photobiont fixes carbon by photosynthesis inside the protective fungal body. Its acids slowly etch the rock, contributing to the earliest stages of soil formation. It is sensitive to sulphur pollution and thins out near heavy industry, and it usually shares its boulders with companions such as Lecanora polytropa, Porpidia, Fuscidea and Sporastatia species.

Separating the Closest Look-alikes

Rhizocarpon lecanorinum is the critical comparison and is often growing on the same boulder. It shares the yellow crust, black prothallus and black discs, but its areoles are lobed and arranged around the apothecia, so that each apothecium is partly or wholly encircled by a crescent of yellow thallus. In Rhizocarpon geographicum the apothecia sit between angular areoles with no yellow rim.

Other members of the Rhizocarpon geographicum group, including Rhizocarpon riparium and Rhizocarpon macrosporum, differ in areole shape and convexity, prothallus development, spore dimensions and chemistry, and many require microscopy and thin-layer chromatography to place with certainty.

Grey and brown Rhizocarpon species such as Rhizocarpon reductum and Rhizocarpon badioatrum have the same black apothecia and prothallus but a grey, white or brown crust with no yellow-green pigment at all.

Pleopsidium species are a warmer, brighter egg-yellow, favour sheltered overhangs and crevices, and have yellow apothecia with a rim of thallus tissue rather than black sunken discs.

Candelariella vitellina forms scattered warm yellow granules with yellow apothecia, lacks any black prothallus, and tolerates nutrient-enriched surfaces such as bird-perch rocks and wall tops.

Acarospora species with yellow thalli are thicker and more scattered or scale-like, with punctiform sunken discs and no black border.

Free-living green algal films such as Desmococcus can look green-yellow on damp rock, but they are powdery, smear when rubbed wet, and have no areoles, no black margin and no apothecia.

Field Checklist

Bright yellow-green crust divided into small angular areoles on hard siliceous rock in the open; a distinct jet-black prothallus between the areoles and forming a border around the colony; black, matt, sunken apothecia with no yellow rim; K-negative cortex and iodine-blue medulla; large brown muriform ascospores; present year-round, growing a fraction of a millimetre a year.