ASCOMYCOTA | LECANOROMYCETES > RHIZOCARPALES > Rhiziocarpaceae

Genus Rhizocarpon, meaning 'fruits with roots' - the derivation of which is unclear

Common name:            Yellow Map Lichen

Synonym:                    Various subspecies, but likely ssp.geographicum

Habitat:                         Montane, but also upland and coastal

Substrata:                     Rock

Growth form:                Saxicolous

Thallus: Crustose

Apothecia:                    Lecideine

BLS identity rating:        Graded 1/5 (shouldn't prove too difficult)

Thallus up to 15cm across, areolate and often mosaic-forming; areoles flat to slightly convex, smooth, yellowish-green to almost pure yellow, seldom with any orange reflections; prothallus black, clearly showing at the margin, also a black hypothallus that's visible between the areoles; apothecia black, irregular in shape, immersed within the thickness of the thallus, discs flat to slightly convex. Often found growing with other crustose lichens.

Rhizocarpon lecanorium is extremely similar, apart from the areoles, which are variably curved, forming arcs around the apothecia. In the field, it might not be immediately obvious that it's a different species but, when you look closely, the difference is easy to see. I've featured the species here for direct comparison with Rhizocarpon geographicum together with the fact that, for some reason, I only have a single close-up photo. The shape of the areoles gives rise to the common name Crescent Map Lichen.

Both species can be found in Britain with Rhizocarpon geographicum being quite widely distributed particularly in Scotland, Wales, the north and the southwest, far less so across the midlands, East Anglia and the south east. Rhizocarpon lecanorium on the other hand is only found in upland areas, mainly in Scotland, Wales and on some northern moors. Rhizocarpon geographicum is very common on hard siliceous rocks in both coastal and inland areas.

In Austria, where these photos were taken, it's listed asa "cool-temperate to arctic-alpine, circumpolar, polymorphic lichen of siliceous rocks wetted by rain". And, Rhizocarpon lecanorium as a "temperate to boreal-montane, circumpolar lichen, most common on stone walls, dust-impregnated siliceous boulders, roofing tiles, but also found in natural habitats; both widespread throughout the Alps".