Description: Much of what was previously recorded as R. obscuratum is this pioneer species of siliceous rock. R. reductum is a crustose, mosaic-forming lichen of very distinct appearance, usually cropping up as one of the very earliest circular growths to develop on acid memorials or garden artefacts carved from sandstone, slate or granite. The thallus can vary from grey to pale brown and is often surrounded by a thin black border. The numerous black, almost flat, discs (to 1mm diam.), when studied through a lens, will be seen to have a margin the same colour as the discs. A squash of the disc under a microscope slide will reveal the spores to be muriform (cracked into little facets like a stained-glass window).

Key Identification Features:

• Mosaic-forming, pioneer lichen of siliceous rock, forming circles of grey or brown thalli sometimes less than 0.5cm diam.
• Discs numerous, black, flat, lying just above the surface. Spores muriform (microscope squash required).

Original text submitted by Vince J. Giavarini

Distribution Map from NBN: Rhizocarpon reductum at National Biodiversity Network mapping facility, data for UK.

iNaturalist: Rhizocarpon reductum at iNaturalist World Species Observations database.