This brown Rhizocarpon is similar to R. polycarpum, from which it is distinguished by much smaller spores that turn brown quickly and by a strong K+ red-purple reaction of the hypothecium (Fletcher et al. 2009). However, R. polycarpum is very variable and its spores sometimes turn brown quickly too. The spore size is also variable and identification mistakes might occur. That is the case of the oldest published record of R. simillimum in the Czech Republic from the Praděd Mt area in the Hrubý Jeseník Mts (Stein 1879), revised by Eitner (1911). Due to the small dark spores it might also be confused with Buellia s. l. species, from which it is easily distinguished by the hamathecium composed of anastomosing paraphysoids (Fletcher et al. 2009).
The lichen is widely distributed in both hemispheres. It is mainly a montane species, growing on exposed, steeply inclined or vertical surfaces of acidic, often nutrient- or calcium-enriched rocks (Nimis et al. 2018). In the Czech Republic it was recently collected only on acidic vulcanic rocks in the Middle Vltava region (Drbákov-Albertovy skály). Historical records are known from the Krkonoše and Jeseníky Mts (cf. Vězda & Liška 1999); the latest record is from the Rychlebské hory Mts (Müller 1951). The identity of these collections has not been verified.
Literature: Stein B. (1879): Flechten. – In: Cohn F. [ed.], Kryptogamen-Flora von Schlesien, 2: 1–400, J. U. Kern’s Verlag, Breslau. Eitner E. (1911): Dritter Nachtrag zur schlesischen Flechtenflora. – Jahresbericht der Schlesischen Gesellschaft für Vaterländische Kultur, Abt. Zool.-Bot., Breslau, 88: 20–60. Fletcher A., Gilbert O. L., Clayden S. & Fryday A. M. (2009): Rhizocarpon Ramond ex DC. (1805). – In: Smith C. W., Aptroot A., Coppins B. J., Fletcher A., Gilbert O. L., James P. W. & Wolseley P. A., The Lichens of Great Britain and Ireland, p. 792–808, The British Lichen Society, London. Nimis P. L., Hafellner J., Roux C., Clerc P., Mayrhofer H., Martellos S. & Bilovitz P. O. (2018): The lichens of the Alps – an annotated checklist. – Mycokeys 31: 1–634.taxonomic classification:
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