File:A lichen - Stereocaulon vesuvianum - geograph.org.uk - 1103503.jpg

English: A lichen - Stereocaulon vesuvianum. This lichen was growing on top of a dry-stone wall: 1050079. It is in the form of a springy mat, and consists of upright stalks called pseudopodetia (compare the podetia that occur in Cladonia species: 1060981). The stalks are densely encrusted with little leaf-like lobes called phyllocladia; see 1103506 for a closer look. Spore-producing discs are sometimes also present; a few of these small dark brown spots can be seen in the upper right part of the lichen.

This specimen was growing on a part of the wall that was beneath some slightly corroded barbed wire. The resulting zinc-rich run-off would be damaging to many lichens, but Stereocaulon species are among those that can tolerate high concentrations of heavy metals. In fact, the species shown here, S. vesuvianum, is "very common in upland regions on well-lit, siliceous rocks, especially if metal-rich, often forming dense mats" ["Lichens - An Illustrated Guide to the British and Irish Species", Frank S. Dobson].

This topic is also discussed in "Understanding Lichens", by George Baron; discussing the ability of some lichens to tolerate high metal concentration, the book notes that "for example, the genus Stereocaulon is found among the debris of old lead mines ... The metal content that can be endured by lichens can be extraordinarily high and would be destructive to most organisms. ... Nevertheless, metal contamination is destructive to most lichens, certainly when constant rainwater drips on to them from copper, barbed wire or zinc, but even that niche is exploited by some lichens."