The 4,500 Year Old Lichen

Lichens aren't the fastest of growers. Then again, they don't need to be, more often than not being the primary colonisers of many substrates where they are unlikely to face much competition such as bare rock and tree bark. Exposed rocky substrates will not tend to accumulate organic or mineral material needed for plant growth so lichens that can withstand the exposure by staying as close to the surface as possible such as crustose species will face very little competition and often uninterrupted growth. Since lichens tend to radial growth they can be used as an indicator or substrate age in areas where they form the dominant vegetation cover. Lichen growth rates vary from one region to another and may decline after initial colonisation to an almost constant value (1). The us of lichens to determine substrate age is known as lichenometry and and has a useful range of up to about 500 years. Some species may be used for up to 4,500 years or more under dry continental conditions.

Map Lichen, Rhizocarpon geographicum

One of these species, and the most commonly used in lichenometry, is the Map Lichen (Rhizocarpon geographicum). Some examples of R. geographicum have been dated to be c. 4,500 years old (2). Its common name is well earned, as cracks in the yellow-green thallus show the underlying black prothallus, giving the lichen a map like appearance (3). A slow growing species, it can be found on hard siliceous on exposed uplands and mountains (4). It also can be seen on rocky seashores, as the example illustrated was, growing on red sandstone.

References:

1. Haeberli et al., 2003. Permafrost (Eds. Phillips, Springman & Arenson) pp. 343-347
2. Beschel, 1958. Arctic 11 p. 254.
3. Phillips, 1980. Grasses,Ferns, Mosses and Lichens of Great Britain and Ireland and p. 165
4. Whelan, 2011. Lichens of Ireland p. 139