A visit to an old graveyard, particularly one that has not been cared for, will generally reveal tombstones covered in lichens. Lichens are composite organisms; they are a symbiotic relationship between a fungus and a green alga (or a cyanobacterium). The fungus provides the physical infrastructure and the algae do photosynthetic duty to supply sugars. The lichen takes a form that resembles neither the fungus nor the alga with the symbiotic partners interpenetrating each other to create a life-form that resembles a primitive plant.
Lichens grow on tombstones because they are adapted to colonizing very dry environments that have very little in the way of nutrients available. They are found on barren rock right up into the polar regions, where the extreme cold adds an additional challenge. In better-maintained cemeteries, the lichens are generally scraped off because they soon begin to obscure the engraving on the stone. In addition, the lichens also chemically degrade the surfaces to which they cling, breaking down the rock into its constituent minerals.
Lichens have many different habits, but most fall into three categories: crustose, foliose, or fructiose. The most well-known fructiose variety is probably the misnamed “Spanish moss,” which hangs in great bedraggled mats from live oaks in the Deep South.
Foliose species are characterized by having leaf-like sheets, often arranged in a rosette pattern. Crustose lichens may look almost painted on to the surface where they are growing or resemble a gray-green stubble.
Austrian botanist Roland Beschel developed lichenometry in the 1950s. He was looking for a method of dating glacial moraines in Alpine valleys. These linear piles of boulders, gravel, and sand stretched across the valleys, marking the location where a glacier had advanced and then remained, its rate of advance equaling the rate at which it was melting back. Eventually the rate of melting exceeded the rate of advance, and the glacier retreated up the valley. Some valleys in the Alps have a whole series of these moraines with the older ones (furthest down the valley) having been deposited before the beginning of written records. Crustose lichens are the slowest growing lichens, and they often grow in regular circles and are therefore relatively easy to measure.
Beschel measured the diameters of crustose lichens that were growing on stone surfaces that could be dated independently. This includes tombstones, as well as very old buildings and bridges for which there are either dated cornerstones or written records for their date of construction. In this way, Beschel established a relationship between the increasing diameter of the lichen and the increasing age of the structure on which it grew. He allowed for a period of approximately a decade between the erection of the structure and the commencement of lichen growth. The next step was to find and measure lichens on rocks that were part of the valley moraines.
Many of these moraines were deposited during the so-called “Little Ice Age,” which came and went episodically from the Renaissance (15th century) through the 19th century. Beschel could measure the diameter of lichens on these natural landforms and then back-calculate an age for them through the curves that he had derived using the man-made structures covered with lichens. In the 60 years since Beschel first published the technique it has been used throughout the world to date Holocene landforms that were too young to date reliably with radiocarbon and too old to date reliably using historical records.
Lichens grow most slowly in the most northern or most southern climates, so the utility of lichenometry is greatest in the polar and subpolar regions. Anyone who wishes to date stone walls, foundations, or other stone structures that have been abandoned and have become encrusted with lichens, needs only to create an age-diameter curve using local dated surfaces like tombstones. It is important to create a age-diameter relationship locally because the rate of lichen growth will vary regionally, giving a different slope to the curve in different locations.