Lichenometry

Lichens are a symbiotic live community between algae and fungi. The alga furnishes nutrients for the fungus, the fungus provide moisture and shelter for the alga. This partnership enables the two partners to colonize habitats, which a single organism couldn’t colonize by itself, and they colonize an extraordinary variety of habitats and surfaces.

Lichens can be found in the death zone of mountains (up to 7400m a.s.l. in the Himalayan), in rainforests, deserts, temperate regions and on the coast of the sea.

The body of lichen that is visible is formed by the fungus, and is called “thallus”. Lichens can divided into three broad groups based on the shape of the thallus: the fruiticose type consists of small tubules and branches, the foliose type which have a leaf-like plant body, and the flattened crustose type. This last group comprises the most common members of the lichen family, and can be found extensively on hard surfaces, including rock outcrops, boulders, tree bark, buildings and gravestones.

Lichens after HAECKEL 1904 "Kunstformen der Natur"

The first study related to Lichenometry, the dating method that use the growth rate of lichens for dating the surface that they colonize, was carried out by the Austrian scientist Roland Beschel in 1950.
Given similar rocks and climatic conditions, the larger the lichen colony, the longer will be the time passed since the growth surface becomes exposed.
Estimating the absolute age of a material from the lichen growing on its exposed surface first requires the determination of the growth rate of the area. After measuring lichen on surfaces of known age (for example by comparing lichens on historic buildings or geomorphic features with known age) it is possible to plot a growth curve that relates lichen diameters to time.
Then is it possible to compare lichen on a surface of unknown age, within the same area, with the grow curve to determinate the surface’s age.

The growth of a lichen proceeds in three different phases:

1) rapid, logarithmic growth

2) linear growth

3) slow growth phase, where lichen growth gradually declines until death.

Only lichen species with a gradual and progressive growth can be used for dating purpose. In a study only the same lichen species can be used. The growth is influenced by local, and regional environmental factors, such as temperature, day length and snow cover.

The photosynthetic productivity is compared to high plants low, only ca. 25% comparing same areas with lichens and plants. This low productivity implies a low growth rate and a great longevity. Some lichen species (like Rhizocarpon geographicum) are estimated to reach (under favourable conditions like in the cold and dry conditions of western Greenland) 5.000 to 9.000 years.

Rhizocarpon geographicum

But because lichens colonies eventually grow together, and can no longer be measured individually, lichenometry as dating tool is used in a range less than 500 years. Under optimal circumstances lichenometry can provide a dating tool accurate to +-5 years over the last 200 years. So these organisms provide accurate dates for young glacial deposits, rockfalls and mudflows – all events that expose new rock surfaces on which lichen can grow.

Brodoa intestiniformis

References

BENEDICT (1990): Experiments on lichen growth. 1 Seasonal patterns and environmental controls. Arctic and Alpine Research 22:244-253

BESCHEL (1961): Dating rock surfaces by lichen growth and its application to glaciology and physiography (lichenometry). In Raasch (ed.) Geology of the Arctic vol.2. Univ. of Toronto Press: 1044-62

BESCHEL (1973): Lichens as a measure of the age of recent moraines. Arctic and Alpine Research 5:303-309

INNES (1982): Lichenometric use of an aggregated Rhizocarpon “species”. Boreas 11:53-57

INNES (1983): Use of an aggregated Rhizocarpon “species” in lichenometry an evolution. Boreas 12:183-190

INNES (1983): Size frequency distributions as a lichenometric technique: an assessment. Arctic and Alpine Research 15:285-294

INNES (1985): An examination of some factors affecting the largest lichens on a substrate. Arctic and Alpine Research 17:99-106

HEUBERGER (1966): Gletschergeschichte. Untersuchungen in den Zentralalpen zwischen Sellrain- und Ötztal. Wissenschaftliche Alpenvereinshefte 20:125

KONRAD & CLARK (1998): Evidence for an early Neoglacial glacier advance from rockglaciers and lake sediments in the Sierra Nevada, California USA. Arctic and Alpine Research 30:272-284

MATTHEWS (1992): The ecology of recently deglaciated terrain. Cambridge University Press

O´NEAL & SCHOENENBERGER (2003): A Rhizocarpon geographicum growth curve for the Cascade Range of Washington and Northern Oregon, USA. Quaternary Research 60:233-241

WALKER (2005): Quaternary dating methods. Wiley Press