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Today a similar approach is used in lichenometry.
The idea to use the growth of lichens as simply and in field applicable method for relative dating of surfaces was first proposed by the botanist Knut Faegri in the 1930s, and in the 1950s developed further by the Austrian botanist Roland Beschel. During lichenological research on cemeteries he noted that on older gravestones larger lichens can be found. Many of the observed species where also found on rocks exposed by receding glaciers, Beschel realized that lichens could be used to relative date the glacial extensions in the Alps during the Holocene.
Despite the first promising results of lichenometry by botanists and geographers, geologists discovered the use of lichens in alpine or arctic environments only in the decade between 1960 and 1970, since then lichenometry was and is used as a simple field method to date moraines, rockfall deposits, debris flows, denudated rockwalls, escarpments and raised beaches.
Lichens are a symbiotic live community between algae and fungi. The microscopic alga furnishes nutrients for the fungus, the macroscopic visible fungus provide moisture and shelter for the alga. This partnership enables the two partners to colonize habitats, which the single organism couldn't possibly colonize by itself, and in fact as pioneer species lichens colonize an extraordinary variety of habitats and surfaces: Lichens can be found on debris and rock walls to an elevation of 7400m a.s.l., they grow on stems and branches of trees from the tropical to temperate forests, they can be found on rocks in the polar and equatorial deserts, and they colonize the boulders on shores of the sea.
The characteristic growth form or thallus of a lichen species is determinated by the fungus. Lichens growth forms can be divided into three groups based on the shape of the thallus:
- the fruiticose type consists of small tubules and/or branches,
Fig.1. Different Cladonia types.
- the foliose type shows a leaf-like thallus,
Fig.2. Umbilicaria sp.
- and the crustose type develops a flattened thallus overgrowing a surface.
Fig.3. A lichen community on crystalline rock - with a prominent specimen of Rhizocarpon geographicum agg.
This last group comprises the most common members of the lichens, and can be found extensively on almost all hard surfaces, like rocks, tree bark and artificial surfaces from buildings and gravestones. Most species however show a preference for a specific substrate, there are for examples differences in the species richness and assemblage found on carbonate and siliceous rocks.
Considering that lichens like all organisms tend to grow and reproduce it is possible observing lichens to relative date a surface. Given similar rocks and climatic conditions, the larger the lichen colony, or denser or richer the lichen assemblage on the surface, the longer will be the time passed since the growth surface becomes exposed and colonized.
Estimating the absolute age of a material from the lichen growing on its exposed surface first requires the determination of the relationship between observed diameter and age of the individual lichen. After measuring lichens on surfaces of known age, for example by studying surfaces on historic buildings, gravestones or already dated geomorphic features, it is possible to determinate the growth rate and plot a growth curve that relates lichen diameters or surface area to time.
Finally it is possible to compare measurements of lichens on a surface of unknown age with the established grow rate, determinate the age of the thallus and interfering a minimal age for the overgrown surface.
Fig.4. Radially grown specimen of Brodoa intestiniformis.
The colonization and growth of a lichen proceeds in four different phases:
1) Surface got exposed and after a while colonized, this ecesis denominated period is not directly determinable with the lichenometric method, however filed-observations of freshly exposed rocks after glacier retreat shows a lag time of 5 to 100 years, depending on the environmental conditions
2) rapid, logarithmic growth of the thallus
3) the growth rate diminishes, and proceeds in a linear manner
4) When lichens grow old, the growth rate gradually declines until death.
Not all lichen species develop a long-lasting phase with relative constant growth; these species can not be used in lichenometry, also the growth rate depends directly from the studied species and the environmental conditions - the growth is influenced by local, and regional environmental factors, such as temperature, moisture, nutrients, day length and snow cover (the latter factors influencing directly photosynthesis of the algae).
To obtain comparable results in an ongoing study it is necessary to measure thalli of the same lichen species under similar conditions, for example choosing a specific species on similar exposed surfaces of stable boulders.
The photosynthetic productivity in lichens is very low compared to "higher" plants, less than 25% comparing the same photosynthetic active areas between lichens and common plants. This low productivity implies a low growth rate, but also an increased longevity. Some lichen species (like Rhizocarpon geographicum) are estimated to reach (under favourable conditions like in the cold and dry conditions of western Greenland) an age of 5.000 to 9.000 years, the theoretical upper limits of lichenometry. However because lichens colonies eventually grow together, and can no longer be measured individually, lichen as a dating tool are used in a range less than 500 years. Under optimal circumstances lichenometry can provide age accuracy with a margin of error of less than 5 years over the past 200 years.
There are also different approaches, how to measure the single lichens specimens.
One of the simplest and fastest methods is to measure the axis of the five largest individual specimen, more elaborated methods increase the number of measurements or vary the measured parameters, like the diameter of an inscribed circle, the surface area, or the outline length of the lichen. Statistic approaches of the data obtained by these different methods showed that they not influence to much the results; therefore the first mentioned method is one of the most popular and most used.
Lichenometry is considered a useful method applicable in an easy and quick way in the field; however there are limitations and some considerations must be mentioned.
For the fundamental principle of the method, the growth rate of a lichen species, it is necessary to find appropriate lichen colonized surfaces of known age. The surface and the single specimens that will be measured and dated must also fullfil some requirements. Not all surfaces are equal, snow cover, sunshine exposure can vary, selected boulders can be instable and their changes of position can influence the growth of lichens.
The large scale climatic factors must be considered during the selection of survey sites, different climatic conditions can it make impossible to use the same growth rate for different valleys, even if geographically they are adjacent.
Despite the restricted numbers of species used in lichenometry, the method at least implies a basic knowledge of the lichen species and their classification. Some lichen species are very similar on a macroscopic scale and differ only slightly in colour or general growth pattern, also the colour of some species tends to change with age.
Despite the omnipresence of lichens, they are often neglected by non-botanists (and even botanists), good books on the matter for non specialists are rare, and important information's are often not divulged. For example one of the most used and well known lichen species by earth scientists, R. geographicum, according to botanist in fact is not a species, but an aggregate of different groups, with slightly different properties, resulting in possible implications on lichenometry not yet fully considered.
Despite these last considerations, lichenometry has proven to be an inexpensive, widely adaptable, and invaluable tool for use in estimating surface ages in lichen-dominated landscapes also for the geologist.
References:
McCARTHY, D.P. (2006): Lichenometry. 1399 - 1404 In (ed): ELIAS, S.A. (2006): Encyclopedia of quaternary science. Elsevier.
WALKER, M. (2005): Quaternary dating methods. Wiley Press: 304
The idea to use the growth of lichens as simply and in field applicable method for relative dating of surfaces was first proposed by the botanist Knut Faegri in the 1930s, and in the 1950s developed further by the Austrian botanist Roland Beschel. During lichenological research on cemeteries he noted that on older gravestones larger lichens can be found. Many of the observed species where also found on rocks exposed by receding glaciers, Beschel realized that lichens could be used to relative date the glacial extensions in the Alps during the Holocene.
Despite the first promising results of lichenometry by botanists and geographers, geologists discovered the use of lichens in alpine or arctic environments only in the decade between 1960 and 1970, since then lichenometry was and is used as a simple field method to date moraines, rockfall deposits, debris flows, denudated rockwalls, escarpments and raised beaches.
Lichens are a symbiotic live community between algae and fungi. The microscopic alga furnishes nutrients for the fungus, the macroscopic visible fungus provide moisture and shelter for the alga. This partnership enables the two partners to colonize habitats, which the single organism couldn't possibly colonize by itself, and in fact as pioneer species lichens colonize an extraordinary variety of habitats and surfaces: Lichens can be found on debris and rock walls to an elevation of 7400m a.s.l., they grow on stems and branches of trees from the tropical to temperate forests, they can be found on rocks in the polar and equatorial deserts, and they colonize the boulders on shores of the sea.
The characteristic growth form or thallus of a lichen species is determinated by the fungus. Lichens growth forms can be divided into three groups based on the shape of the thallus:
- the fruiticose type consists of small tubules and/or branches,
Fig.1. Different Cladonia types.- the foliose type shows a leaf-like thallus,
Fig.2. Umbilicaria sp.- and the crustose type develops a flattened thallus overgrowing a surface.
Fig.3. A lichen community on crystalline rock - with a prominent specimen of Rhizocarpon geographicum agg.This last group comprises the most common members of the lichens, and can be found extensively on almost all hard surfaces, like rocks, tree bark and artificial surfaces from buildings and gravestones. Most species however show a preference for a specific substrate, there are for examples differences in the species richness and assemblage found on carbonate and siliceous rocks.
Considering that lichens like all organisms tend to grow and reproduce it is possible observing lichens to relative date a surface. Given similar rocks and climatic conditions, the larger the lichen colony, or denser or richer the lichen assemblage on the surface, the longer will be the time passed since the growth surface becomes exposed and colonized.
Estimating the absolute age of a material from the lichen growing on its exposed surface first requires the determination of the relationship between observed diameter and age of the individual lichen. After measuring lichens on surfaces of known age, for example by studying surfaces on historic buildings, gravestones or already dated geomorphic features, it is possible to determinate the growth rate and plot a growth curve that relates lichen diameters or surface area to time.
Finally it is possible to compare measurements of lichens on a surface of unknown age with the established grow rate, determinate the age of the thallus and interfering a minimal age for the overgrown surface.
Fig.4. Radially grown specimen of Brodoa intestiniformis.The colonization and growth of a lichen proceeds in four different phases:
1) Surface got exposed and after a while colonized, this ecesis denominated period is not directly determinable with the lichenometric method, however filed-observations of freshly exposed rocks after glacier retreat shows a lag time of 5 to 100 years, depending on the environmental conditions
2) rapid, logarithmic growth of the thallus
3) the growth rate diminishes, and proceeds in a linear manner
4) When lichens grow old, the growth rate gradually declines until death.
Not all lichen species develop a long-lasting phase with relative constant growth; these species can not be used in lichenometry, also the growth rate depends directly from the studied species and the environmental conditions - the growth is influenced by local, and regional environmental factors, such as temperature, moisture, nutrients, day length and snow cover (the latter factors influencing directly photosynthesis of the algae).
To obtain comparable results in an ongoing study it is necessary to measure thalli of the same lichen species under similar conditions, for example choosing a specific species on similar exposed surfaces of stable boulders.
The photosynthetic productivity in lichens is very low compared to "higher" plants, less than 25% comparing the same photosynthetic active areas between lichens and common plants. This low productivity implies a low growth rate, but also an increased longevity. Some lichen species (like Rhizocarpon geographicum) are estimated to reach (under favourable conditions like in the cold and dry conditions of western Greenland) an age of 5.000 to 9.000 years, the theoretical upper limits of lichenometry. However because lichens colonies eventually grow together, and can no longer be measured individually, lichen as a dating tool are used in a range less than 500 years. Under optimal circumstances lichenometry can provide age accuracy with a margin of error of less than 5 years over the past 200 years.
There are also different approaches, how to measure the single lichens specimens.
One of the simplest and fastest methods is to measure the axis of the five largest individual specimen, more elaborated methods increase the number of measurements or vary the measured parameters, like the diameter of an inscribed circle, the surface area, or the outline length of the lichen. Statistic approaches of the data obtained by these different methods showed that they not influence to much the results; therefore the first mentioned method is one of the most popular and most used.
Lichenometry is considered a useful method applicable in an easy and quick way in the field; however there are limitations and some considerations must be mentioned.
For the fundamental principle of the method, the growth rate of a lichen species, it is necessary to find appropriate lichen colonized surfaces of known age. The surface and the single specimens that will be measured and dated must also fullfil some requirements. Not all surfaces are equal, snow cover, sunshine exposure can vary, selected boulders can be instable and their changes of position can influence the growth of lichens.
The large scale climatic factors must be considered during the selection of survey sites, different climatic conditions can it make impossible to use the same growth rate for different valleys, even if geographically they are adjacent.
Despite the restricted numbers of species used in lichenometry, the method at least implies a basic knowledge of the lichen species and their classification. Some lichen species are very similar on a macroscopic scale and differ only slightly in colour or general growth pattern, also the colour of some species tends to change with age.
Despite the omnipresence of lichens, they are often neglected by non-botanists (and even botanists), good books on the matter for non specialists are rare, and important information's are often not divulged. For example one of the most used and well known lichen species by earth scientists, R. geographicum, according to botanist in fact is not a species, but an aggregate of different groups, with slightly different properties, resulting in possible implications on lichenometry not yet fully considered.
Despite these last considerations, lichenometry has proven to be an inexpensive, widely adaptable, and invaluable tool for use in estimating surface ages in lichen-dominated landscapes also for the geologist.
References:
McCARTHY, D.P. (2006): Lichenometry. 1399 - 1404 In (ed): ELIAS, S.A. (2006): Encyclopedia of quaternary science. Elsevier.
WALKER, M. (2005): Quaternary dating methods. Wiley Press: 304
Very interesting post, David. I studied lichens back in the 1970s, but you have provided great detail on this interesting aspect of their use in geology.
ReplyDeleteLichens are fascinating organisms... thanks for promoting them!