A Study of Lichens and Lichenometry
BY MEGAN NYLUND
There are certain problems with this method of dating. One also has to take into consideration that the lichen did not start growing as soon as the wall was built, it takes years for this to happen. Only a relative date can be determined, and even then there may be some errors. Sometimes a wall will be built with rocks that already had lichens growing on them. This may cause the date to be inaccurate. There are certain ways in getting around these setbacks like looking to see if lichens are growing across the entire wall or just a small portion of it.
One way to be more precise is to see if there is more than one colony of lichens on the wall. Once one colony of lichens has started it is easier for others to develop. One can then measure numerous lichens across the wall to see if they have similar measurements. If the measurements are similar it is a good indication that the estimated date is somewhat accurate. On the other hand, if there are no other colonies of lichens, or the measurements are significantly different, it might be hard to get an accurate date.
Pollution may also hinder one's results. If the artifact one is studying is near an industrial park or factory, the results may be incorrect. Lichens are very sensitive to pollution from the atmosphere which causes them not to grow in highly polluted areas. An artifact might not have any lichens on it, yet it could still be much older than expected. Lichenometry is often the only way to find out when a stone wall was built.
Lichens are easiest to "read" on wet, rainy days. On dry days, lichens can become inactive and their metabolism can change significantly.' Lichens can live and grow almost anywhere under very harsh conditions. They can even live in places like the Arctic and almost any other solid surface. It is amazing to look at a stone wall and see all different colonies of lichens across it.
Lichens are a symbiotic relationship as the fungi and algae feed off each other in order to ensure survival. The fungi are responsible for protecting the photosynthetic organism by producing a vegative body called a thallus. The photosynthetic alga produces carbohydrates and provides energy for itself and the fungal organism. These plants inhabit rocks and other surfaces that are exposed to air and take a long time to develop lichen colonies. Once one colony has been established, it makes it easier for other colonies to grow and flourish.
There are three main types of lichens in New England: crustose (see Figure 1), foliose (see Figure 2), and fructicose (see Figure 3). Within these three categories of lichens scientists have found about 13,500 species of lichens on earth. Each specie looks different and has distinguishing characteristics which differentiate the different types.
Crustose lichens comprise of about 75% of all the lichens in the world. They vary in color like orange, yellow, green, brown, gray, or black (see Figure 1). These lichens are encrusted in the stone. They grow so tightly to the rock that it is nearly impossible to pull them off. Crustose lichens are the most commonly used in dating a stone wall because they have the slowest growth rate of all lichen species. 'They grow about one millimeter per year, making artifacts easier to date.' Sometimes crustose lichens can show no increase in size for ten years. 'In dating a lichen, for example, if the radius is about fifty millimeters across, or five centimeters, one could assume that the rock had been there for at least twenty five years.'
Foliose lichens connect loosely to stones and have a leaf-like shape (see Figure 2). The bottom part of a lichen is a root-like structure called a rhizine. These 'rhizines are what attach the lichen to the rock.' Since these lichens are not sufficiently attached to the rock, they can be easily separated by a knife or other instrument. Certain types of foliose lichens are among the fastest-growing species which have the ability to grow up to one centimeter per year. This growth rate is unusual in most lichen species. Foliose lichens are usually yellow-green, orange, brown, or gray in color.
Crustose and foliose seem to be more abundant in
New England than fructicose. Fructicose lichens appear more branch-like
and bushy in appearance. Many fructicose lichens stand upright on
a leaf-like base (see Figure 3). Many times, they
do not have a distinguishing top or bottom. 'Fructicose is not attached
to its surface as securely as the others. These lichens are easily
pulled off by hand.'
One can also measure the growth of lichens on gravestones. This helps to determine an accurate date for surfaces. When the lichen is measured an approximate date can be estimated. The date can then be compared to the actual date on the gravestone. If it is close, it proves that lichens can be an effective way of dating. This becomes useful when the date on a gravestone is no longer visible. Gravestones are ideal for checking accuracy. If a lichen growing on a gravestone is measured, the date can be calculated and then compared to the actual date on the stone. If there is a relationship between them, then lichenometry worked.
After a personal experiment, several lichens were measured on gravestones and a stone wall in the same area. On one of the gravestones the lichen measured 8.5cm. This translates to 42.5 years. The actual date on the gravestone is 1760. The dating is 201.5 years off. Another gravestone that was measured was 3cm which means it has been there for about 15 years. The actual date of the gravestone is 1805. The difference in the time is 184 years. The last gravestone to be measured calculated to be 3.5 cm meaning it has been there for at least 17.5 years. The actual date was 1769, making a difference of 217.5 years. There are many factors that can account for this error. The most probable explanation is that the lichens took a long time to commence growing.
The stonewalls which surrounded the area also had many lichens on its surface. During the experiment, three different lichens were measured on different parts of the stone wall. The first lichen measured 4.5 cm which converts to 22.5 years. The second one measured 4 cm translating to twenty years. The last lichen to be measured was 5.5 cm, meaning it had been there for at least 27.5 years. The estimated dates using the lichens for the gravestones and the wall are pretty close. This shows that there is probably a correlation between when the wall was built and how old the gravestones are. It also shows a relationship between the growth of a lichen and the dates (see Figure 5). Although the dates compared to the actual dates on the gravestones are significantly different, a comparison can still be seen between the ages of the lichens. It is likely that the reason for this difference is because it took a long time for the lichens to begin to grow.
Lichens can be used as a means of a relative dating method. Each type of lichen varies and no two lichens are the same. They live as one symbiotically, although they really consist of two different plant forms. Once one lichen is produced there will, most likely, be many more to follow. Lichenometry can be a fairly accurate way to date certain surfaces. Depending on the different conditions, such as the environment, lichenometry can give a relative date for artifacts. Lichens have proved to be a successful way of dating artifacts such as stonewalls and gravestones.
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Bednarik, Robert G. "Lichenometry," http://mc2.vicnet.net.au/home/date/web/lich.html (4 Nov. 2004)
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Santis, Salvatore De. "An Introduction to Lichens," 1999, http://www.nybg.org/bsci/lichens/lichen.html (5 Nov. 2004)