Different coloured lichens on a rock surface.
WE often read about Carbon 14 dating of human remains in archaeological sites, which are uncovered when excavating to lay the foundations for high rise buildings in inner city areas abroad. This method of dating is based upon the rate of decay of radioactive isotopes.
It was in 1933 that Norwegian botanist Knut Faegri, whilst exploring the rate of ice melt of the Jostedalsbreen glacier in Norway, discovered a new method of dating rocks revealed in that glacier’s moraines since the glacier receded. World War 2 intervened and so it took until 1950 until an Austrian, Roland Beschel, published a paper on the dating of rocks since glacial times in the Austrian Alps. This method of dating is known as lichenometry, using the rate of lichen growth on bare rock or stone surfaces to determine the length of time they have been exposed to the elements.
How is this measured?
This dating is based on a measured (calibrated) rate of specific lichen growth, taking the average radial measurement from the centre of the lichen to its extremities. The speed at which lichens grow is dependent on the climate, the rock type to which the lichen may anchor itself, and to the amount of atmospheric pollution. Thus, lichens are reliable indicators of climate change and air pollution levels. Preserved on rock faces for 11,000 years, lichen growth is now considered a very accurate of climate changes over the last 1,000 years.
What are lichens?
Recently, scientists at Exeter University, UK, based on dated fossil evidence, have found that lichens and algae were the first colonising organisms on our planet from the time when the continents emerged from the oceans. Worldwide there are over 18,000 known species of lichen with 1,700 of these identified today in the United Kingdom. Each lichen is in a close partnership between a fungus and an alga, depending on the type of alga with which the fungus associates itself. These two organisms are so closely interwoven that they appear as a single structure not unlike coral. Each lichen is a mixture of different species.
This symbiotic relationship sees algae cells, through the process of photosynthesis, providing the fungi with organic nutrients by ‘fixing’ atmospheric nitrogen. In exchange, water, gases and nutrients absorbed from the environment by fungi are shared with algae.
The fungi provide shelter to the algae from excess sunlight and water-loss. Neither the fungi nor the algae could share the same spot on freshly exposed rock surfaces without each other’s help.
Types of lichen
Simple lichens have a crust-like structure but others are more complex with leafy or shrub-like forms with specialised appendages to anchor themselves to rocks or plants. The three most common types of lichen are:
- Crustaceous lichens – encrusting forms which spread over a surface and can easily be removed without crumbling.
- Foliose lichens – characterised by leafy lobes spreading out in a horizontal layer with root-like threads. These can be removed with a knife.
- Fruticose lichens – shrubby forms with many branches which can be picked off a rock face by hand.
The rate of lichen growth is dependent on the species and its environmental conditions, with some species having a growth rate of one millimetre per year and others at two centimetres a year.
Photo shows ‘old man’s beard’ lichens.
Dating of tombstones
Over the years, I have dwelled upon the realms of morbidity to not only look at tombstones in order to calculate the life expectancy of humans during the last three centuries and to note the number of deaths caused by a local plague but also to compare the sizes of lichen growth relative to the time of the deceased. These engravings are often almost barely decipherable through weathering.
On a recent second visit to St Paul’s derelict church in Melaka I carefully looked at the tombstones of Dutch and Portuguese colonists dating back many centuries. Regrettably some of the headstones were encrusted in lichen and the dates on them worn away. Today, tombstones of highly polished marble or granite show no recent lichen growth.
Sabah and Sarawak
On Mount Kinabalu, the upper montane or cloud forest’s stunted trees (between 2,200 and 3,300 metres) see them encrusted with the lichen Usnea ssp. This lichen, often known as ‘old man’s beard’ or ‘old woman’s hair’, dangles down from the tree branches and feeds on the moist atmosphere from the cloud cover. It blows like hair in the wind. Further up the mountain in the sub-alpine zone and on the bare granite surfaces of the summit plateau, above 3,300 metres, crustaceous lichens precariously cling to the bedrock. The rate of growth of these lichens has been used to calculate the amount of time these bare rock surfaces have been exposed, since the melting of their previous coverage of an ice cap with feeder glaciers, some 3,000 years ago. Overall, some 286 species of lichen have been recorded on this mountain.
In Sarawak, an extensive survey of lichens in the Bario Highlands was made in the 1990s by Universiti Malaysia Sarawak in conjunction with Australian National University. Nearly 40 lichen species were observed in the kerangas forests. Seven of these were newly discovered Bornean species. Further research is in progress.
Pollution indicators
These organisms play a central role as environmental indicators for they are sensitive to air pollution and especially sulphur dioxide (SO2). Lichens derive their water and nutrient supplies from rainfall and thus their rate of growth is dependent on the elements. Should the rain be contaminated by polluted air in terms of obnoxious gases or metals then the lichen’s absorption rate slows down. Lichens have also become excellent local long term indicators of global warming with their growth rates controlled by climate changes.
Practical uses
The ancient Egyptians used lichens in their packing material before closing the lids of their deceased Pharaohs’ coffins. Before chemical dyes were invented lichens of various coloured varieties were utilised for dyeing materials. In my schooldays, I never realised that the litmus paper I dropped into a solution to determine its acidity or alkalinity was made from lichens. Acids, produced from species of lichen, are used in drugs and yet other species have both antibiotic and antiseptic properties.
Just think of me, later this summer, when I return to the UK with plans to scrape all the lichen growth off the red tiles on the roof of my Victorian aged house.
If I don’t do this, then the lichens will harbour mosses to be later colonised by wind-blown grass seeds. Should I let nature take its course and have the first eco-friendly green rooftop in my hamlet? I fear, much will depend on how the extent of these one and a half century old lichens have spread.