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Lichens

What is a Lichen?
Despite having plant like features, lichens are not plants at all. They are actually compound organisms made up of two, or even three, very different partners, none of which is a plant.

The partners exist in a symbiotic relationship which means they all benefit from the relationship. The dominant partner in the lichen symbiosis is a fungus with a colony of algae or cyanobacteria, sometimes both - supplying food by photosynthesis.

Fungal hyphae enveloping algal chains in the antler-horn and bearded lichens.

Structurally, the algal cells are sandwiched by fungal tissue and therefore protected from intense light, drought and heat. Thus the alga is sheltered and can live in extreme conditions where it might otherwise be destroyed by the harsh environment. The fungus also could not live alone under such conditions as it cannot produce carbohydrates. These compound organisms can colonise habitats otherwise unoccupied, which gives lichens a decisive advantage in competition for space.

Lichen Survival
Lichens can endure physiological desiccation which enables the species to survive alternate drying and wetting of its tissues and to colonise hostile environments. Lichens use this feature to their advantage in competing with other plants on the bog. The drying of the lichen and its shrinkage effectively strangles germinating seedlings allowing the lichen to form thick carpets on the bog surface.

Lichens have the ability to accumulate nutrients from rain or runoff even though such solutions are very dilute and may only be in contact with the lichen for a very short time. Lichens possess a number of chemicals and acids that can cause weathering especially of rocks. The nutrients released from this process are used by the growing lichen. When the lichen dies these nutrients are released and help to form soil in rock crevices.

Reproducing Lichens
Lichens reproduce in two ways. The fungal partner produces spores in sacs which are forcibly ejected from a fruiting body on the lichen. The spores are light and may be carried some distance before alighting to germinate. If the algal or bacterial partner happens to be present, a new lichen is produced. If not, the germinating fungal spore dies. The second method of reproduction is more reliable from structures called soredia. These consist of both fungal and algal material and appear as a powdery deposit on the lichen surface. The soredia are dispersed by wind, rain or grazing animals. They germinate to form a new lichen.

Identifying Lichens Lichens are divided into three main growth forms: Top: Crustose (crusty) Lichens are closely attached to the substrate like paint on wood. You can barely scratch some off with your nail. Middle: Foliose (leafy) Lichens are leaf like but still grip tightly to the substrate. You can just about get your finger underneath them to remove small bits. Bottom: Fruticose (bushy) lichens are bushy or form clumps. They easily detach from the substrate.

Animals and Lichens
Long-tailed tits, goldcrests and chaffinches all incorporate lichens into their nests to help camouflage them in the trees. Slugs, snails, moth caterpillars, nematode worms and bark lice all graze lichens. Moth caterpillars feeding on lichens during the day mimic the grey-green colours of their food source for camouflage. Moth larvae of the foot man group feed exclusively on lichen epiphytes such as Evernia, Hypogymnia, Peltigera and Usnea. Adult moths resting on lichen encrusted trees or shrubs also mimic their surroundings.

Lichens on Bogs
Most of the lichens on the bog belong to the genus Cladonia. These conspicuous lichens in shapes of matches, cups, antler horns and corals grow in patches, like a small garden on the bog. The commonest species in bogs are Bearded lichen (Cladonia portentosa), Antler-horn Lichen (Cladonia uncialis), Matchstick Lichen (Cladonia floerkeana) and Cup Lichen (Cladonia pyxidata).

When a bog has been burned Cladonia lichens are quick to recolonise the bare peat. Early colonisers are matchstick lichens, followed by cup lichens and finally bearded lichens which reach their maximum development and cover after 10 years. Sensitive to light, lichen cover decreases as they are increasingly shaded by heathers. Never the less cushions of lichens are often seen to be very dominant on the bog. Scientists have discovered that chemicals in lichens can inhibit seedling germination. In addition alternating expansion and shrinkage of lichens following wetting and drying can strangle any seedlings which do germinate in a lichen patch.

Mature heather bushes and stands of birch trees in bogs support conspicuous tufts of epiphytic lichen species such as Ramalina, Usnea and Parmelia.

The types of Cladonia lichen found in Irish bogs.

Uses of Lichens
Over 600 substances have been extracted from lichens which help them to survive in marginal environments and ward off attacks by bacteria, other fungi and grazing herbivores. These substances include pigments used in dyeing, toxins and antibiotics used in medicines.

Lichens to dye for Human beings have used lichens for centuries in dyeing. The Romans produced a purple dye from lichens for their togas. In Scotland the traditional kilts were dyed from lichen extracts while native American Indians use lichen dyes for colour in their rugs.

Medicines Teas and salves made from lichens have been used in traditional medicine throughout the world. Lichens are also used in modern medicines in antibiotic creams.

Perfumes Lichens are used as a fixative in the perfume industry which means that they allow added scents in the perfume to be released slowly. Extracts from Oak Moss (Evernia prunastri) have a haunting mossy odour and are used as a base for many perfumes.

Air Monitors Lichens are sensitive to precise levels of pollution (from sulphur dioxide) and are used as biological indicators of air quality by environmentalists. Lichens act like sponges, taking up pollutants that come their way. By analysing them chemically, scientists can tell what's in the air.

Food Lichens are a major source of food for caribou and reindeer and for smaller creatures such as moths.

Dating Some lichens are among the world's oldest living things and have been used by archaeologists to date artefacts and track geological events since the retreat of the glaciers.

Conservation
Pollution is a serious threat to lichens especially in urban and industrial areas. Loss of habitat is another serious threat to lichens from activities such as drainage, afforestation, hedge and scrub clearance, peatland exploitation and the widespread use of chemicals for fertilisation and weed control.

Further Reading
Dobson, F. (1979) Lichens An Illustrated Guide. The Richmond Publishing Company Limited, Surrey.
Gilbert, O. (2000) Lichens. HarperCollins Publishers, London.

Dr Catherine O'Connell