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Lichens are ancient organisms; some colonies are over 8,000 years old. There are around 1,700 species in the British Isles.

What are lichens?

Lichens are made up of two or more organisms working in symbiosis; a fungus and an alga or cyanobacterium. Fungi have a long history of interacting with plants. It is thought lichens may be the first example of fungi teaming up with photosynthesizing organisms like green algae.

The algae are usually trapped and protected by the fungal layers. It is believed by some that the lichenous fungi actually cultivate the algae to produce carbohydrates from photosynthesis.

Lichens grow on substrates like bark and rock. They mainly extract water and essential nutrients from the atmosphere around them. Because of this, and their differing sensitivities, they are useful indicators of air quality and pollution. Sulphur dioxide from coal-burning power stations can be harmful to some but others are tolerant. While nitrogen enrichment from fertilisers can damage some but benefit others.

Loss of ancient trees and woods can have serious negative consequences for lichen communities, especially rare species. Lichens grow extremely slowly, and ancient trees and woods offer good continuity of habitat. They may hold species that are internationally threatened by intensive forestry.

Lichen structures

There are several types of lichen: leprose, crustose, placodioid, squamulose, foliose and fruticose. The three main types are:

Crustose (crusty)

Flat lichens that cover substrate like a shallow crust. They cannot easily be removed without damaging the substrate or lichen. Such as the pin-head lichen, Calicium viride.

Foliose (leafy)

Leaf-like lichens with lobed edges and distinct upper and lower surfaces. They are attached to the substrate by the lower surface. Such as the tube lichen, Hypogymnia physodes.

Fruticose (shrubby)

Shrub-like or wispy lichens with rounded or flattened branches. They are attached to substrate by a single point. Such as the oakmoss lichen, Evernia prunastri.

Lichens you may spot on woods and trees

Tree bark can offer lichens a good substrate to grow on. The pH or acidity of tree bark differs between species and will dictate what lichens colonise it. Bark pH will also be affected by atmospheric pollutants.

The pepper pot lichen, Pertusaria pertusa, is a crustose type most commonly found on open-grown trees in the UK. Another crustose form is the orange fruited elm lichen, Caloplaca luteoalba. This is a priority species that has seriously declined through the loss of elms from Dutch elm disease.

Tree lungwort, Lobaria pulmonaria, is a foliose lichen that grows on the bark of mature deciduous trees, most commonly in central and western Scotland. It is particularly sensitive to atmospheric pollution and loss of old-growth habitat. The foliose golden shield lichen, Xanthoria parietina, prefers nutrient enriched bark in sunny areas. It is tolerant of nitrogen pollution and is common on farmland.

Dotted ramalina, Ramalina farinacea, is a shrub-like fruticose lichen that is widespread on trees due to its tolerance to acid and nitrogen. The protected, fruticose golden hair lichen, Teloschistes flavicans, is very sensitive to pollution and is rare in the UK today.