Step outside the door and look closely at the surfaces around you. Look out for what look like brightly coloured circular painted patches on the pavement, on walls, on the bark of trees and on roofs. These are almost certainly lichens. There are approximately 2,271 species of lichens in the UK, occurring in most habitats. In the national Species and Habitats Review 2007, a substancial number of lichen species were added to the UK list of priority species for biodiversity action.
Humans have a long history of using lichens for different purposes. Lichens are used to produce herbal medicine like antibiotics, dyes including the litmus dye for indicating pH levels, they are a valuable food source for animals in arctic regions and they have even been used as packing material for ancient Egyptian mummies!
A lichen is not a single organism like most other living things, but rather it is a combination of two organisms, a fungi and an algae (a type of plant). The fungus forms most of the body of a lichen (called a thallus), providing a layer around the algae, protecting it from extremes of temperature and drought. The fungus is unable to produce food so it relies on the algae to produce nutrients for both organisms by photosynthesis. Lichens are highly sensitive to subtle changes in environmental conditions, especially air pollution. As a result the presence of some species is an indicator of clean air and the presence of others is an indicator of pollution.
There are three main growth forms. Foliose lichens have a leaf-like thallus which is attached to the soil, plant or rock it is growing on by root-like rhizines. Fruticose lichens (also known as bearded lichen) have a branched, shrub-like thallus which is attached to the soil, plant or rock it is growing on by a sucker-like holdfast. Crustiose lichens have a crust-like thallus on the plant or rock it is growing on.
Brief descriptions of lichen species growing in different habitats in the Yorkshire Dales are given below:
Lichens of heath and bogs
Fruticose lichen. Photographby Roger Hensen.
Cetraria islandica, also known as the Iceland moss, is a fruticose lichen. In Britain it is common in northern England and Scotland among heather on moors and peaks. It is well adapted to growing in high winds in harsh environments, is an important food source of caribou elsewhere in the world and has been used in Chinese medicine and as a source of antibiotics. It has a cushion-like growth, forked branches 2-6cm tall, up to 1cm across and edged with a row of small spines. The upper surface is dark chestnut brown and the lower surface is pale. Colonies are 6-24cm across.
Cladonia portentosa is a fruticose lichen common on moors and heaths. It forms patches among heathers stems and on bare ground. The hollow stems have many branches and the tips point in all directions. It contains ursnic acid which imparts a yellow dye.
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Lichens of rocks and walls
Crustiose lichen. Photographby Roger Hensen.
Solenopsora candicans is a crustiose lichen found commonly in England and Wales on limestone. In the Yorkshire Dales National Park the best places to see it are in the south-west of the Park within the Craven District. It forms white or pale grey circular crusts with lobed edges, a cracked centre and black spore producing discs.
Collema crispum also known as the crinkled jelly lichen has a foliose habit. Worldwide this lichen occurs in Europe, North Africa, the Middle East, North America and New Zealand. In the Yorkshire Dales it is common on limestone rocks, walls and lime mortar, especially where it is moist and shaded. It is olive-black in colour and forms densely packed, rounded, overlapping lobes. Spore producing discs have granular or wavy edges.
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Lichens of woodlands
Fruticose and folise lichens.Photograph by Roger Hensen.
Hypogymnia physodes is a foliose lichen which is an indicator of moderate air pollution. Its worldwide distribution is Europe, the Himalayas, East Africa and North America. It occurs throughout Britain especially in the North and West. It grows on tree branches, twigs, wood, rocks, walls and soil, forming great masses or small patches of narrow smooth glaucous-grey lobes. The lichen appears slightly inflated with a dark bare lower surface and vegetative structures can often be seen at the ends of the lobes which are white, powdery and lip-shaped.
Usnea subfloridana is a fruticose lichen and an indicator of clean unpolluted air. It is common on trees and sometimes on rocks, especially frequent in the hilly districts of the north of Britain. It has many branches, it is green-grey in colour, it is 3-8cm long and has a blackish base. The branches are rounded in cross-section and the colour is the same on the upper and lower surfaces. Small rod shaped growths are also common.
Cladonia pyxidata also known as the cup lichen, is a fruticose lichen growing in dry places on banks, walls and rocks within woodland. It can grow on mineral soils and is tolerant of zinc and copper. The lichen has broad warty granular cups which grow up from basal scales. The basal scales are grey or brown-grey with white under surfaces.
Pyrenula nitida is a crustose lichen commonly seen in the Yorkshire Dales. It is found on smooth-barked trees, forming a smooth, waxy crust. It is green-grey when damp and brown when dry. Neighbouring colonies are divided from each other by dark wavy thallus lines. The circular spore-producing discs are blackish and become domed with a central opening when mature.
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