Lichens

Lichens are a large and successful group of plants. Each species consists of a fungus living in very close association with an alga. This relationship, known as symbiosis, produces a more elaborate and longer-lived plant than either partner can form alone. Lichens occur in a great variety of forms and range from dull grey-brown to brilliant yellow or orange-red in colour. Lichens are often the first living things to colonise new surfaces. They grow on many surfaces including trees, rocks, soil and buildings. Lichens get their energy from sunlight in the same way as green plants. They do not 'feed' on the stone, bark or soil upon which they grow. They are unlikely to damage the surface on which they are growing, and in some cases may protect it from damage caused by weathering.

 Most lichens are highly sensitive to air and soil pollution, and are therefore used as environmental indicators. In highly urban areas, few lichen species are found since most species cannot tolerate even moderate levels of air pollution. However, a few species are able to grow in areas with appreciable pollution levels and are commonly found on pavements, walls and tree bark in cities. If you have lichens growing near your house consider yourself lucky as it means you breathe clean air.

Current Status

More than 18,000 lichen species have been described, but it is estimated that there are about 30,000 species world-wide. There are over 1,500 lichen species in Britain, but between 5 and 10 new species are discovered each year. Most lichens grow better in moist conditions so in Britain there are more species in the north and west. Since industrialisation many lichens have become extinct in large areas of lowland Britain. Lichens are returning to many inner city areas following introduction of stricter pollution control measures. This trend is mirrored in many parts of Europe, North America and Japan.

Until very recently, little was known about the lichens that grow in Hull, although there are numerous records for the surrounding area. A wide variety of the habitats in the city were surveyed in 2002. The survey identified over 50 species, this number compares favourably with the 147 species recorded for the rural areas surrounding the city. Although there are no lichens in Hull which require special protection, there are habitats supporting relatively rich and improving lichen assemblages, such as the mature trees and thickets on golf courses and roadsides with wide verges. Many of the older buildings in the city have a diverse range of associated lichens and should be considered in terms of their plant interest as well as historical value.

Current Factors Affecting Lichens

• The range of lichen species and their distribution within the City is mainly dependent on the air quality. The city can be split into zones: an inner zone of 1 to 4 species, an intermediate zone of 5 to 8 species, and towards the boundary of the city an outer zone of more than 8 species.
  
  
• The level of nutrients available also determines the range of species that can grow. Lichens are often found in areas where high nutrient levels accumulate, such as bases of posts or trees used by dogs, places where bird droppings build up, or cracks where water runs down trees. High levels of nutrients can also build up from use of garden and agricultural chemicals or form industry.
  
  
• Many of the commonly planted trees in the city, such as Plane, Beech, Cherry and Hawthorn are poor for lichens.
  
  
• Vandalism, in some cases, also affects the lichens presently.

Current Action

Legal Status

None of the lichens found in Hull are specifically protected, although all plants are protected by the Wildlife and Countryside Act (1981). It is illegal to uproot any wild plant without permission from the landowner or occupier. The Countryside and Rights of Way Act (2000) increases the penalties for these offences.

Management, Research and Guidance

The British Lichen Society publishes a range of information on lichens, including leaflets on 'Churchyard lichens' and 'Lichens on man-made surfaces'.

The Hull Biodiversity Partnership recently commissioned a lichen survey for the City. The study identified 52 lichen species that occur in Hull. Many different species were found on tree trunks and twigs, and also on stonework, brickwork, cement and other man-made surfaces. Fewer species were found to occur over mosses or on timber, and no species that grow on soil were found. The study provides clear evidence that the city's lichen flora reflects atmospheric improvements. The range of lichens particularly those which grow on trees, noticeably improved (i.e. increase in variety, cover and luxuriance) with distance from the city centre in all directions, with the exception of the easterly route where only a marginal improvement was detected. This study provides baseline information that can be used to evaluate future impacts, whether positive or negative, on Hull's lichens, and the quality of our environment.

Action Plan Aims

1. To use the baseline data to determine changes over time in the lichen species present in Hull.
2. To highlight the link between lichens and air quality.
3. To continue survey work within the City to identify lichens in a wide range of habitats.
   

What We Are Going to Do

What We Can All Do

• Do not remove lichens, they take a very long time to grow, but add character to buildings and stonework and are unlikely to damage the surface.
  
  
• If you want to encourage lichens to grow on new surfaces send for the British Lichen Society leaflet 'Lichens on man-made surfaces' which suggests a number of ways to encourage lichens, including painting new buildings with yoghurt, beer, porridge and even cow slurry!
   

Links With Other Action Plans 

This plan should be considered along with those for The Built Environment, Parks, Cemeteries and Golf Courses, Grassland and Trees, Scrub and Hedgerows. Wall Ferns often share similar habitats and these species may require similar action.

References

Dobson, F.S. (1996) Lichens on man-made surfaces: Encouragement and removal. British Lichen Society, London.

Dobson, F.S. (2000) Lichens. An illustrated guide to the British and Irish species. Richmond Publishing, Slough.

Chester, T., Palmer, K. (1994) Churchyard lichens. British Lichen Society, London.

James, P. (1982) Lichens and Air Pollution. British Museum (Natural History) and BP Educational Services, London.

Seaward, M.R.D. (2000) The lichen flora of Hull, with particular reference to their zonal distribution and use in environmental monitoring. (Unpublished report for Hull Biodiversity Partnership).

The Natural History Museum (2002) Lichens: silent witnesses of air quality. (Retrieved February 2002 from http://www.nhm.ac.uk/botany/lichen/air/)