January 2014 - Lichens
Lichen growing on a tombstone, Bawburgh churchyard
Common Bawburgh lichen growing on old fruit tree trunk
Winter is generally a time of dull colours and dismal weather, but, once the trees have shed their leaves, it is a good opportunity to see lichens in all their glory. Lichens are small, unique plants, made up of two or more closely interacting organisms, a fungus and an alga and/or cyanobacteria. All the organisms in the association are dependent on the others and all benefit from the other/s. The fungal element provides structure and stability to the association, whilst the alga and/or cyanobacteria are capable of photosynthesis and hence provide the food.
Lichen species can be found growing on pavements, fences, rocks, stones, mature trees and they form a variety of different shapes; some appear leaf-like, some crusty or scaly, some are hair-like and some jelly-like. The colours of the different species can be as varied as the shapes i.e. all shades of green, red, black and orange may be found. They generally grow very slowly (some less than ½ mm a year) so are more evident on old structures and mature trees (they do no harm to the tree just using it as a place to grow on). In churchyards, and this is certainly evident in our own churchyard of St Walstan's, lichens form mosaics of colour, especially on the stonework. Indeed of the approximate 2000 British species, over a third have been found in churchyards with almost half of these species being rare and some seldom, if ever, found in other habitats. Many churchyards are found to have more than 100 species.
Each stone type, for example granite, slate, limestone, marble, sandstone, brick and mortar have their own distinctive lichen communities as do different trees for example oak or hawthorn. Shaded or sunny areas again show different lichen species and whether dry or wet conditions prevail.
We know that many lichens are sensitive to pollutants such as sulphur dioxide in the air, and hence their presence is usually recognised as being an indicator of air quality. The British Lichen Society has worked closely with OPAL (the Open Air Laboratory) Air Centre at Imperial College London to develop the OPAL Air Survey, a citizen science project to collect data on the distribution of common lichens which could then be related to air quality in local areas. This has already shown an increase in lichens in towns and cities, presumably because of the decrease in air pollution in recent years.
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