By Anthony John
Lichen
Lichen
As you walk around towns, villages, woodland, moorland or the seashore, you may have noticed small greyish-green or orange-yellow plants growing on walls, rocks, roofs, trees, the soil, gravestones, or seashore rocks; many of these are lichens. Lichens are a close association or a ‘symbiosis’ between a fungus and an alga in which both organisms benefit. There are many different types of lichens, including over 1900 in Britain of which around 900 are found in Devon. To fully enjoy them you need to use a magnifying glass, or a jeweller’s loupe (available from opticians), or better still a X10 hand lens. If you look closely at them, you will see that they are truly beautiful.
They occur in several forms: crust-like (crustose, tightly attached to trees or rocks), leafy (foliose) or shrubby (fruticose); some even produce tiny ‘pixie-cups’. They are found growing on almost all types of surface, including metal, PVC plastic, and glass; some even grow on my car! Lichens can grow in very harsh environments such as deserts, mountain tops or the high Arctic. In the Himalayas they have been found growing at 7400 metres (over 24,000 feet).
Lichens are highly sensitive to atmospheric pollution, particularly sulphur dioxide (SO2). In industrial city centres, there are very few lichens as a result of high concentrations of SO2; but as the air becomes progressively cleaner the variety of lichens increases, until in very clean air, such as we have in many parts of Devon, rare species may be found – such as the ‘string of sausages’ lichen (Usnea articulata) which is fairly common on Dartmoor. As well as being indicators of air quality, the presence of certain species of lichens is a sign of ancient woodland. Whereas on the rocky seashore there are several lichen ‘zones’; the lowest tide-washed zone is black, looking tar-like (this is Verrucaria); above this is an orange zone (with Caloplaca), and higher still is a grey zone with ‘sea ivory’ (Ramalina).
In earlier centuries, lichens were used extensively in dyeing woollen clothes, and even today a few people still use small amounts for dyeing; ‘cudbear’ (Ochrolechia) produced a crimson or purple dye (extracted by soaking it in urine!), while ‘crottle’ (or crotal, Parmelia) gave a brown colour. A few lichens are also used to make perfumes with a musk-like fragrance. The Saami people who inhabit Lapland (northern Norway, Sweden, Finland and Russia) traditionally have herds of reindeer which they live with all year round. In these Arctic areas lichens such as ‘reindeer moss’ (Cladonia) are an important food source for reindeer and make up 60-70% of their diet in winter.
Lichens are very slow growing, some growing less than 1 millimetre (mm) per year, others up to 5 mm or more per year. By studying the growth rate of different lichens, it is possible to estimate the age of a lichen and therefore the age of its ‘host’. For instance, the stone sculptures on Easter Island are estimated to be 400 years old, while the oldest alpine lichens may live up to 1000 years!
Lichens, known as dog lichens (Peltigera), also sometimes grow in lawns, as do weeds. Gardeners often worry about ‘weeds’ – but they are basically beautiful wildflowers which appear, according to some gardeners, in the ‘wrong’ place. Many flowers such as dandelions, daisies, self-heal, speedwell, hawkbits, buttercups, primroses, nettles, etc., are foodplants for insects such as hoverflies, bees, butterflies and moths. As gardeners, we can take a positive step towards helping maintain insect populations by allowing just a part of our lawn to become a wildflower meadow, simply by not mowing it for weeks or even months until the wildflowers have finished.
If you wish to find out more about lichens, the Field Studies Council publishes fold-out charts (available from local bookshops) showing many of the common lichens; much useful information can also be found on the British Lichen Society website - www.britishlichen society.org.uk.