Wildlife: In praise of lichen
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These fascinating organisms live on bare rock and help create the soil other plants need to thrive, explains Bob Gilbert.
I recently had the opportunity of working alongside a scientist, a lichenologist to be precise, as she surveyed these fascinating organisms in one of our local churchyards. We must have looked a suspicious pair as we scoured the tombs and combed the church walls for what is sometimes no more than a thin crust of colour on the stone. Simple though they appear, every lichen is actually two unrelated organisms that remain inextricably linked. Each consists of an outer layer, a fungus, that provides the protection, and beneath it, an algae that produces the food. With this clever combination they are able to survive on bare surfaces, absorbing nutrients from the rainwater that runs over them. They occupy therefore a hugely significant place in the story of life on earth; they are the colonists, the first-comers, spreading across the inhospitable rock and forming soil from the fragments they flake off and from their own forms when they die and decompose. Without lichen the planet would have remained infertile.
The lichen’s dependence on rainwater means that they are particularly susceptible to pollution, especially in the form of acid rain. They fared badly in London over the last couple of sooty centuries and, despite the improvements in air quality since the Clean Air Acts, their recovery is proceeding only slowly – unlike the wall ferns which have staged a rather more dramatic come-back. There are, however, a couple of lichen that have prospered, the most notable among them being a species by the name of Xanthoria parietina. Most lichen must do without common English names, but not so Xanthoria. It is, from its colours, the ‘golden lichen’, ‘golden shield’, ‘yellow scale’, ‘sunburst lichen’ or ‘bronze moss’. As my lichenologist companion pointed out to me, it is actually different colours in different situations, being a pale lime green when it grows in shade and a golden-orange colour where it is exposed to light, the result of chemicals which it produces to protect itself from sunshine. It is probably, therefore, the earliest recorded user of sun-screen.
You can find Xanthoria on many of our city trees and I have it in my own back garden. I see it most often on the smaller twigs when they blow down from the plane tree after a windy spell. Its success here is due to the fact that it is a lover of nitrogen, collecting it from airborne urban dust as it filters through the upper branches. Still, its growth remains remarkably slow, expanding at less than 0.2 millimetres a year once it is mature. Despite this slow growth rate, lichen has a long history of use in dyeing, particularly in the Hebridean islands where one species was used on an almost industrial scale in the preparation of Harris tweed. Xanthoria itself seems to have been put to more frivolous uses. In Derbyshire it was applied in well dressing, whilst in the Scottish borders it was used to stain eggs as part of the preparations for Easter.
We have long practiced egg staining in my own household, painting them afterwards or even decorating them with glitter. The results are used in our annual Easter egg-rolling competition, though, since there are no hills in our area, we have to construct one in our garden, a Heath Robinson affair of ladders and planks, discarded guttering or old bits of timber. I have usually used food dyes to stain the eggs but this year I got good results by following the advice of a Polish friend and using the skins of red onions. Emboldened by this I may well experiment next year with Xanthoria –though I would need to start collecting those fallen twigs right now if I am to amass anything like a sufficient supply in time for next Easter.