Lichens are important as indicators of pollution because they readily absorb heavy metals into their tissues, mopping up elements like lead and cadmium. Lead was formerly present in appreciable quantities in gasoline. In Britain, it was practical to assay the damage done to the environment by mapping lichen species; most are unable to tolerate lead pollution for long, but those that can proliferate at the expense of others. The species plot out the state of the environment. It is extraordinary to see how precisely lead pollution traced the course of major roads, forming a series of corridors with low lichen diversity crisscrossing the landscape. The introduction of lead-free fuel received a boost from such findings.

These humble living patches also faithfully record far wider regional pollution across industrial areas. Species diversity can be very low in inner cities because of the pollution from Blake’s “dark satanic mills” of the last 150 years. But as you move westward across the British Isles the number of lichens increases. This is because the dominant weather systems move in from the Atlantic Ocean, flushing most pollutants eastward, and the lichens revel in the moister atmosphere in the west. In the old oak forests in North Wales every boulder is dappled with lichens, while the twisted oak branches are heavily draped with leafy and feathery forms. Around the old mines on the hillsides nearby, other lichen species take up noxious elements or pollutants, for which they seem to have a particular affinity. These substances can now be accurately assessed using modern mass spectrometry. They are useful things, these unspectacular lichens, for diagnosing the health of the planet.

Many lichens also grow very slowly, some just a few millimeters a year, or less. One way to estimate average growth rates is to use gravestones. These memento mori are a favorite habitat for lichens — indeed, there are some species that are now hardly found outside old churchyards. This is partly because many of these sacred acres have escaped chemical spraying or artificial fertilizer for decades or longer. Some churchyards are little patches of medieval habitat. Lie back beneath an old yew tree, and imagine priests and squires and villeins going about their Sunday business.

A gravestone provides that very useful piece of information — a date. When erected they are pristine, but soon time and lichens make their mark. Lichens on flat gravestones tend to grow outward in a regular circle, so the diameter of the circle is proportionate to its age. The largest circle found on a gravestone of a given date will give an approximation of the maximum rate of growth. There will be a certain range of variation as a consequence of local conditions, and variation in the time of first colonization. Furthermore, as time passes, new species of lichen will join the gravestone habitat — and younger rings will cut through older ones as they grow, thus revealing the order of succession of colonization. A good gravestone will accordingly yield a complex narrative, and many gravestones will provide usable statistics. Growth rates can then be applied to other sites.

The use of lichens in dating is known as lichenometrics. Although not without its critics, lichenometrics has revealed some interesting figures. It seems that lichen growth rate has accelerated in high latitudes since the industrial revolution, and that this may be connected with global warming. Because they can be found nearly everywhere and grow so slowly, lichens are potentially a ubiquitous biological diary that records changes to the environment and atmosphere on the century scale.