A lichen is not a plant in the usual sense. It is a partnership: a fungus that builds the structure, working with a partner that makes sugars from sunlight — usually a green alga, sometimes a cyanobacterium. There are no roots, no waxy cuticle and no way of closing up when the air is poor. Rain, dew and mist are absorbed straight through the surface, and whatever is dissolved in that water comes in with them.
That is exactly why lichens make such good indicators. A tree can drop its leaves, a moss can shelter in a damp hollow, but a lichen simply sits there and accumulates what the atmosphere delivers. Sulphur dioxide, ammonia, nitrogen oxides and heavy metals all leave their mark. Read a wall or a hedgerow carefully and you are reading the air above it.
Older walkers will remember how bare urban trees once were. In the industrial era, sulphur dioxide from coal smoke stripped lichens from most of our cities and valleys, leaving little beyond a grey-green crust on the most tolerant bark. The Clean Air Acts of the late 1950s and 1960s changed that, and lichens have been creeping back ever since.
This gives you a genuinely useful yardstick. If you find bushy, shrubby lichens — the stringy beard lichens and the pale, strap-like oakmoss — on twigs along a lane, that is a decent sign of sustained clean air rather than a good week. Those species took decades to return. Around heathland fringes and old parkland, a rich beard-lichen flora on oak and birch is one of the quiet rewards of a windy, well-washed landscape.
Bark chemistry matters as much as air quality. Oak, birch and pine are acidic and support a modest, specialist community. Ash, willow, elder and sycamore are richer in nutrients and carry more species. Look at the sheltered, damp side of a trunk first — often the north or west — and start at knee height.
Heathland is superb lichen country, because bare sand, pebbles, gateposts and boundary stones all offer stable surfaces. On old woodland edges — particularly in the damp west — you may find the big, lettuce-like foliose species that only persist where woodland has been continuous for centuries.
Before you celebrate a hedge covered in orange, look again. The bright orange crustose lichen that coats twigs and kerbstones thrives on nitrogen enrichment, and it is now spreading along busy roads and around intensive poultry and livestock units. A hedge plastered with it is not a clean-air hedge; it is a well-fertilised one.
So judge the whole community rather than a single species. A diverse mix of growth forms, including the bushy ones, points towards cleaner, well-flushed air. A dull, uniformly orange or grey-green flora with nothing shrubby suggests the opposite. Aspect, altitude, rainfall and bark type all sway the result, so treat every reading as a clue, not a verdict.
Winter is the lichen-watcher's season. Leaves are off the trees, light reaches the trunks, and damp, mild days bring out the colours. Many lichens look like dull grey smudges when dry and transform after rain or a light misting — a small spray bottle in your pack is a genuinely useful bit of kit.
Late autumn and early spring suit heathland visits, when the ground is soft but the Cladonia cups stand clear of the heather. High summer is the least rewarding time: everything looks parched and similar, and you will misidentify more than you record.
You do not need a laboratory to make this useful. A x10 hand lens, a notebook and a consistent route will do. Pick a hedgerow, a churchyard wall or a stretch of heath and repeat the same circuit through the year.
Over a few years, a simple record like this tells you something no weather report can: whether the air over your favourite walk is getting cleaner or richer in nitrogen. It is slow, quiet citizen science, and it fits neatly into a morning's ramble.
The Golden Heath shares practical, down-to-earth guidance on a uk blog about heathland walks, wildlife, seasonal countryside living, and practical outdoor skills. for readers across the UK.
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