What the Bark Is Telling Us: A Field Guide to Lichens in Scottish Woods
Photo: lichen-covered oak bark in Scottish Atlantic temperate rainforest, via i.etsystatic.com
A partnership written on bark
Lichens are among the most articulate witnesses to the state of the air. They have no roots and no waxy cuticle of the kind that protects a flowering plant’s leaves. Instead, they absorb water, gases and dissolved nutrients directly from the atmosphere. That makes them vulnerable. Pollutants that pass almost unnoticed through other organisms can accumulate in lichen tissue or disrupt the delicate partnership between the fungus and its photosynthetic partner, whether an alga or a cyanobacterium.
That partnership is the basis of the lichen. The fungus provides structure and shelter; the alga or cyanobacterium supplies sugars through photosynthesis. The result is a life form that can colonise bark, stone, soil and even roofs, and that often responds to environmental change more quickly than the trees beneath it. In a Scottish wood, the lichens on an old oak or birch are not mere decoration. They are a record of the air, the humidity, the age of the tree and the continuity of the wood.
Yet a lichen is not a simple pollution meter. Its presence depends on light, shade, bark chemistry, tree age, rainfall and woodland history. Reading lichens well means reading the wood as well as the bark.
Begin with growth form, not species names
Species identification can be daunting. There are hundreds of lichens in Scotland, many requiring a hand lens or chemical tests. For a beginner, the more useful first step is to recognise growth form. Three broad categories will take you a long way.
| What you see | What it often suggests | Examples to learn |
|---|---|---|
| A flat crust tightly attached to bark or stone, sometimes like a thin paint or a scribble of lines | Crustose lichens. Some are pollution-tolerant, others are sensitive; context matters | Lecanora species, Graphis scripta |
| Leafy lobes that can be lifted slightly at the edges | Foliose lichens. Many tolerate nutrient enrichment or moderate pollution, though not all | Parmelia sulcata, Hypogymnia physodes, Xanthoria parietina |
| Shrubby, beard-like or hair-like growths hanging from bark or twigs | Fruticose lichens. Many are associated with cleaner, humid air and older woodland | Usnea species, Bryoria species, Ramalina species |
The useful shorthand is that many leafy lichens tolerate more polluted or nutrient-enriched air than many beard-like lichens. But shorthand is not a rule. Xanthoria parietina, the orange wall lichen, is a well-known indicator of nitrogen enrichment, while some fruticose species can persist in less than pristine conditions. The skill lies in looking at the whole assemblage, not one conspicuous species.
The clean-air signal: beard lichens and their allies
Beard lichens in the genus Usnea, and the darker hair lichens in Bryoria, are often associated with low sulphur dioxide levels and humid, long-established woodland. In Scotland, their abundance in western oakwoods, birchwoods and pinewoods is one reason those habitats matter internationally. They are not universal indicators, and their presence also depends on suitable bark, light and moisture. But a wood rich in beard lichens, lungworts and other sensitive species is telling you something important about the atmosphere above it.
That message is not always comfortable. Sulphur dioxide from fossil-fuel burning once stripped many woods of their sensitive lichens. Since the clean-up of industrial emissions, some species have recovered, but nitrogen pollution from agriculture and transport is now a widespread pressure. Nitrogen-loving lichens can spread at the expense of species adapted to nutrient-poor conditions. In lowland and peri-urban woods, a shift towards orange and grey-green leafy lichens may be a sign of enrichment rather than health.
Scotland’s Atlantic rainforest: a lichen hotspot
On the west coast and in sheltered glens, Scotland holds fragments of temperate rainforest, often called Atlantic woodland. These woods are globally rare. They depend on high rainfall, mild winters, clean air and a long continuity of trees. Oak, hazel and birch are typical hosts, and the lichen communities they support are exceptional.
Species such as tree lungwort, Lobaria pulmonaria, and various pseudocyphellas are among the best-known indicators of this habitat. They are joined by a rich assemblage of crustose lichens that few visitors notice but which can be internationally important. These communities are fragile. Overgrazing, invasive rhododendron, canopy loss, pollution and the fragmentation of woodland cover all take a toll. Conservation here is not only about planting trees. It is about retaining old trees, varied age structure, damp sheltered glades and the stable humidity that lets sensitive lichens persist.
A half-hour recording method
Citizen recording has real value for lichens because they remain under-recorded. A phone, a spare half hour and a little care are enough to make a useful contribution.
Choose a tree you can reach without damaging the bark. Photograph the whole lichen growth, then a close-up of the lobes and, if possible, the underside. Include a coin or ruler for scale. Note the tree species, bark texture, aspect, height above ground, shade and a grid reference from your phone. Record several trees of the same species rather than one conspicuous specimen.
Submit your records to iRecord, your local environmental records centre, or a recording scheme run by the British Lichen Society. Follow the society’s guidance on sensitive species: do not collect from rare lichens, and avoid disturbing bark or woodland in wet conditions when damage is more likely. A well-made record with a photograph and a grid reference is more useful than a long list of uncertain names.
Reading the wood, not just the lichen
A walk through a Scottish wood can tell you a good deal about the atmosphere above it, but the reading must be cautious. Compare several trees of the same species. Look for gradients: more beard lichens on damper, older trees; more nitrogen-loving species near farmland or roads. A lowland wood dominated by Xanthoria and a few leafy species may reflect enriched air. A western oakwood with Usnea, Lobaria and a rich crustose flora may reflect clean, humid conditions and long continuity.
Absence is the hardest signal to interpret. A lichen may be missing because of present pollution, but also because of past pollution, lack of old trees, dense shade or simply a lack of nearby sources. Lichens respond slowly, and woods are not uniform. The most honest approach is to record what you see, note the conditions, and let the pattern emerge over time. In that sense, every woodland walk is a small act of monitoring.
