Reading the Bark: Why Lichens Reward a Closer Look

A lichen is not one organism but two, living as one: a fungus that builds the structure and an alga or cyanobacterium that photosynthesises inside it. The partnership is ancient, slow-growing and extraordinarily fussy about where it settles. That fussiness is what makes lichens so useful. A trunk coated in a rich, varied community is usually a tree in clean air, in a wood that has been there a long time. A trunk with only a few tough, dust-like patches tells a different story.

Better still, lichens are visible all year, and winter is arguably the best season for them. With the leaves gone, light reaches trunks and branches, and damp days plump the thalli up so their colours and textures are at their most legible. You do not need a laboratory, just patience and a hand lens.

A Few Habits Before You Begin

• Carry a 10× hand lens. Most identification happens at this magnification, where discs, hairs and powdery granules become visible.
• Note the tree species first. Oak, birch and pine have acidic bark; ash, elder and sycamore are richer in nutrients and carry very different communities.
• Check aspect and height. Sheltered, damp, north-facing sides and the bases of trunks hold the most diverse growth; exposed crowns are harsher territory.
• Take a small spray bottle of water. Dampening a dry patch can transform a grey smear into something identifiable.
• Photograph with something for scale — a coin works well — and resist the urge to scrape. Lichens grow by millimetres a year, and a mark made in a second can last decades.

Crusty Lichens: Flat Against the Bark

Crustose lichens are the most common and the easiest to overlook. They spread as a thin, often cracked crust that follows the contours of the bark and cannot be lifted away without taking wood with it. Look for the discs — the fungal fruiting bodies — because they are usually the most distinctive feature.

• Lecanora chlarotera forms a smooth, pale grey crust dotted with pale brown discs, and is abundant on the twigs and branches of ash and sycamore.
• Lecidella elaeochroma is similar but bears black discs, often on smooth bark.
• Graphis scripta, the script lichen, is unmistakable once seen: thin black lines writhe across a white crust like tiny handwriting. It favours the smooth bark of hazel, beech and young holly, and is a good sign of long-established woodland.
• Pertusaria species look like white paint with raised warts, sometimes dusted with chalky spores.
• Lepraria incana is the powdery, blue-grey film on shaded trunk bases — no structure, no discs, just dust.

Leafy Lichens: Lobes and Ruffles

Foliose lichens lift away from the bark at the edges and are the most satisfying group to identify, because you can see their shapes clearly without a lens.

• Parmelia sulcata is grey-green with a network of ridges across the surface and powdery soredia along them. It is common on almost any tree.
• Flavoparmelia caperata is brighter, yellow-green, with broad, wrinkled lobes that often curl at the edges.
• Hypogymnia physodes has swollen, inflated lobes, grey above and brown beneath, with lip-shaped openings at the tips.
• Xanthoria parietina is the vivid orange-yellow lichen of field-edge trees, elder and nutrient-enriched bark. Its presence usually signals nitrogen pollution rather than especially clean air.
• Physcia tenella is small and grey with fine hairs along the lobe margins, frequent on twigs.

Shrubby Lichens: Tufts and Beards

Fruticose lichens grow away from the surface in three dimensions, as tufts, straps or hanging beards. They are the group most sensitive to air pollution, so finding them is a genuine pleasure.

• Evernia prunastri, oakmoss, forms soft, strap-like tufts on oak and ash. It is grey-green above and paler beneath, dividing into two neat forks at the tips.
• Ramalina farinacea is more rigid, with flattened straps bearing neat oval patches of soredia along the edges.
• Usnea species, the beard lichens, hang in stringy tassels from branches. Gently stretch one: if a white elastic cord shows inside, it is Usnea.
• Cladonia species — the pixie cups and trumpet lichens — belong mostly on rotting stumps, heath and soil rather than living bark, but they often fringe woodland clearings.

What the Lichens Are Telling You

Taken together, lichens make a rough but remarkably reliable map of the air and the history around you. Sulphur dioxide, the great killer of lichens through the twentieth century, has declined sharply, and beard lichens are recolonising woods where they were absent a generation ago. Nitrogen compounds from farming and traffic have increased, however, and the orange flare of Xanthoria now spreads along hedgerows and field edges.

Then there are the specialists. Species such as Lobaria pulmonaria, the lungwort, with its deep-ridged, lung-like lobes, and the script lichen Graphis scripta are strongly associated with ancient woodland — sites that have been continuously wooded for centuries. Finding them, on a damp western oak or a hazel coppice stool, is as close as most of us get to reading a wood's age at a single glance.

Start with one trunk. Learn three species well, then five. Within a season you will find that the grey and orange and green patina on every tree has stopped being background and become a story — of clean air, old trees and slow, patient growth.