Small plants, large consequences

Stand in an old English wood in February and you will notice the green before the leaves. Mosses fur the bases of oaks, fill the gaps between roots, and soften the crumbling edge of every stone. Lichens crust the bark in greys, greens and pale yellows, and dangle from branches like frayed ribbon. Neither group is glamorous, but both are quietly doing the work that keeps a wood functioning. Mosses are non-vascular plants that drink straight from the air. Lichens are partnerships between fungi and algae that grow where nothing else can. Together they form the living skin of a woodland.

Clean air, written on bark and stone

Lichens are among the best air-quality recorders in Britain. They absorb water and nutrients directly from the atmosphere, which means they also absorb pollutants — and respond to them. The bushy beard lichens (Usnea) and oakmoss (Evernia prunastri) are sensitive to sulphur dioxide, and their disappearance from much of lowland England during the twentieth century was an early warning about acid rain and industrial smoke.

Since the Clean Air Acts, some have begun to creep back, and that is genuinely good news. Others tell a different story. The orange crust Xanthoria parietina and the grey rosettes of Physcia species thrive where nitrogen is high, so a trunk thick with them usually points to agricultural ammonia or heavy traffic nearby. A wood with a rich mix of pollution-sensitive lichens is a wood with clean, damp air — a condition well worth protecting.

The registers of ancient woodland

Some species spread so slowly that they can only recolonise a wood over centuries. Tree lungwort (Lobaria pulmonaria), with its ridged, lettuce-like lobes, is a classic ancient woodland indicator, almost always found in long-established pasture woodland in the wetter west. So is the barnacle lichen (Thelotrema lepadinum), which studs old bark with tiny craters, and the moss Hookeria lucens, a glossy, translucent cushion of shady, humid banks and gullies.

These are not just botanical curiosities. Their presence tells you the wood has been continuously wooded, and that its microclimate — its humidity, shade and shelter — has remained intact. A wood full of them is a wood with continuity behind it, and that should shape how gently you manage it.

What mosses and lichens actually do for a wood

The contributions are practical, not merely decorative.

• Holding water. Sphagnum mosses hold many times their own weight in water, keeping wet woodland and boggy flushes damp through dry spells and steadily releasing moisture. Ground mosses slow rainfall, reduce splash erosion on bare soil and keep the leaf litter beneath them cool and moist.
• Feeding the system. Some lichens — tree lungwort, dog lichens (Peltigera) and others — contain cyanobacteria that fix atmospheric nitrogen, converting it into a form plants can use. They are fertilising the wood slowly, from the air.
• Housing wildlife. A moss cushion is a whole habitat: tardigrades, rotifers, springtails and mites live in the film of water between its stems. Wrens, long-tailed tits and other small birds strip moss for nesting material, and lichens are gathered too. Several moths and spiders are coloured to match lichen-covered bark and depend on it for camouflage.
• Nursing new trees. Moss carpets are excellent seedbeds for birch, willow and rowan seedlings, protecting them from drought and frost. In wet woods, Sphagnum helps raise the water table and build the peaty soils that eventually support alder and willow carr.

Encouraging them in your own patch

The good news is that you do not need a wood to help. Mosses and lichens respond quickly to small changes in dampness, shade and air quality, so a garden can become a genuine refuge.

• Leave fallen lichen-covered branches, standing dead wood and old stumps in place. Dead wood is where many of the best species live.
• Stop jet-washing paths, patios, walls and headstones. That green film is not dirt; it is a community.
• Avoid sulphur-containing fungicides and garden chemicals, and keep manure, compost and fertiliser heaps away from tree trunks and old walls, where excess nitrogen will let coarse species swamp the specialists.
• Keep the damp, shaded side of your plot undisturbed. A north-facing wall, a stone trough, or a few logs kept moist in shade will colonise surprisingly fast.
• Plant or retain native trees with rough, neutral bark — oak, ash, birch, rowan, willow and field maple all carry good lichen floras. Young trees do too, if you are patient.
• In a wet corner, block a drainage channel and let Sphagnum establish. It will do the rest.
• Buy a hand lens. Ten times magnification turns bark into a landscape and will teach you more about your patch than any book.

Stewardship, not tidiness

Most woodland damage in Britain has come from doing too much, not too little. Lichens grow a few millimetres a year, so the grey-green crust on an ancient oak may be older than the house you live in. Mosses take a season to recover from a scrubbing brush, but decades to return to a drained bog.

So the advice is simple: leave what looks untidy, keep the air and water clean, and let damp shade do its slow work. The reward is a patch of Britain that holds its moisture, shelters its insects and birds, feeds itself from the air, and carries the quiet evidence of a healthy, well-established wood.