Why a dead tree is still a working tree

A living oak is a fine thing, but it is only half the story. For the beetles, fungi, lichens and bats that make a British wood rich, a tree becomes far more interesting once it stops growing. Decay is not decline. It is a process that feeds almost everything else, and the hollow trunk, the crumbling stump and the log half sunk in leaf litter are precisely where woodland life concentrates.

In a truly old wood, a good share of the standing timber is dead at any one time. Most of our woods now carry a small fraction of that, and the species that depend on it have dwindled with it. Putting deadwood back is one of the cheapest and most rewarding conservation acts available to anyone who looks after a few acres of trees.

The insects and fungi that do the work

Deadwood is broken down by fungi, and fungi in turn feed an enormous chain of invertebrates. The word for these creatures is saproxylic — dependent on decaying wood — and Britain has well over a thousand of them. Some are specialists you will never see without a torch and patience: the violet click beetle, the oak pinhole borer, a host of tiny fungus gnats and hoverflies.

The fungi arrive first and prepare the way. Candle snuff spreads its pale fingers over rotten stumps. Dead man's fingers clusters on elder. Beefsteak fungus and chicken of the woods stain the trunks of old oaks and yews. As the fungi soften the heartwood, beetles and their larvae tunnel deeper, and the wood becomes porous, damp and full of holes.

• Fresh deadwood — bark still tight, favoured by bark beetles and the larvae of longhorn beetles.
• Partially decayed wood — soft enough for stag beetle larvae, which spend years inside a single rotting stump.
• Well-rotted crumbly wood — the stage that supports the greatest variety of invertebrates, and the stage most often cleared away.

Standing dead trees, rot holes and cavity nesters

A standing dead tree — often called a stag — is the most valuable piece of deadwood you can leave. Fungi work on it from within, softening the heartwood until a woodpecker can chisel out a nest chamber. Great spotted woodpeckers usually do the excavating, but the tenancy does not stay with them. Nuthatches, treecreepers, marsh tits, pied flycatchers, stock doves and tawny owls all move into holes they could never make themselves. The woodpecker is effectively the landlord of the wood.

Bats use dead trees too. Barbastelles and Natterer's bats roost under loose bark and in cracks and rot holes, often switching trees every few nights. A single standing dead oak, riddled and hollow, may hold more roosting value than a dozen healthy ones.

Standing deadwood also feeds the soil beneath it, dropping a slow rain of frass and fragments that builds the deep, crumbly woodland floor that wildflowers and tree seedlings need.

How much to leave, and where

In a managed wood, a reasonable target is the equivalent of twenty to thirty cubic metres per hectare — several mature trunks' worth, scattered rather than heaped in one corner. Variety matters more than volume. If you achieve nothing else, aim for this:

• Some deadwood in full sun, along a ride edge or clearing; invertebrates bask and develop faster in warm timber.
• Some in deep shade, where it stays damp and rots slowly.
• Some touching the soil, so fungal threads can move in from below.
• Several decay stages at once, from a limb that came down last winter to a log you can crumble in your hand.
• Brash and smaller branches left in loose piles rather than chipped or burned. Hedgehogs, toads and hibernating butterflies all use them.

Leave deadwood where it falls if you possibly can. Moving it between woods risks carrying tree diseases and invertebrates that will not survive the journey.

Leaving deadwood safely

Dead trees do fall — that is rather the point — so decide with your eyes up and your feet on the ground. Only leave a standing dead tree if it cannot reach a footpath, road, building, power line, fence or boundary. Check the base for hollowing, cracking and fungal brackets such as brittle cinder, and check the crown for hanging limbs. If in doubt, fell it and leave the timber where it lands. That is a perfectly good outcome.

Revisit your dead trees each year, ideally after gales, and keep records so the next person knows what was left deliberately. Where a log pile sits beside a well-used path, keep it low so it is not a climbing temptation, and avoid stacking deadwood against the trunks of young or valuable trees, where it can harbour pests and damp.

Starting this winter

Thinning and felling happen between October and March, which makes winter the ideal moment to plan. Leaves are down, so dead trees show themselves plainly against the grey sky. Leave stumps taller than usual — a metre or more, not flush with the ground. Push a few cut lengths into the shade off a ride, and leave a couple of branch piles in the sun.

Then go back in May with a torch, on a mild, still evening, and look at what has arrived. The beetles first, then the fungi, then the birds. It rarely takes long, and it is one of the most satisfying things you can do for a British wood.