Why dead wood is alive with life

Walk through a wood that has been thoroughly tidied and you will notice something missing: the litter of twigs, the crumbling stump, the leaning dead tree still standing among the living. To a tidy eye these look like neglect. To wildlife they are the richest habitat in the wood.

Around 1,700 British species are described as saproxylic — dependent on dead or dying wood for at least part of their life cycle. That includes beetles, flies, hoverflies, moths, solitary bees and wasps, fungi, lichens, mosses and the birds and bats that feed on them. Dead wood is also a slow-release larder: as it decays, the nutrients locked up in it return to the soil, feeding the next generation of trees.

Leaving it in place is one of the cheapest, most effective things a woodland owner or gardener can do for biodiversity. Nothing needs planting, nothing needs watering, and the benefits build year on year.

Standing dead trees: the high-rise habitat

A standing dead tree — sometimes called a snag — decays from the inside out or the outside in, depending on the species of fungus at work. Bark loosens and flakes, heartwood hollows, and cracks open. Every stage suits a different tenant. Bats, including the barbastelle, roost behind loose bark and in splits. Great spotted woodpeckers chisel out nest cavities that are later used by starlings, nuthatches and tawny owls. Willow tits excavate their own holes in soft, rotten stumps, a habit almost unique among British tits and one reason the species has declined so sharply as woods have been cleaned up.

The upper parts of a standing dead tree stay relatively dry and sun-warmed, which suits beetles needing warm conditions, while the damp base suits an entirely different set of species. A single mature oak can support hundreds of species across the decades it takes to fall apart.

Fallen logs and the woodland floor

Once timber reaches the ground, the pace changes. Logs in contact with damp soil rot quickly and become home to woodlice, millipedes, centipedes, slugs, beetle larvae, frogs and newts. Stag beetle larvae — confined largely to southern England — spend up to seven years chewing through buried, decaying wood, often in gardens and hedgerows as much as in woods.

Not all dead wood should be shaded, though. A few logs left in a sunny spot warm up nicely and are used by basking reptiles such as slow worms and adders, and by solitary bees and wasps that nest in old beetle holes. Grass snakes will use warm, rotting material to incubate their eggs.

Size matters more than most people expect. Thin brash dries out and disappears within a couple of years, while a log of 30cm diameter or more holds moisture for a decade or longer. Aim for a mix.

Fungi: the recyclers that make it all possible

None of this would happen without fungi. They do the chemical work that no animal can manage, breaking down cellulose and lignin and softening timber so that insects can get in. Brown rots crumble wood into cubical fragments; white rots leave it pale, stringy and spongy.

Some fungi are generalists, but others are remarkably fussy. The oak polypore is largely restricted to oak, and several species need dead wood to be continuously present in a wood for centuries. The fruiting bodies — brackets, candles, jelly-like blobs and the sulphur-yellow tiers of chicken of the woods — feed invertebrates in their own right. Many are inedible and some are poisonous, so enjoy them with your eyes rather than your plate.

What to do: practical steps for your patch

• Where it is safe, retain three to five standing dead trees per hectare, favouring the largest and most decayed.
• Leave fallen branches and brash roughly where they land rather than clearing them to a bonfire site.
• Build a few log piles of mixed sizes — some in shade, some in sun, and a couple of logs partly buried in soil.
• Keep timber on site. Saproxylic species disperse poorly, so moving logs to another wood rarely helps the species that need them.
• If a tree must be made safe near a path, consider reducing it to a standing monolith rather than felling it to the ground.
• In a garden, a short log pile behind a shed, a dead hedge along a boundary or a patch of ivy left on a tree will all do good work.

Tidiness, safety and finding a balance

Safety has to come first, and no one is suggesting you leave dangerous timber beside a footpath or a road. Inspect trees near boundaries, rides and access points, and deal with genuine hazards. But when a tree does come down, resist the urge to burn it. Logs stacked in a quiet corner, out of the way of walkers, will support wildlife for years and can be moved later if needed.

It is also worth talking to neighbours and visitors about why the wood looks the way it does. A simple explanation that dead wood is habitat, not untidiness, changes how people see a scatter of branches.

Finally, be patient. Decay is slow, and the species that specialise in it take time to find new habitat and build up numbers. Leave the wood, mark the spot and check it in five years. You will almost certainly find something unexpected living in it.