Life In a Dead Wood
Although an old log is itself lifeless, a close look may reveal within it a living community of invertebrates and fungi, all deriving sustenance from the wood.
Long before the arrival of man, forests covered most of lowland Britain. There were literally billions of trees. At any one time, huge numbers of them were reaching old age and dying, making dead wood one of the most abundant habitats in Britain at that time. Animals and fungi have thus had millions of years in which to evolve a large number of species adapted to feeding on, and living in, dead wood, promoting its decay in the process.
This is why there are in Britain as many as 1000 species of animals that have been recorded living in dead wood. Most are tiny, of course. Insects include many types of beetles and flies; among other invertebrates are slugs and snails, woodlice and centipedes. Many hundreds of species of fungi, too, are found on dead wood.
Today dead wood is relatively scarce because man has changed the character of the forests. Trees are often felled before they reach old age, and many pieces of woodland are kept so tidy that fallen logs are a rarity. It has been estimated that a tidy forest, free of dead wood, may be impoverished by up to a fifth of its fauna.
The animals and plants that depend on dead wood have an important role to play in the forest ecosystem. The living tree ‘locks up’ quantities of minerals and nutrients in its bark and wood. When a tree rots, its valuable chemical substances are returned to thewhere they can be recycled into the next generation of trees. A similar process, maintaining the fertility of the soil, occurs with leaf fall.
The decay of dead wood can be very slow unless it is speeded up by the growth of fungi and the activity of insects. This can best be appreciated if you look at the gaunt white trunks of elms, killed by Dutch elm disease, or the standing victims of a stroke of lightning. In exposed conditions, the dead wood is baked by the rays of the sun, which sterilise it and virtually stop all biological activity. The bark soon peels off, and the wood becomes too dry and, at times, too hot to support fungi and insects.
A log which lies in the shade and remains moist, on the other hand, is in an ideal condition for wildlife to thrive on it. It will retain its bark and support a varied range of fungi and many species of invertebrates, and soon the signs of decay are plentiful.
A sequence of decay
It takes about 20 years for a large log to decay completely to the final stage in which all the nutrients have been used up and the wood disintegrates. The time varies with the type of tree, and also depends on whether decay had already started on the live tree.
The creatures that colonize the wood appear in a natural sequence: first are those that invade a dying tree, followed by those that specialise on recently dead timber, then others, through progressive stages of decay.
To the species that predominate when the wood has reached a crumbly texture. The most specialised species (those most restricted to the habitat) are the early colonizers, while in later stages of decay many types of invertebrates that also live in leaf litter or soil become residents.
A tree trunk consists of an inner layer of heartwood, and an outer layer of sap wood which is richer in nutrients. Some species of fungi develop in the heartwood. Others in the sapwood. Their presence may not be easy to detect: for much of the time they consist of tiny transparent threads known as hyphae, which are normally invisible to the naked eye. Only when the hyphae produce fruiting bodies – toadstools or bracket fungi, for example – can the species be identified. The larger species tend to be seen in the earlier years of decay while the wood still contains the plentiful supply of nutrients they require.
The insect pioneers
Bark beetles are often among the first colonists. The female bores a tunnel and lays her eggs along it; when these hatch, the larvae burrow outwards in a radiating pattern.
The bark beetles loosen the bark and enable other species to invade, such as the flattened larvae of’ cardinal beetles. Then woodlice, centipedes and other species are able to establish themselves, and these creatures dominate the community that lives in the area just below the bark.
Deeper under the surface, the wood is attacked first by the larvae of the larger beetles, for instance some of the longhorn beetles. The stag beetle has larvae which take several years to grow, preferring wood which has already started to rot. The more rotten wood is the home of various click beetle larvae and some uncommon cockchafer larvae.
Flies also breed in dead wood, for the most part under the bark or within partly decayed wood. The larger species of flies include a number of mimics, for instance hoverflies that look like bees and wasps, or craneflies that have a similar appearance to some of the large ichneumon wasps.
Also occupying the dead wood are the wood wasps; the largest species of these are found in conifers. One of our largest ichneumon wasps is a parasite of wood wasps, its long egg-laying tube or ovipositor being designed to penetrate deeply into wood containing wood wasp larvae. It is one of nature’s mysteries that the wasp can locate the larvae so accurately.
Some habitats on old trees
Whereas a log provides a habitat during a period of some years as it decays, an ageing tree is a longer-term habitat. There are even veteran trees that take a century or so to die. Hollow trees sometimes shelter bats and owls. The dead wood on the inside may be riddled with beetle burrows, the insects taking advantage of the fact that new dead wood is being added to their habitat as the live trunk grows outwards.
However, to many naturalists, old trees are of interest because of the birds which nest in natural hollows, or woodpeckers which chisel out their own nest holes. There are some 20 species that regularly nest in tree hollows: in large ones, birds such as the tawny owl are found, while the smaller ones are occupied by pied flycatchers or blue tits. Various insects specialise in living in the nests of these birds.
The forest web of life
The dead wood fauna are an integral part of the forest wildlife community. They play a general role as a source of food for insect-eating creatures such as shrews, hedgehogs and some birds; and at the same time other roles are played in the life of the community. Hoverflies emerge from the wood and visit wild, as do the longhorn beetles, to obtain energy from the nectar, providing the service of pollination in return. Another role is that of the predator – some species of solitary wasps. For example, catch flies and other insects. Storing them up as a food reserve.
Dead or rotting wood is nowadays surprisingly vulnerable as a natural habitat – to see why, you only need look at the number of’ hollow trees wrecked by the bonfires of vandals. Even those interested in observing all the activity of the dead wood fauna should heed a warning: patience is required, and the dead wood should always be kept intact. Break it open, and you have already destroyed the habitat that you are investigating.