The Rotten Life of Trees

An organism that recycles itself from its core in order to keep growing outwards? It sounds like part of a sci-fi film plot. Not so, such organisms are all around us. They have the capacity to outlive man more than tenfold, and we have developed great affection, even reverence, for some of the most ancient specimens.

Trees are believed to have first evolved on earth around 400 million years ago. They survived the meteorite that ended the era of the dinosaurs and have continued to provide their numerous life-giving services to our planet ever since.

Man relies on trees for his continued survival: for renewing and conditioning the air we breathe, for food and fibres, for shade and many other vital services. There are also countless species of wild plants and animals that are dependent on trees, including a rather overlooked but far from insignificant subset of creatures that are wholly or partly reliant on dead or dying wood.

And it is now evident – to tree scientists at least – that trees themselves are dependent on certain fungi and invertebrates if they are to realise their natural inclination to live long enough to be one of those gnarled and stout veteran trees that have witnessed a continuum of British history: the march of Roman legions, royal hunting parties, the distant boom of the Blitz air-raids and, now, the roar of another jet taking off from Heathrow.

HEART OF THE MATTER

Experts in arboreal matters are revising their understanding of the way that trees age and decay, and the relationship between trees and rots. There is increasing evidence that, far from signalling the demise of a tree, fungal disease and rots are essential in the tree’s ageing process – vital elements in its ambition to stay alive for centuries.

Veteran trees do not soar to awesome heights with cumulous clouds of foliage atop. They are stooped and cranky-looking. They sprout whiskery, epicormic growth from warty bosses on their trunks. They are full of untidy holes and scars where whole limbs have been ripped away. Their remaining branches stick out at ungainly angles, are of uneven lengths, some dead, some half-dead and others rotting back into the trunk.

Most significantly, such trees have hollows in their trunks. In some, their heartwood is entirely gone. Detection of fungal disease in the heartwood of a tree is, too often, swiftly followed by the sound of a chain-saw kicking into action. This may be deemed necessary for safety’s sake, but tree specialists now appreciate that heartwood rot does not necessarily mean that a tree is dying – at least in our sense of the word.

Trees age and die in their own way and in their own time. Losing limbs and rotting away inside are just part of growing old gracefully – and more securely. The tree gradually lowers its centre of gravity and spreads its grip wider across the ground. Such a tree may lose a limb or two in a gale, but will rarely be ripped up by its roots, whereas the relatively slim, towering trunk of a tree reaching maturity – say between 100 and 300 years old – with copious, billowing foliage like full sails on a yacht, is at far greater risk in that situation.

This was illustrated in the Great Storm of October 1987 when most of the victims were tall, graceful trees in rude good health. On the day following the storm, London’s parks were a tragic sight; huge trees sprawled on the ground wherever one looked. But among the carnage were veteran trees looking perhaps a bit “morning after the night before” with splintery gashes on their trunks where a branch had gone, but still standing and still very much alive. And, they most likely still are, some two decades later – a mere blink of an eye for a major oak contemplating its 500th birthday.

Biologists are also beginning to see the importance of how a tree feeds itself using its own leaves, twigs, fallen branches and heartwood. With the vital assistance of bacteria, fungi and invertebrates processing wood back to soil, a tree is constantly recycling itself. “Every farmer understands the need for fertiliser-rich soil for his crops, but this is not generally realised in forestry where too much material, including stumps and roots, is removed as standard practice,” says tree expert Ted Green, a founder of the Ancient Tree Forum. Zealous clearance of dead and dying timber not only impoverishes woodland soils but also reduces woodland biodiversity, he points out.

The organisms responsible for recycling the tree are just a fraction of the complex web of life dependent on dead wood. “Micro-organisms and fungi are the bio-engines of trees and woodland, but there are more than 1,700 species of invertebrate alone in Britain and Ireland that need dead wood – that’s about six per cent of total invertebrate fauna. On top of that, there are many, many species of birds and mammals dependent on trees. Where would woodpeckers and bats be without hollowing trees?” he asks.

Indeed. But it is the invertebrate life in dead wood that most fascinates Green. “The most important wood-decay resource for them is a large, standing, living tree in which there is decay in the heartwood,” he explains.

ROTTEN IDEAS

“Few invertebrates are capable of attacking and digesting healthy timber,” Green continues. “This is done by fungi and micro-organisms, which convert cellulose and lignin to more digestible materials. Early decay is termed ‘white rot’.”

In a beauty contest of British invertebrates, the vote, according to many entomologists, would go to some of the creepy-crawlies dependent on white-rotted heartwood. Many are notoriously fussy about how rotted the wood is, where it is, what species of tree, and even the level of moisture, thus some are very rare. Examples include the red net-winged beetle and brassy tortoise beetle, which inhabit white-rotted oak heartwood.

Moving on a stage, red rot supports quite different fauna including species that feed on the fungi rather than the wood,such as the hairy fungus beetle, which eats the mycelium of sulphur polypore fungus. And the predators begin to move in, such as the larvae of the ominously named stiletto fly.

The process of rotting produces a crumbly,black mould that tends to collect in the base of crevices in the tree. Full of nutrients, it becomes further enriched by the droppings of cavity-nesting birds and other detritus. Enter Limoniscus violaceus, the violet click beetle – it requires exactly this habitat but since such conditions are not common the beetle is so rare that it has legal protection in Britain. The larvae of the rare noble chafer (Gnorimus nobilis) prefer similar habitats.

LIVE AND LET DIE

Britain is home to more dead-wood invertebrates than any other European country for the simple reason that we are also guardians of the lion’s share of Europe’s veteran trees. Royal hunting forests and deer parks survived as expansive settings for grand houses, where Gainsborough seated his subjects beneath a veteran oak to help convey the impression of a family of substance and long ownership of land.

For a healthy stock of dead and dying wood, long continuity of landownership and an appreciation of the aesthetic appeal of half-dead hulks on rolling pastures and in dappled glades seem to be roughly what is needed. And a more laissez-faire attitude towards fallen branches and fungi-infested trunks would be the best news for our rarest beetles.

The Guild of Agricultural Journalists of Great Britain