Wildwoods don’t need our help to survive ‘apocalyptic’ diseases

Wildwoods will survive without the misguided efforts of humans to halt the destruction wrought by tree diseases
Engravings left by elm bark beetles that spread Dutch Elm disease

Listen to this article


It has become a nervous May ritual, scanning the tops of our ash trees for signs of a premature and possibly terminal autumn. We have eight big trees, and if the dieback fungus, Chalara, hits them it will dramatically change the space we live in.

At the moment they are huge weathervanes. Any breath of wind sends their elastic, fish-bone branches into spasms, leaf-waves breaking across the wood. They set the mood, the rhythm of the garden, and if they go, everything from summer suppers to winter bird-roosts will be different. When is probably more likely than if.

The wood where Chalara was first recorded in the wild in the UK in 2012 (the wood’s name, ironically, is Ashwellthorpe) is only 12 miles to the north, and some of its big trees are already pale skeletons. So I try to be grown-up and plan for our ashes’ demise. I fuss about the trees growing round them — oak, cherry, maple — lopping overhanging branches to ensure they’ve got enough light, and realise I’m slipping into an untypically paternal role.

But is there any point? To judge from current wisdom we’re in the midst of an arboreal apocalypse. The ancient denizens of the wildwood are being ravaged by nasty immigrant parasites. The names alone sound like a sorcerer’s incantation: sudden oak death; cherry leaf scorch; chestnut blight. And as if Chalara isn’t enough, the deadly emerald ash borer is poised to cross the Atlantic.

Our response to these plagues has an embedded pattern: panic, chiefly, followed by a search for panaceas. For months the media insisted ashes made up 30 per cent of national tree cover (its 5 per cent). Antiseptic footbaths were installed at wood gates. Pre-emptive felling began before the realisation this was the worst option, risking knocking out immune trees with the rest.

The damage wrought by a horse chestnut leaf-miner

An insidious anthropomorphism governs our relationships with trees, from beliefs about their conception to judgments on their health. We persist in regarding them as frail humanoids in need of intensive care, not as autonomous organisms. For at least two centuries trees have been rebranded as the products of human enterprise, and their existence predicated on our behaviour as surrogate parents. We must plant, stake, weed them, employ hygienic or cosmetic surgery if they are to survive, and put them out of their misery when they don’t pass our tests of worthiness. What we shut our eyes to is that this pattern of ubiquitous, regimented intervention is part of what makes them susceptible to trouble.

I hope I’m not sounding complacent. Chalara especially is a nasty, ugly infection. Stricken trees develop weal-like gashes on their limbs, and the wilted leaves hang on through the summer like brown fever flags. In the short term it may be as pervasive as Dutch elm disease. And its progress is providing an object lesson in how tree diseases can spread, and how ineptly we deal with them, emotionally and practically.

No one is certain about its exact origins. The best theory envisages a non-lethal ash parasite from Asia mutating or hybridising with a similarly benign European species, Hymenoscyphus albidus, generating a new form with a byzantine reproductive life. The sexual stage of its life cycle is embodied in tiny white cup-fungi that develop on fallen leaves, and whose spores are whirled upwards by the wind to settle on growing leaves and twigs. The disease stage seems to be an asexual mould-like organism, Chalara fraxinea, that develops on the leaves and then works its way down into the woody parts of the tree.

The trunk of an elm tree killed by Dutch elm disease

It was first noticed in the early 1990s and began advancing inexorably across Europe. In Lithuania it has killed 60 per cent of ashes, in Denmark 90 per cent. Tree pathologists in Britain were sounding warnings a decade ago, but were ignored by both the government and commercial forest sector. The disease may have arrived in Britain by spores blown across the Channel, but most evidence points to saplings imported by tree nurseries. That nurseries should be selling ash at all, let alone material from areas that should have been in quarantine, is baffling, given that ash, after sycamore, is the most prolific self-seeder in Britain. Foresters call it “the weed tree”.

It’s this massive seed production, not fungicide assaults or genetic manipulation in the lab, that is likely to be the saving of the British ash. Wild ashes are hugely diverse genetically. Varieties that grow in limestone are not the same as those that have evolved in deep loams, and one type can’t be successfully transplanted into the other’s habitat. The likely scenario is that resistant trees will be found in pockets scattered across the country. Even in the worst case — with, say, just 5 per cent of trees remaining, some of whose seeds carry genes for resistance — the country could be recolonised by ash within a century.

A woodland trust worker disinfects his boots in a wildwood near Ipswich, in the UK, where many trees have been hit by ash dieback disease

Ash dieback isn’t a typical disease, and the plethora of tree scares over the past 50 years show other sides of the infection-and-response pattern. In the 1970s beech, incredibly, was predicted to become extinct in southern England by now, as a result of aphid attacks on drought-weakened trees. Ten years on, stag-heading in oaks, in which the top branches die back to resemble antlers, began to be regarded as a disease, and the prelude to “the End of England’s Oaks”. It is nothing of the kind. Cutting back on foliage is a sensible budgeting move by a tree having trouble with its supply of water and nutrients (which is why such oaks are most commonly found on roadsides, where their roots are compacted, salt-soaked and ditch-cut.) Then the invasion of acorns by an exotic wasp, producing “knopper galls” was predicted by the Forest Authority to end in the emasculation of the national tree’s reproductive powers. In 2006 the press prophesied “autumns without conkers” because of the horse chestnuts’ invasion by canker and a leaf bacterium. Many trees were felled as a precaution. The remainder live on. Their leaves brown early like a dose of summer flu, but the spring candles flare and the conkers continue to fall.

The reason these catastrophes didn’t materialise is that the trees adapted — and we don’t like the idea of adaptation because it violates our notion of trees’ dependence on us. Even diseased elms — a salutary argument against optimism being flourished indiscriminately — have adapted to a degree. The Dutch elm fungus has invaded British trees many times, and they have always recovered. The variety that dealt the most recent, near-fatal blow was carried in with imported timber, foreshadowing the entry route of Chalara. Our East Anglian elm reproduces by sucker not seed (its weakness — when one individual contracts a disease so does the rest of the clone). But the clones are diverse (their strength). Several “village elms” have proved immune. In Boxworth, in Cambridgeshire, for example, there is a clone unaffected by Dutch elm — many magnificent trees with gracious tapering leaves tinted with tangerine in autumn. Elsewhere, stricken elms don’t actually die. The fungus doesn’t invade the roots, and once the trunk expires, the tree sprouts again from the base. So we now have elm bushes that may grow up to 20 feet before being infected again.

A chestnut tree

One of the errors we make in making sense of infirmity in trees is to envisage it as analogous to ageing and affliction in ourselves. The manikin form of trees makes analogies with the human frame irresistible. We anthropomorphise them, seeing the trunk, the body of the tree, as its spine, and its loss as equivalent to death. But plants have compensated for immobility by evolving a modular structure with no essential organs. Most plants can regrow after losing 90 per cent of their tissue. Trees can lose all their above-ground growth on a regular basis without harm (the reason coppicing works). Oak leaves can be defoliated by tortrix caterpillars in the spring and grow back strongly with beautiful red “lammas” leaves in midsummer.

A wood without any diseases or parasites would be a lifeless cohort of leafy poles. No leaf-eating insects, therefore no insect-eating birds. No rot-holes for bats and owls. No timber recycled back into the soil. Trees are social organisms. They tend to live with other trees, in a complex network of mutual dependency. They are linked by chains of benign underground fungi that distribute nutrients and information about insect predation, and which one ecologist has nicknamed “the wood-wide web”. If one species of tree succumbs to stresses, other species take its place. In individual trees, reduced vitality prolongs life. What we regard as “diseases” are often just the intricate exchanges and workings of the forest food-chain.

Exotic diseases, to which these exquisite networks are not adapted, are the exception, and there is no conceivable argument against a total ban on imports of tree material from areas where non-native afflictions are rampant. But we should reflect on how the ways we manage trees and woods provides conditions as conducive to epidemics as an overcrowded hospital: battery-grown saplings with minimal genetic variety; dense block planting with single species, often in unsuitable sites; suppression of natural regeneration; ignorance of trees’ natural immune systems and survival mechanisms. At every point we are the cause and aggravator of malignant tree diseases, but it is natural woodland that is likely to be the remedy.

Photographs: Philip Scalia/Alamy; GAP Photos/Simon Colmer; Elmtree Images/Alamy; Bethany Clarke/Getty Images; Getty Images

Copyright The Financial Times Limited 2017. All rights reserved. You may share using our article tools. Please don't copy articles from FT.com and redistribute by email or post to the web.