How To Build A Thousand-Year-Old Tree

Even in early summer, when Sherwood Forest is thick with lime-colored new leaves, you start to see it from a few hundred paces away. Its trunk is 36 feet around and its canopy stretches for almost three bus-lengths. Its broad, bowl-shaped crown is propped up by a ring of metal columns, like walking sticks measured to fit each of its groaning boughs. The Major Oak, as it’s called, is surrounded by a fence that keeps its many visitors from tramping too close. On the summer day I visited, Reg Harris, a 50-something arborist in thick utility trousers and a sun-bleached polo, invited me to hop over and stand for a while at the foot of the giant tree.

The Major, Harris told me, sprouted from an acorn here at least eight centuries ago. No one knows exactly when. Some estimate it has seen the arrival of a thousand summers. It’s shorter than it once was, Harris said; the tree would have reached its peak height sometime around Christopher Columbus’s arrival in the Americas, but its upper limbs have long since dropped off. In its later years, it has developed a fat, furrowed trunk that has twisted and fissured through storms and grown boulder-size calluses, which bulge where wounds were torn and healed lifetimes ago.

Place a hand against its lichen-crusted bark, and its hard flesh feels as cool and sturdy as a cathedral’s timbers. Up close, it’s possible to see signs of rot and inhabitation between the band-aids Harris’s predecessors have placed to try to keep the tree alive. The walking sticks, which replaced a system of iron chains (some of them are still stuck fast in the limbs), protect the tree’s trunk from being torn apart by the cantilevered weight of its branches. A macabre process of “cannibalization” began centuries ago, during which time the tree has basically consumed its own rotting, pulpy core, creating a 10-foot high cavern in the trunk. Harris is one of the few people alive to have been in there, where bugs crawl and beefsteak fungus erupts. Some previous caretaker lined parts of the cavity with lead, and you can see into the lower branches, which are hollow and have been reinforced with planks. Harris likened it to being inside the prow of a ship.

Faced with this strange, Frankenstein-ish scene, I wondered if I should feel saddened by the decline of this long-lived beast. But no. Harris introduced me to the Major not to show me a withering geriatric, but rather a sort of living blueprint. It’s a template or “inspiration,” he said, that’s guiding a set of experimental techniques and technologies that aim to recreate some of the Major’s rich collection of scars and wrinkles, hollows and decay. 

These “veteran features” of the rarest, oldest trees are what brought me to Sherwood in search of an answer to an impossible challenge that Harris and other expert “arbs” are collectively facing: to artificially recreate in a fraction of the time the things that only the slow movement of eons is known to make. Across the planet, forest managers, ecological designers, mycologists and others are embarking on new experiments that attempt to analyze and mimic the characteristics of old trees: scanning their architecture with LIDAR equipment, injecting young trees with microbes, constructing artificial trees from prosthetic trunks and limbs, and much more. It’s beginning to show promising results. As Harris joked: “We’re trying to create some sort of arb time-machine.”

A Crisis In Sherwood Forest

As the world faces an accelerating crisis of biodiversity loss and spiking rates of extinction, Britain’s protected natural areas are getting an explicit new assignment. For 75 years since the country’s national parks were first established, their purpose was chiefly to serve humans and their desire for green places rather than the nonhuman species that need them for food and habitat. Today, the U.K. ranks as one of the most nature-depleted nations in the world, in the bottom 10% of nations for biodiversity. To rectify such a dismal situation, British scientists and environmentalists are demanding that an old idea — parks as living ecosystems — ought to be written into law at last.

Britain’s oaks are both symbols of national pride and vital infrastructure in its forests. Collectively, the country’s oaks support about 2,300 species of mammals, birds, invertebrates, mosses, fungi and lichens. Many of them — like the family of bats that live high in the Major’s shattered trunk — find shelter only in old trees and feast on the deadwood-loving creatures that crawl the dark bark canyons and damp cavities that the tree builds up over countless human lifetimes. On their branches and trunks, squirrels make their dreys (dens) and woodpeckers their nests. (Harris said a very rare Lesser Spotted Woodpecker was seen in a tree nearby.) So-called secondary cavity nesters — including the petite pied flycatcher and the stocky, foot-tall tawny owl — later take over the vacated hollows that the woodpeckers excavated. Taken altogether, trees like the Major are complex mountains of microhabitats. 

Unfortunately, they are roughly one in a million. The U.K. has an estimated 170 million oaks, but only 115 ancient giants the size of the Major were found in a census by the University of Oxford. In total, Sherwood Forest has about 380 trees older than 400 years — usually the minimum age to have developed veteran features and be classified by arborists as “ancient.” Few places in Europe can boast these numbers. The rest of the Continent combined has fewer ancients than the U.K.

Partly, this is a consequence of the U.K.’s long history of ring-fencing vast areas away from the common folk as private reserves for kings and queens. Sherwood Forest has been recognized as a protected area since it became William the Conqueror’s hunting ground in the 11th century. Thousands of its trees have been hauled away over the centuries, destined to be used to build ships during the Napoleonic Wars or cathedrals like St. Paul’s in London, or else cut down in some other wave of logging. The handful that survived, like the Major, were usually spared the axe because they were considered too old, gnarled or rotten to be of use.

On the day I visited, a haze of tree pollen hung overhead as the forest began its most active season of growth and reproduction, but what Harris wanted to show me was rot and decay. “That one died,” he said, enthusiastically pointing to an oak that still had a forking silhouette of branches but was entirely without leaves. “And yet it’s absolutely full of life.” 

Maintaining a supply of dying wood, and exploiting trees’ ability to exist in a state of spooky half-death, is critical for the unique communities of inhabitants that depend on it. Columns of dead and decaying tree matter called “tree soil’ are the single most important nutrient resource for deadwood-loving insects and can remain supported in “deadwood chimneys” by the rest of the living trunk, Harris explained. “That’s where we get the beetles,” he said.

Deeper in the backwoods, Harris pointed out dead branches hanging from still-living joints: places beloved by a moth that lives behind the bark. He stopped to explain “ram’s horn callusing,” a wave-like shape of “wound wood” where the tree scabs around injuries: a favorite sleeping spot for bats. There were split trunks, where the core of the tree had been suddenly exposed by a storm or lightning strike, and “walking trees,” where a tree cracked or was blown down and rerooted itself a few feet away. All of it is habitat for something, even if it’s microscopic fungi invisible to the naked human eye.

Today, just about everyone who works with these gnarled survivors agrees that keeping them going is critical for the health of the forest overall. As the ancients die, the many species that live and depend on them must move to survive. Most of the available oaks in the surrounding landscape are inhospitable: nearly identical in size and only a century or two old. “Boring trees,” said Harris. “Featureless trees.” Without action, he continued, the number of ancients will dwindle to zero before new ones can be minted, leaving many species “like polar bears on an ever-diminishing iceberg.” 

“At Sherwood, it’s not an exaggeration to say that there’s now an age gap, conservatively, of 500 years between the oldest trees and the next cohort,” he said. “So, what we’re really after is something to bridge that gap.”

Hacking Trees

An hour into our walk, by then deep in Sherwood, Harris found an unremarkable oak he’d been hoping to show me. About three feet up its trunk, there was an unnaturally rectangular hole that looked like an upright mailbox. An inch or two of wound wood curled around the edges. Harris grinned with pride.

Squeezing his hand into the slit, he turned over a leaf that had fallen inside and found two woodlice rummaging through a dusting of decaying tree soil. Two years ago, he’d found a darkling beetle here, one of the wood mold specialists that are fond of hanging around deadwood chimneys.

Without help, a tree like this — “a very uninteresting, very boring, very small tree, maybe 25-30 years old” — would not form a dank, bug-crawling crevice for perhaps a few hundred years. So, to defeat time, arbs like Harris have put the tools of tree maintenance to work on surgical tree-wounding. 

Sherwood is now the site of the U.K.’s largest program of “veteranization,” as the process is known to arborists. The goal of veteranization, which is only ever done to young trees, is not to kill them but to leave them living with features like storm damage or damp hollows that would usually not form until much later in life. Armed with a chainsaw, an arborist might make a slice akin to a lightning strike, carve out an artificial “woodpecker hole” or make a series of plunging cuts into a trunk to create a “nestbox.” 

Other veteranization techniques are even cruder than rough cuts with a chainsaw. Harris has ripped off branches using a winch attached to his pickup, mimicking the way strong winds tear at trees during storms. Walloping the base of a tree with a sledgehammer has turned out to be a surprisingly efficient way to cause a column of rot to form above. This “horse kick damage” replicates impacts by roaming herds of horses and extinct megafauna like aurochs or elks, whose creative disturbances have been disappearing or missing entirely from the British Isles for millennia.

To the uninitiated, it can be surprising that caring for a forest might involve yanking trees’ limbs off. And there’s something slightly unsettling about seeing a skinny, green tree with an ancient hollow in it — a bit like a toddler with a long beard. But it is a good sign, Harris said. If you can detonate the beginnings of deadwood ecology in a tree that’s “as ridiculously young as that, then we’ve definitely bridged the gap,” he said. “It’s tiny but it’s a step in the right direction, isn’t it?”

Vikki Bengtsson, a trailblazing arborist who taught Harris, marveled at how arborists who have spent their lives doctoring trees enthusiastically took to damaging them. “It gained momentum so fast — faster than I’m comfortable with,” she told me on a video call. Many of them consider it a low-cost and “creative” way to do something to benefit local wildlife. 

Bengtsson first veteranized trees at an experimental plantation near London’s Stansted Airport in the late 1990s; now, she leads research on it across Europe. The first major European study on veteranization, which began in 2012 and is focused on around 1,000 trees at 20 sites across Norway, Sweden and England, has so far found encouraging results, including that veteranizing trees very rarely kills them. Remnants of eggshells, feathers and nest materials were found in a third of artificial woodpecker holes and two-thirds of nestboxes. Researchers have seen indications that microhabitats are emerging more quickly than would have occurred naturally and seen bats, tree ants, bees and wasps — all positive signs that veteranization features are effectively mimicking naturally formed features. 

The work in Sherwood is specifically focused on some of the forest’s most overlooked wildlife. Thanks to its unique stock of ancient trees, the forest is one of the best places in Europe to find deadwood-loving invertebrates like the red-robed cardinal click beetle and wood-boring anobiid beetles, whose backs look like they are carved out of wood. Known as “saproxylic invertebrates,” these species recycle many of the forest’s resources, provide an important food source for birds and mammals and carry tiny organisms around the forest that are important for its overall health, like mites and bacteria. 

As old trees have vanished, saproxylic communities have vanished from much of Europe and are considered in danger of imminent collapse in areas where only a few ancient trees remain. Around a fifth of Europe’s saproxylic species are now threatened with extinction. Crucially, some of the rarest beetles, such as those that live their lives in the core of oaks, struggle to move around in the outside world, so once the continuity of ancients in a forest has been broken, its saproxylic biodiversity might never return. 

In Sherwood, an initial survey by entomologist Adrian Dutton found half of the forest’s 350 species of saproxylics around trees that had been veteranized. A good sign, but it’s too early to say for sure that veteranization is helping, Dutton cautioned. “Something’s happening,” he explained on the phone. “But would I bet my house on it?” he laughed. “Hm … possibly.”

Accelerating Age

Three hours south of Sherwood is Windsor Great Park, another former royal hunting ground turned park that’s become a forest ecology test site. One day in the summer last year, mycologist Matthew Wainhouse drove a flame-sterilized drill bit into the bark of an oak in search of what he referred to as a “needle in a haystack”: fungus that had made it into the core of the tree and begun breaking down the oak’s skeletal heartwood.

Trees spend their lives under constant attack, with fungi, insects and other animals attempting at every minute of the day to invade their leaves, bark and interior. At Windsor, Wainhouse and a team of mycologists aim to harness fungi to fast-track the creation of hollows, a holy grail for veteranization. Such damp little caverns within old trees are living miniature worlds, where saproxylic invertebrates find food, shelter, mates and more. Over time, “the cavity naturally expands so it is used by a succession of species with increasing body size, all the way up to bears in places like Canada,” Wainhouse told me. Nesting birds leave behind detritus, waste and microbes — “this really gungy wood-mold stuff that many insects like,” as Bengtsson put it. Thermally insulated by living wood, hollows also maintain a steady temperature for birds and other animals looking to incubate eggs or survive chill winters.

That all starts with a fungal infection, which turns impenetrable, inedible wood into an exploitable resource with benefits that cascade to “all these other communities downstream,” Wainhouse said. Wainhouse’s approach to veteranization, which he developed with fungal pioneer Lynne Boddy, bypasses a tree’s boundary defenses and directly introduces its enemies into its vulnerable core: A square of wood is cut out from the trunk and a new chunk — one that has been sitting for months in a box of sawdust impregnated with fungus — is inserted in its place. 

In Windsor, these blocks of wood have been inoculated with four different types of specialist heart-rot fungi gathered from oaks on-site, which Wainhouse called the forest’s “least-recognized keystone species.” Even though the fungus was directly introduced into the tree, the researchers expected it to be many years before hard oak heartwood broke down. But when Wainhouse first came to test for progress in 2021, only three years after the first inoculated blocks had been inserted, he found signs of incipient decay that may evolve into hollows and heart rot.

Although oak woodlands are among many people’s most familiar environments, efforts to return them to health often run into unknown territory at the frontier of fields like mycology and entomology. As Emma Gilmartin, who trained as a fungal ecologist and now works with arbs at the Arboricultural Association, explained, restoration often forces scientists and practitioners to work in new ways that cut across fields — to develop a common language between specialists who often work in silos in order for them to understand larger systems.

Advanced technology is helping that effort. In recent years, digital sensors have become commonplace in some woodland environments, turning them into bleeping hubs of arboreal data or smart forests. (Some trees even used to tweet out their daily movements.) Since 2023, for example, researchers from University College London have used terrestrial LIDAR (laser imaging, detection and ranging) technology to scan the Major and 40 other oaks, allowing them to assess the trees’ volume and calculate their carbon content. The resulting scans digitally reproduced the trees for use in science education and as a resource bank for arborists who care for ancients.

Digital scans promise to begin revealing the cryptic language of trees’ structure, shape and developmental history into usable insights, while artificial intelligence can assess how various creatures use old oaks, Gilmartin said. “We might be able to automate analysis of which trees are more valuable, which habitats are more common, which hollows develop faster or slower. So we can use it to assess the quality of woodland as a whole or individual trees,” she said.

On the other side of the planet, one group of researchers is already putting this kind of approach into practice. The grassy woodlands of the Molonglo region of Canberra, Australia, are visited by numerous transitory bird species, many of which exclusively perch in the canopies of mature eucalyptus trees. Such forests have dwindled to around 5% of their historic extent. Today they are fragmented and damaged, with large old trees increasingly hard to find.

Over the past few years, architect Stanislav Roudavski and colleagues at the University of Melbourne have been experimenting with “more-than-human” environmental design like artificial or “prosthetic” structures that can expand a young forest’s functionality, including hollows custom-designed for owls. 

In partnership with Canberra’s parks service and the Australian National University, Roudavski’s group is using AI, LIDAR and long-term observations of birds to work out the hard-to-comprehend features that attract birds to certain eucalyptus trees. They’ve found, for example, that most birds prefer small horizontal branches for perching and nesting. He and his team developed a statistical model that allowed them to predict bird behavior and then used generative AI to produce designs for “artificial tree crowns” — lightweight tensile structures of cables and rods — that, when built, will maximize the perching space available to birds.

Artificial 1,000-Year-Old Trees

In 1970, the poet W. S. Merwin published a four-page instructional manual for putting a tree that had been felled back together. The first step is to stick each leaf back to its twig, he wrote, then attach the bark and branches that had come loose. “Unchopping a Tree” is a fable written in meticulous, matter-of-fact steps: 

In a tense final act, the reader is commanded to remove the supporting scaffold of “chains and struts” that prop up the rebuilt tree until it is left standing, at last, on its own: 

I had this image in mind as I walked through Sherwood, studying the Major, propped up on metal sticks, and the various trees bearing band-aids from preservation or toolmarks from veteranization. It’s hard not to feel that conservation and restoration often turn wilderness into artifacts like Merwin’s tree — no longer their wild selves, but artificially remade. As we protect what’s left of nature, we seem to end up paradoxically making its vestiges more designed, covered in human fingerprints and nested in systems of artificial interventions and technologies proliferating through organic fabric. During my research on veteranization, I read about researchers attempting to restore a temperate rainforest in Scotland by painstakingly sticking moss and lichen back onto old trees. And Wainhouse told me about a Douglas fir forest in Oregon where fungi were administered somewhat less delicately with a 12-gauge shotgun. 

But I was reminded by Harris that artificial interventions are just the latest chapter in the age-old human management of trees and forests. There’s much more to Sherwood’s story than the ancient royal decree that the commoners should be kept out. For generation upon generation, people have been active among the trees of the forest, harvesting branches for firewood, grazing livestock and drilling into trunks to assess timber. Even before the Norman Conquest in 1066 (around the time the Major germinated), virtually all woodland in England had already been highly modified by human action. 

Many visitors today arrive at Sherwood expecting a pristine forest densely packed with unmolested trees, but that’s not what they find. It’s “a rich mosaic of closed-canopy oak-birch woodlands, interspersed with huge areas of lowland heathland and acid grassland,” as Louise Hackett, a trustee of the Sherwood Forest Trust Charity, told me when we met under the crown of the Major. Indeed, it is the open fields cleared for farming that have allowed giants like the Major to grow so broad and old without being crowded out by competitors.

Across the planet, forests are hybrid places: Not perfectly protected away from human impact or wholly manufactured piece by piece, but intertwined to one degree or another in our lives. Once-heretical practices like controlled burning are making a comeback — indeed, being understood as essential to forest health. So long as they can prove their value to ecosystems, emerging practices like Harris’s veteranization techniques and Roudavski’s abstract structures will follow this same path. 

Those who know the Major say it teaches a lesson in humility: Human management of something as complex as an old tree, let alone a whole forest, is a chastening ordeal. The oak’s own ingenuity constantly surprises arborists and forest biologists. With our current technology at least, humans could never hope to build an artificial 1,000-year-old tree. “All we can do is provide something in a younger tree that might simulate some of those properties,” Wainhouse told me. “Just to give enough time and space for some of these things to have a future.”

Merwin came to a similar conclusion. For 50 years after he published “Unchopping a Tree,” he labored to reforest three acres of agricultural scrubland on the Hawaiian island of Maui. He carted seaweed from coastal coves to spread on the barren land, heaved wheelbarrows of manure from neighbors’ cows and goats and planted some 14,000 native and imported palms. Many of them died, and the resulting collection of trees was not much like a Hawaiian forest. “Only a forest knows how to grow a forest,” he eventually concluded. 

Amid unfolding and intertwined ecological crises, we are chasing eons we do not have. “The trouble with trees is that everything takes a long time,” Bengtsson told me. Several people I interviewed for this story mentioned their own deaths as moments in the overall arc of the work they were doing, signifying that it will be decades before success (or failure) has become clear. What alternative is there but to keep going? “There’s hope,” Harris said as he looked into the cavity in the unremarkable young oak, observing the woodlice and a teaspoon or so of tree soil. “I think that’s what I take from this: There is actual hope.”