London’s most emblematic tree is under threat. Is biodiversity the answer?

One rainy morning this April, the residents of Senlac Road in Lewisham, south London, gathered for a funeral. The ceremony was not for a person but for a London plane tree that had been growing happily in the pavement for 70 years. It was the only mature tree on the road and, for some in the neighbourhood, took on a totemic role. Kids met to play under it. In summer, community meetings were convened in its shade. Then an insurance company decided the tree’s roots were undermining the foundations of a nearby house. Despite the locals’ best efforts, which included a petition with almost 50,000 signatures and a rota of people to park their cars under the canopy to hamper the chainsaws, the council cut it down. The residents felt its loss keenly. “We are in mourning for the tree,” said Andrew Edkins, whose driveway was overhung by its branches.

Their grief is testament to the outsized role that planes have played in shaping the cityscape. According to the London i-Tree Eco Project, an arboreal survey of the capital, there are about 8.5mn trees in the city. Only 1.36 per cent of them are planes. Plum, cherry, oak and ash all grow more profusely. But planes occupy all the best real estate, casting their dappled shade over Berkeley Square and Park Lane, Westminster Abbey and Aldwych. They are the tree of choice for public spaces, where they help to conjure a sense of occasion and theatre. Planes add a ceremonial dash to the Mall, where they are lined up as neatly as soldiers on parade. On the Strand they encircle the church of St Clement Danes, marking a boundary between the bustle of the street and the stillness of a sacred space.

Their vaunted position in the middle of town carries over to residential districts, where their characteristic mottled trunk and broad-leafed canopy have long been markers of aspiration, lining South Kensington’s Queen’s Gate, Islington’s Highbury Crescent and Camberwell Grove. In his 1961 book Victorian Suburb, Harold Dyos wrote that “the choice of trees, too, had its social overtone”: planes for the well-to-do, limes for middle incomes, bare tarmac for the working class.

But the London plane tree faces a problem. According to climate models from the Met Office, if global emissions continue to rise, the planet’s mean temperature could increase by as much as 4C by 2100. “This will push the London plane to the edge of its range,” says Kevin Martin, head of tree collections at the Royal Botanic Gardens at Kew. In this scenario, London would see temperatures of over 40C every three to four years.

The consequences for the city’s treescape would be dire. The heatwave of 2022 killed hundreds of planes, Martin says, and the rise in mortality will last for years. Extreme heat destroys trees’ vascular systems, he explains, which means they can’t pull water and nutrients from the ground. Although they can rely on energy stored in their roots for a while, if the damage is severe they will eventually go into decline: “Some trees that came into leaf this year just won’t have the energy to carry on and will die.”

If climate change wasn’t enough, London planes are facing a pandemic, too. In France, a disease called canker stain has killed tens of thousands of planes, denuding the once-shady banks of the Canal du Midi. According to Martin, it’s only a matter of time before the infection arrives in Britain, imported through the plant trade. “It’s when, not if,” he says. “In some parts of London, we will lose all the plane trees.” With them will go the environmental benefits they confer. A 2021 study by researchers at ETH Zürich, Switzerland’s top university, found that urban trees reduce land-surface temperatures in cities by up to 12C. “Without the trees,” Martin says, “liveability would change very quickly.”

If this happens, the grief felt by the residents of Senlac Road will be widely shared. At the funeral, they gathered around the plane tree’s stump, before laying a wreath of cuttings from their gardens. Then one of the congregation read E Nesbit’s poem “Child’s Song in Spring”:

The chestnut’s proud and the lilac’s pretty,
The poplar’s gentle and tall,
But the plane tree’s kind to the poor dull city –
I love him best of all.

“Now this is a whopper,” says Todd Longstaffe-Gowan, a landscape architect and historian, as he admires his favourite London plane. It stands in the large oval garden in the middle of Bedford Square in Bloomsbury. The garden contains several planes, which tower over hollies, dogwoods and viburnums. But this one is easily the biggest. Its trunk is gnarled and bulbous, so thick that the wrought-iron railings that enclose the garden have been cut away to accommodate its growth. “When they planted this tree,” Longstaffe-Gowan says, “I don’t think they ever thought it could get this large.”

He has brought me here because, as he explains in his book The London Square, it was in places like this that the city’s first-ever trees were planted. The square was laid out between 1776 and 1780, during the reign of George III, on an estate belonging to the Duke of Bedford. He developed the site to increase the value of the land, and trees were part of the sales pitch to potential property buyers. This was a period of high romance in landscape design, the era of Capability Brown and his successor Humphry Repton, who created rolling, tree-studded parkland for country estates. When the rich bought property in town, they wanted the same thing in miniature. “They expected this rural idyll in the middle of the city,” Longstaffe-Gowan says.

A 1911 map of Bedford Square in Bloomsbury, home to several historic London planes © ‘Bedford Square: an Architectural Study’ (1990) by Andrew Byrne (The Anthlone Press)

At that point, the London plane was yet to acquire its name or status within the city. The species is a hybrid of the American sycamore and the Oriental plane, whose native territory is south-eastern Europe. Nobody knows for certain how their offspring tree arrived in the UK, but it was first described here by a botanist in Oxford in 1666. Back then it was known simply as the “hybrid plane” and to the garden designers of Georgian London it was nothing special. It was one of dozens of species they planted in the city’s new squares as they attempted to emulate the variety of natural woodland.

But the planes quickly outgrew everything else. Although botanists didn’t yet understand their biology, they were peculiarly well adapted for city life. Planes shed their bark, enabling them to expel pollutants. They are one of the few species to photosynthesise through their trunks as well as their leaves, making it easier for them to produce the sugars they need to grow, even in poor conditions. So while other trees choked and withered in the soot-filled air of late-Georgian London, the planes flourished. Not everyone appreciated their success. By the 1820s, London’s planes were so big that homeowners began to complain that they were obstructing the view from their living-room windows.

Their scale and resilience soon became a boon rather than a bugbear. As London industrialised, its population grew fast, as did the quantity of coal smoke in the air. Faced with the prospect of mass asphyxiation, in the mid-19th century the government decided to clean up the city.

Planes became one of their most potent tools. In 1868, the civil engineer Joseph Bazalgette installed 250 plane trees on Victoria Embankment along the Thames. He had reclaimed the land from the river in order to house part of his revolutionary new sewer system, which comprised more than 1,000 miles of subterranean pipes. Just as Bazalgette’s sewer dealt with the filth underground, his planes cleansed the air above, giving wheezing pedestrians respite from the smog. Towering over the street, their thick summer canopy filtered noxious fumes and offered shelter and shade.

Bazalgette’s planting scheme wasn’t original: he copied the idea of planting street trees from the boulevards designed by the French and the Germans. But it proved so successful that it spread across the city. “He started a craze,” says Paul Wood, the author of London is a Forest. Soon, avenues of planes were sprouting all over the place, from Chiswick in the west to Mile End in the east. In the 18th century, urban trees had been aristocratic baubles designed to beautify London’s poshest enclaves. Now they were democratic amenities, planted to improve life for everybody. They were so ubiquitous that by the turn of the century the hybrid plane was renamed, and the London plane was born. ce44 4ddd a8a0 1e5c75b436ba

© Gem Toes-Crichton 295f 4d48 a7ad eb46d6cafffd

London plane tree leaves on a lightbox © Gem Toes-Crichton

Their utility has since been quantified. As well as acting as natural thermostats, trees trap thousands of tonnes of particulate pollution every year and suck in vast quantities of carbon to build their timber as they grow. As the largest trees in London, planes are especially efficient environmental regulators.

There’s a price put on the service they provide. In 2008, in a bid to protect urban trees from rapacious developers, the London Tree Officers Association devised a system to put a monetary value on street trees as public amenities. Their formula considers a tree’s size and location, and the benefits it offers to the local community. If a developer wants to cut down a tree, the court can order them to pay the price (a deterrent that has been successful). According to their measure, the most valuable tree in Britain is an 18th-century plane in the grounds of St Mary Magdalene Church in Islington, measuring over 25 metres tall. Thanks to its prime location and monumental scale, it is worth more than £1.5mn. Added together, London’s trees are valued at £43bn.

Martin, who runs the tree collections at Kew, is working to ensure their survival in the long term. One morning in July, I join him in his laboratory at the Royal Botanic Gardens. On his workbench is a bucket of leaves from a variety of species: the golden raintree from China, the silver lime from the Balkans and Turkey. All the varieties have one thing in common. They come from regions of the world with climates like the one London could have 100 years from now.

Although the timber industry has been gathering data on forestry trees for decades, the same isn’t true for nursery trees like those planted in cities. “There is a huge gap in knowledge,” Martin says. “Most of the guidance today about what to plant is based on opinion and observation.” The research he’s been doing is designed to change that. Over the past six years, Martin has been quantifying which species are best adapted to grow in London, both now and in the future, as temperatures go up. If our street trees can weather the increase, they will help regulate temperature overall and keep the city cooler.

The Corinthia Hotel

This morning he is testing the Persian ironwood, whose native territory stretches across northern Iran and Azerbaijan. First he takes a tiny sample from a leaf. Then he wraps it in silver foil and plunges it into a canister of liquid nitrogen, chilled to minus 200C. The experiment is testing the ironwood’s resistance to drought: the liquid nitrogen damages the leaf’s cell structure much as prolonged summer heat does.

Finally, Martin runs the leaf sample through a machine called an osmometer, which measures how much water it can retain despite the damage he has inflicted. The Persian ironwood is among the most drought-resistant trees in the world. According to Martin’s results, it can go weeks without water. In the desiccated London summer of 2022, it would have been just fine.

The data Martin collects will help to shape a new tree-planting strategy. Many of the species that he is testing are not yet commercially available, the Persian ironwood among them. “We can produce guidance on species selection, and advise nurseries on what trees to cultivate,” he says. They will pass their findings on to councils and tree officers in the relevant boroughs, too.

So does this mean that planes are about to be usurped? Quite the opposite. Martin’s research has turned up two big advantages for London’s most emblematic tree. First, it will help to increase the biodiversity of the city’s treescape. This in turn should protect the planes from canker stain, the disease that threatens to wipe them out. “We need diversity to create breaks in the canopy,” he says. “If you have this break, the spread of disease will be broken.”

Secondly it will help to preserve the magic that planes confer on London’s streets and squares. It turns out that, although the London plane will suffer as the climate heats up, one of its parents, the Oriental plane, is already perfectly adapted for any future change. The Oriental plane evolved in the Caucasus and central Asia, where it learnt to weather hot, dry summers. And for anyone who loves the appearance of London’s plane trees, the Oriental variety ticks another important box. It may be genetically different, but it looks exactly the same.

Simon Willis was formerly an editor at The Economist and Granta

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