Black poplars, once common in our landscape, are now a mere few thousand. Tree enthusiasts in Britain are finding creative ways to boost the genetic resilience of their trees and replant large numbers of ‘ballerina Poplars’
Jamie Simpson was in the late 20s when his interest in black poplars began to grow. In 2008, he says, London authorities threatened to cut down several black poplars that were growing along the Thames towpath at Barnes, where the river curves into a dramatic curve.
Simpson, an arborist who had been familiar with the local population, believed that it was unmatched in the UK where black poplars are usually found as lone sentinels on remote fields. A few dozen trees were growing in an unnatural way along a concrete section of the Thames, much to the dismay of the Port of London Authority. They were also destroying the revetment, which protects against erosion. Simpson rallied opposition and successfully lobbied to keep the trees.
He was energized by his victory and sent some cuttings immediately to the Forestry Commission which had just begun offering genetic testing of trees. Simpson recalls that “all three of them were unique”.
Barnes black poplars are the most genetically varied population of Populus Nigra Betulifolia (a subspecies native in Britain). It’s also Britain’s rarest hardwood, with only 7,000 mature trees in Britain.
Native black poplars were once widespread across the country. They were known as ‘ballerina trees’ because of their acrobatically-angled limbs. John Constable’s quintessential English scene, The Hay Wain, features several of them. Its wood was prized for its durability and resistance to fire. It was used in everything, from wagons to scaffolding. Many of its waterlogged habitats were drained, and few native species were planted, after a faster growing hybrid became popular during the 19th century. When landowners planted native species, they preferred males without the cottony seed fluff produced in female flowers. Wild specimens are therefore rarely able to reproduce themselves.
“They’ve been through a tough time,” says Chris Jenkins. He is the nursery manager at the Royal Botanic Garden, Kew in Wakehurst. The Royal Botanic Garden, Kew has maintained a black-poplar stool bed, a collection coppiced trunks that produce new sprouts every year, since the 1990s. The program has produced thousands for planting in Sussex. Similar efforts have been made across the country to save the species.
This is a good way to preserve a local population of black poplars. It also means that the same clones are now spread everywhere, as cuttings create exact copies of genetic material.
In 2018, a team at Forest Research, an agency of the Forestry Commission, published findings of DNA analyses of 811 native black poplars sent in by landowners for more than a decade. They identified 87 genotypes or genetically distinct clones. The most common clone, also known as ‘Manchester Poplar’, was found in a fifth sample. Joan Cottrell is the head of forest genetics at the agency. She says that some clones were found everywhere.
This can be a problem, as it makes the species more susceptible to disease and other threats. In order to make them more resilient, it would be ideal if new trees were grown from seeds, and not cuttings. Seeds can be difficult to find, however, because cross-pollination between black poplars is rare in nature. Kew’s Millennium Seed Bank, a biodiversity backup facility, has only four collections of black-poplar seeds. Two of the seeds are from a single tree in Surrey.
Ted Chapman coordinates Kew’s domestic conservation partnerships. He says, “Black poplar seeds are like hen’s teeth.” “We’re still searching for them.”
Pollinating using paintbrushes
This conundrum has sparked a passionate drive to provide what the natural world cannot. Zeke Marshall, Forest Research scientist, started artificially reproducing the black poplars during his spare time, after finding old research papers about controlled pollination. He placed cuttings from a male tree in Durham and a female in Darlington in vases filled with distilled water in front of his south-facing home office window earlier this year.
He used a small brush to transfer male pollen from the branch to the female flowers. He harvested 205 seeds, and sent the majority to Kew where Jenkins, the nursery manager, has grown 30 seedlings, each of which is a genetically unique mix of its parents.
Jenkins says that “we can propagate black poplars quite easily vegetatively.” “But the seeds are what’s most important for the species.”
It’s like hens teeth. We’re on the hunt for them
The pair will continue to experiment with artificial pollination methods. They’d like graft cuttings onto rootstock next season, hoping that it could support more vigorous flowering.
Marshall says that “ideally, we’d be able to get some funding for the establishment of a seed orchard.” “That’s exactly what we need to do.”
Cottrell says that the work of Marshall and Jenkins could be vital. This would allow for a wider range of specimens to be available for natural selection. This is especially important in a time of climate change and increased threats from pests and disease. “Species cannot adapt if you don’t have diversity,” she says.
Simpson, who grew up in Barnes, knows first-hand how fragile Britain’s Black Poplars are and how important preservation can be. Years ago, he set up a nursery for hundreds of saplings. This proved to be a wise decision: a fifth of veteran poplars on the towpath are now gone due to storms and age. He says that many local genotypes can be represented by just one tree, which makes them vulnerable.
He can be assured that the rarest of the individuals will not die out easily, as they are now all backed up in the nursery and in replica populations as distant as Devon and the Lake District.
He says that if they don’t, “they will just fall down.” Once they’re gone they’re never coming back.
Image: Richard Allenby-Pratt