Are there any old elm trees left? You may be forgiven for thinking there aren’t, as since the 1960s, more than 20 million mature elms have died in the UK from Dutch elm disease.
However, younger trees have survived, and so have some mature trees. But how long have they got, and how many are there?
You can help scientists find out more about elms in the UK by taking part in the Natural History Museum’s urban tree survey.
And the Conservation Foundation’s Ulmus Londinium project, which launched this week at the Museum’s Angela Marmont Centre for UK Biodiversity (AMC), is asking people to record elms they spot in London. The Museum is working with them to include this data on the urban tree survey’s interactive map.
Elm, Ulmus glabra, leaf. Where the leaf joins the stem is not symmetrical and this is a good indicator that the plant is an elm.
To identify an elm at the moment, look out for small clusters of reddish and purplish flowers (shown in the image at the top). Later come the leaves. The base of the leaf where it joins the stem isn’t symmetrical (shown in the image on the right). In a few weeks, the pale green fruit will appear.
You can get more help identifying elms from the survey’s interactive tree identification key and elm factsheet, the Museum’s online ID forum, and by dropping in at the AMC, bring along elm leaves and receive expert help identifying the elm species.
Rather than referring to where the disease came from, the ‘Dutch’ in its name relates to important early research that was carried out in the Netherlands. In fact, Dutch elm disease was accidentally introduced from the USA in the late 1960s, although it isn't native to the USA and its origins are actually unknown.
‘It isn't easy to know whether an elm has been infected,’ says Rumsey, ‘ until you see the classic signs of die-back - the topmost shoots withering, the leaves yellowing and falling long before they naturally would, and the plant obviously not looking healthy. The time between this happening and the plant dying often isn't particularly long.’
Dutch elm disease is caused by a fungus of the genus Ophiostoma. It blocks the water transport, or vascular, system causing the branches to wilt and die. The most recent outbreak was caused by a new highly virulent species, Ophiostoma nova-ulmi.
The fungus is spread by the elm bark beetle, Scolytus scolytus, and other beetles in the genus Scolytus. The beetles tend to breed on mature elms, which is why they were the ones lost.
Many of the mature elms that died produced suckers from the remains of the roots and these plants have survived, as Rumsey explains. ‘These elms have survived as small plants in hedgerows and thickets.
'However, once they get to a certain size with bigger stems and deeper bark, after about 15 years or so, they are once again attractive to the beetles, which burrow into them carrying the fungus that are responsible for the trees dying.’
Some trees seem to be resistant to Dutch elm disease and have survived the last 60 years unscathed. There may be hope in finding a disease-resistant species or a cause of this resistance.
The Conservation Foundation’s Great British Elm Experiment is using saplings grown from cuttings (propagated) from mature elm survivors and getting schools and other organisations to plant them around the country. They’ll then map and monitor them over the coming years.
Elms belong to the genus Ulmus. There are probably 4 species native to Britain, although this number is up for debate. ‘Not only is the taxonomy contentious but also the native status is often in dispute too,’ says Rumsey.
Upper side of male white-spotted pinion moth, Cosmia diffinis. Elms are the only known home of this moth.
This is because, instead of reproducing naturally (sexually), most of the trees we see have been propagated by humans. The selection process may give the impression there are different tree species.
Elms are important habitats for all sorts of insects, fungi, lichen and mosses and are also the only known home to a species of moth, the white-spotted pinion moth, Cosmia diffinis.
The elm is the Museum’s first Tree of the month. It will feature a range of trees that the Museum would particularly like the public to survey this year: from ones that capture the essence of a season, such as cherries in spring; to those, like elms, that are at particular risk from disease; to non-native species that may cause issues if they spread from gardens into the countryside. And it will highlight the key seasonal features to look out for.
Discover the Centre for UK Biodiversity. It offers a drop-in identification service, research facilities, and online nature resources. Watch a video and meet the team.