ROOTING FOR OUR TREES
Tens of thousands of our best-loved British trees are under attack from a growing army of devastating pests and diseases, but you can help them fight back
Join the fight to stop the epidemics blighting British woodlands
Do you have a favourite tree? One that dominates a view, punctuates the landscape on a daily walk or simply exists in childhood memory, along with scraped knees from climbing its branches or long summer picnics under its canopy? Mine is an oak tree, alone at the end of a long field, tall and imposing, a king among trees. That it could disappear – along with its equally distinguished cousins – seems unthinkable. And yet it is all too possible.
The threat to our trees is real and increasing, from climate change but also global trade practices, which, if unchecked, enable the fast-spreading of pest and disease. There are 27 notifiable suspects in the UK. Some will eventually kill a tree outright; others will weaken it to make way for another pest, pathogen or period of drought to finish it off. The impact is not just on the look of the landscape. It is also financial – larch and pine plantations have suffered devastation from disease – and environmental, with habitat loss for invertebrates, lichens, mosses and birds. A butterfly, the white-letter hairstreak, depends on the elm for its home. No elm, no butterfly.
Professor Nicola Spence, chief plant health officer at Defra (Department for Environment, Food and Rural Affairs), is on the frontline battling the pests and pathogens that threaten our landscape. Her role is to command the national response through policy and partnership. As she says, it’s all about vigilance: “Prevent. Check, check, check. And invest in science and research.” Charities, landscape architects, horticulturalists and landowners all have a part to play – as does every one of us.
All is certainly not lost: we can help to protect our trees. The Asian longhorn beetle, a threat to many broadleaved trees, has been eradicated after a six-year trapping and monitoring campaign. Strict biosecurity operates both for shipments of living trees and consignments of timber, while scientists are leading research programmes, mapping genomes and developing varieties of disease-resistant trees.
First, we must know what to look out for: the more we all monitor the situation, the more we can curb transmission. Spring 2021 will see the launch of a Tree Health Centre at The Yorkshire Arboretum, beside Castle Howard, to educate everyone, from homeowners with a few garden trees to highway-maintenance teams and railway workers. In the classroom, the laboratory and the arboretum, students will learn the hallmarks of a healthy tree, the symptoms of pest and disease and how to respond. “This is a critically needed public outreach programme,” says Dr John Grimshaw, director of The Yorkshire Arboretum. “Everyone has a stake in tree health. We need to promote knowledge and we need to change behaviour.”
The beleaguered ELM
Dutch elm disease, a fungus spread by a beetle, almost wiped out the mature elm in the 1970s. Millions of trees died. Some Scottish and north-western populations of wych elm survived, as did the roots of the old trees from which suckers sprang. The disease still poses a threat, as do other new enemies such as the elm zigzag sawfly. Its larvae chew through elm leaves, leaving a zigzag pattern, eventually destroying all foliage.
HOW TO SPOT IT
In early summer, the leaves of a tree with Dutch elm disease will turn yellow and wilt, then brown and fall. Twigs twist into shepherd’s crooks. In a younger tree, break one off and peel away the bark: dark purple-brown streaks indicate that the disease is present. To spot sawfly, look out for the adults (small and black with white legs) and the caterpillars (small and green).
HOW TO HELP
Prevention is key: keep young hedgerow elms trimmed – Dutch elm beetles are thought to be more attracted to larger trees. If planting new trees, choose ones that are Dutch elm resistant: Ulmus ‘New Horizon’ (hillier.co.uk) or Ulmus lutece (barcham.co.uk). If you find hints of disease or pests, log your findings through Treealert (forestresearch. gov.uk) or Treecheck (treecheck.net). Don’t take matters into your own hands by getting rid of the pests: if it’s a case of misidentification, it could be disastrous for a perfectly benign species.
“Everyone has a stake in tree health. We need to promote knowledge, and we need to change behaviour”
The not-so-mighty OAK
Acute oak decline affects our two native species of oak – sessile and pedunculate or English – and is caused by a mix of three bacteria helped by a native beetle with wood-boring larvae. Trees die within six years. As if that were not enough, there are other threats, too, including the oak processionary moth, which arrived in Greater London from Central Europe with imported trees 15 years ago. The adult moth is harmless, but its caterpillar will strip an oak tree of its leaves, seriously weakening it.
HOW TO SPOT IT
Acute oak decline causes black liquid to seep from the bark (below right). Underneath, the tree will be rotting and struggling to move water and nutrients through its trunk. As this can signify other tree problems, also look out for 4mm-wide, 3mm-high, D-shaped holes in the bark made by the emerging beetles. As the disease advances, the tree’s canopy begins to thin. Oak processionary moth, on the other hand, reveals itself in silky white bulbous nests in the branches and canopy, as well as arrow-shaped columns of caterpillars (above right) processing down trunks or travelling along the forest floor.
HOW TO HELP
As with the other pests, report any findings through Treealert or Treecheck. Avoid contact with the moths: the hairs are toxic to humans and animals. They can cause rashes, sore throats and breathing problems, which may need urgent medical attention.
The afflicted ASH
The Asian fungus that causes chalara ash dieback has been devastating to species in Europe, and is expected to wipe out 95 per cent of Britain’s trees. To stave off new threats such as the emerald ash borer, currently not present in the UK, ash imports are banned. Native to east Asia, this beetle has been hitchhiking west, decimating populations of ash as it travels with its tunnelling larvae.
HOW TO SPOT IT
Look out for the borer’s metallic-green adult form, up to 13.5mm long, and for horizontal fractures and D-shaped holes in bark. Chalara causes a blackening and wilting of leaves and shoots in summer, premature leaf fall and dark, elongated, diamondshaped lesions on the bark (visible year-round).
HOW TO HELP
Use Treealert or Treecheck for possible sightings of emerald ash borer. Chalara is notifiable only in areas where it is new (see chalaramap.fera.defra.gov.uk). Sadly, these are few and far between. Keep an eye on progress at the Living Ash Project (livingashproject. org.uk), which has planted 3,000 disease-tolerant saplings in Hampshire, using citizen scientists to help identify resilient strains.