An event organised by the Institute of Chartered Foresters explored the possibilities for the return of the elm tree to the UK’s urban and rural landscapes.
TAKING place at RBG Kew in June, the ICF’s ‘The Future of the Elm Tree’ conference began with remarks from director Richard Deverell, who said: “Once ubiquitous and iconic in our landscape, the sudden loss of the elm is a stark example of the unintended impact of globalisation and climate change and a reminder of the challenges faced to halt and reverse this.”
The next decade is critical for environmental action – what we do now impacts the next 1,000 years – and plant health is integral to developing and implementing solutions.
The elm, a valuable landscape tree growing in hedgerows and in woodlands, is an environmentally tolerant timber tree, hosting up to 80 species of invertebrate.
Since the 1970s, Dutch Elm Disease (DED), a vascular wilt caused by a fungal pathogen spread by two species of Scolytus bark beetles, has decimated elms. The few mature ‘survivors’ have natural resistance, as exhibited in collections at Kew and Wakehurst in Sussex.
READ MORE: Royal Forestry Society reveals Excellence in Forestry Award winners
Launched in May, the Centre for Forest Protection (collaborative virtual research network) aims to further the protection of forests, woods and trees against environmental, socio-economic and emerging threats. “As part of this, Kew’s senior research leader Professor Richard Buggs, and Dr Joan Webber OBE of Forest Research, will lead a new elm project, laying the foundations for elm restoration in Britain.”
A farmer and forester himself, DEFRA’s Minister for Biosecurity Lord Richard Benyon, said: “I chair monthly biosecurity meetings discussing what affects our trees now and what is coming our way. It is a stark story. We must be better at keeping these islands safe and seeing how we can restore tree species such as the elm.”
RBG Kew, the epitome of ‘soft power’ and respected the world over, “reminds us of the importance of plants and fungi, with UK woodlands removing around 18 million tonnes of CO2 equivalent annually, as well as supporting biodiversity, air filtration, water, recreation and health benefits.
“It is ironic that a tree associated with the underworld has been decimated by a deadly epidemic, DED killing an estimated 60–100 million trees across Britain.”
Young elms do establish in the countryside, but “few mature specimens remain. Given what they provide for biodiversity, you can see the impact this has had on our natural world.”
He believes there is hope. 2021’s England Trees Action Plan and Net Zero Strategy include protective measures, planting targets and establishing new woodlands across the UK, supported by £750 million from the Nature for Climate Fund.
For a resilient treescape, through the Tree Planting Innovation Fund and Tree Production Capital Grants DEFRA will support £11.6 million to 2025 suppliers, public and private sector nurseries and seed suppliers to increase the quantity and diversity of forest reproductive material and planting stock.
For safe sourcing, “a new ‘biosecurity procurement requirement’ has been added to the English Woodland Creation Scheme and tree restocking pilots, stipulating that plants and tree suppliers must meet biosecurity requirements set out in the just published Plant Health Management Standard.”
Urging nurseries and traders to sign up to the Plant Healthy certification scheme and tree buyers and planters to source stock from certified businesses, he wished the conference every success.
SESSION 1: SETTING THE SCENE
Forty years ago, host David Shreeve, director of the Conservation Foundation, planted out elm cultivar ‘Sappora Autumn Gold’ (Japanese). Some still survive.
Professor Juan A. Martin, from the Technical University of Madrid, said that since 2002 the Spanish breeding programme has concentrated on conserving the native U. minor’s genetic resource (700 genotypes so far), identifying DED-resistance mechanisms and obtaining resistant elms by breeding.
In 2014, the first clones resistant to DED exhibited 30 per cent crown wilt when inoculated with the pathogen. In field trials, cultivars ‘Ademuz’ and ‘Dehesa de Amaniel’ displayed few or no symptoms, and distinct (and differing) resistance mechanisms. Crossing ‘Ademuz’ x ‘Retiro’ resulted in 20 per cent crown wilt after the first inoculation.
Future research includes pre-screening 70 new U. minor genotypes for resistance, incorporating crown microbiome endophytes into plant breeding and creating a reference genome of U. minor to assist molecular breeding.
DED is so named because epidemic research began in Holland, explained Dr Joan Webber OBE, principal pathologist at Forest Research.
“DED, a pathogen fungus, produces fruiting bodies in elm bark. In Britain, beetles Scolytus scolytus (GB/Europe), Scolytus multistriatus (Southern Britain) and occasionally Scolytus labis (Scotland/northern Europe) transfer the disease to healthy trees.”
There have been two epidemics. “Ophiostoma ulmi in the 1920s declined unexpectedly.”
Since the mid-1950s, a second wave, Ophiostoma novo-ulmi, has swept across the northern hemisphere and shows no signs of abating.
“The DED cycle hinges on maturation feeding. The beetle, emerging in spring and early summer, makes pit stops to feed on twigs in elm crowns exposing the xylem (feeding wound). Fungal spores (carried or ingested) are introduced into xylem-conducting vessels. Over a couple of years, symptoms begin (canopy wilt) and the tree slowly dies, becoming suitable for breeding and feeding in. In autumn and winter, females burrow in thicker bark and excavate longitudinal galleries. The eggs hatch and larvae feed on fresh bark. If the pathogen overtakes the larvae and kills the bark, feeding stops. Less than five per cent of feeding grooves result in infection.”
Beetles prefer feeding on U. minor ‘Atinia’ than U. glabra, emerging and dispersing in temperatures of 20 degrees C or more. Climate change, causing higher temperatures earlier in the year, could be why “the disease is increasing in Scotland”.
There are 100–300 million elms in the UK, clones regenerating from roots, even after the main stem dies. “Few survive to maturity. Most reach 20 or 30 years old when they are stuck down, having become large enough (30–45 cm stem diameter) to sustain breeding beetle populations.” Survivors displaying field resistance are unpalatable to the beetle for reasons unknown.
For more elms to survive, an integrated approach to sanitation felling and knowledge of vector behaviour is needed. “Resistant elms from breeding programmes are available. Are they affordable?” Not yet.
Dr Andrew Hirons, senior arboriculture lecturer at Myerscough College, said Preston’s avenue of ‘New Horizon’ exhibit good establishment rates at years 5 and 10. In Sheffield, the tight, upright forms of ‘Columella’ complement housing development landscaping. Along the River Severn, elms planted riverside displayed excellent tolerance following weeks underwater in winter floods.
The commercial sector offers a range of resistant cultivars, Hillier Nursery offering five, Moreton Arboretum (with Chicagoland Grows Inc) offering five (Asian U. davidiana parent) also resistant to elm yellows (or look on www.resistantelms.co.uk).
SESSION 2: DEVELOPING DISEASE RESISTANCE
In ‘What the life cycle and epidemiology of DED tell us about future options’, Professor Dr Clive Brasier, Emeritus Mycologist, Forest Research, said O. novo-omni is hybridizing in Europe, with resistant white elms (U. laevis) in Holland and the UK succumbing to DED.
“Pathogen attenuation is unlikely.”
That more elms have not succumbed is because there are few mature elms: young (recruitment) elms in the landscape means that native gene pools remain. Buying an elm tree is expensive. Planting resistant varieties (European most suitable) needs testing in differing UK conditions, monitoring changes in pathogen and vector populations. The return of Scolytus scolytus has possibly helped the pathogen’s increasing spread (in Scotland).
European research concentrates on xylem resistance mechanisms. For the UK, research at the feeding wound stage, considering beetle feeding preferences, investigating a possible separate resistance mechanism in the phloem (as compared to the xylem) and the role fungal (RNA) viruses play in preventing pathogen infection, are areas ripe for investigation.
In the 1990s, genetic modification research helped to develop an anti-fungal protein and the creation of a resistant U. minor ‘Atinia’. Field trials never happened due to public outcry against GM.
Dr Joukje Buiteveld from Wageningen Environmental Research and the Centre for Genetic Resources said the Netherlands’ reintroduction of elms to urban, coastal and rural landscapes is the result of breeding programmes, field adaptability trials, sharing information and municipal cooperation.
Between 2009 and 2018, 18 cultivars were planted in demonstration plots on an artificial island in east Amsterdam, their characteristics printed in factsheets and distributed to urban foresters for informed decision-making: ‘Columella’, good for planting in wet, dry and poor soils, pavements, narrow and wide streets, parks. Properties include good wind resistance.
“We need the elm, it is a climate-proof tree.” Breeding resistant clones took 40 years.
“This timescale is not feasible any more.”
Dr Allison Oakes, research scientist, SUNY College of Environmental Science and Forestry, announced that sequencing the American elm (U. americana) genome should be complete in around six months. The information will be freely available.
As well as DED, the American elm suffers elm yellows, a bacterial infection of the xylem (vascular disease) transmitted in leafhopper and spittlebug spittle during feeding. Infection spreads to the roots.
With no cure, a tree dies within one–three years.
Once DNA is sequenced, research will determine which genes are expressed differently between species and between infected trees and trees displaying resistance. “We can look for novel mutations and loss of function. Has there been a promoter alteration – a gene that did work and now does not – so we need only edit existing genomic information rather than add new genes. GM is just one tool in the box.”
SESSION 3: PRACTICAL EXPERIENCE
Head gardener at Roughlow Farm in Cheshire and Young Horticulturalist of the Year 2021 Tim Stafford said seedlings propagated from U. leavis (Asian) seeds should grow within a month. “It can survive infection, so offspring will be resistant.” For back garden use, vegetative propagation of elms from shoots, softwood cuttings (from hardwood shoots) and root cuttings is viable.
He suggested that to reproduce vegetative clones, grafting, possibly using an apical wedge graft (taking softwood cuttings in the same season, as done at the University of Minnesota), is probably the way forward.
Andrew Brookes of Butterfly Conservation discussed ‘the potential of disease-resistant cultivars in the conservation of the White-letter Hairstreak’. Between 1984 and 2009, Hairstreak numbers declined by 98 per cent. They are now on the IUCN Red List of Threatened Species.
Hairstreak eggs, laid in July, hatch in early March, the larvae feeding on flowers and samara (immature fruits). The butterflies live in elm canopies feeding on honeydew. “The elm suckers populating UK hedgerows are of little use.”
The UK has no propagation programme for the “tallest, fastest growing broadleaf in Europe”.
Of European DED-resistant cultivars trialled in the UK, 5,000 French ‘Lutece’ planted on the Isle of Wight were found to host the butterfly. Frank P Matthews (nursery) acquired the British propagation rights for ‘Lutece’ and ‘Varda’, offering the first resistant cultivars to be raised in the UK as bare-root whips. They have the capacity to propagate (only) 10,000 trees a year.
‘New Horizon’, propagated in Germany, is sold as a root ball or container tree, prices starting at £360. However, 36 planted in London’s Vauxhall Pleasure Gardens were colonised in 2017.
UK trials (planted eight years ago) of the Spanish DED-resistant U. minor ‘Ademuz’ (derived solely from European field elm) have yet to reach maturity or host butterfly populations (as in Europe). Andrew hopes a UK nursery will acquire propagation rights. “Spanish imports are prohibited due to Xylella.”
Dr Leila Pinchot, research ecologist, US Forest Service Northern Research Station, Ohio, spoke about American elm resistance breeding and restoration.
From the 1930s, rural and urban elm populations succumbed to two waves of DED. As wetlands are threatened by invasive non-native pests, pathogens and plants, grazing and changing land-use, a quick-growing, shade-tolerant, riparian-zone-loving DED-resistant American elm would be ideal to plant, as part of a larger suite of species, in ecosystems decimated by the emerald ash borer.
Producing a genetically diverse, locally-adapted DED-tolerant tree is a collaborative effort. Native survivor elms (over 100 cm in diameter) in rural areas showing some tolerance have been identified. Information on new ‘survivors’ is collated on a central database.
SESSION 4: PANEL DISCUSSION
The panel: Professor Richard Buggs, Kew’s senior research leader plant health; land manager Matthew Bennison MICFor, Strutt & Parker (sponsor); Professor Nicola Spence, chief plant health officer, DEFRA (sponsor); hobbyist Dr Alec Gunner.
Q. Estate woodlands have seen the gradual erosion of disease resistance on many species available to plant. Will technology ensure we can find resistant clones of all the productive species we need to plant to satisfy timber needs as well as all others?
A. RB: We live in a globalised world, stirring up pests and pathogens that locally have co-evolved to live with native species. Breaking those relationships introduces microbes into naive populations. In the long term, if we do nothing, we lose all trees for planting. Mix up the genetics, globally there is resistance. Technology (GM) could help, depending on whether we embrace it or not.
A. NS: We need a genetic map for elm, investing in science, genetic resistance and novel solutions, possibly technology (eg. parasitoids) and strategies to cover the impact of the pest. GM is part of this. We must have resilience in mixed woodlands with species that can adapt to future climates and P&D.
Q. What are the panel’s thoughts about elm as a timber?
A. MB: Landowners want an end product in addition to biodiversity. It is a good building material with dense grain. Homegrown timber limits our exposure to the outside world.
A. AG: We don’t know the timber colour of new resistant strains, their heartwood is not yet of sufficient size to work with.
A. RB: Colour can be predicted genomically as with disease resistance.
A. AG: For resilience and also for good form, there are less than 20 options. Limited variety (U. minor ‘Atinia’) caused their quick demise. Survivor populations tend to be diverse, diversity offers levels of protection against how pathogens develop.
Closing the session and the day, Sir Harry Studholme thanked the speakers and the audience, online and live at Kew.
Comments: Our rules
We want our comments to be a lively and valuable part of our community - a place where readers can debate and engage with the most important local issues. The ability to comment on our stories is a privilege, not a right, however, and that privilege may be withdrawn if it is abused or misused.
Please report any comments that break our rules.
Read the rules here