The arrival of a new pest or disease into the UK does not necessarily have to mean the complete eradication of the affected tree population. A sensible and carefully considered approach to protection can work wonders, as the case of Brighton’s elm trees proves.
IF you want to take a step back in time to when the landscape was covered with millions of elm trees, then take a day trip to the seaside and visit Brighton and Hove in Sussex. I did just that in 2012 and was privileged to meet and speak with Rob Greenland, senior arboriculturalist on Brighton and Hove City Council. Rob was primarily responsible for the design, development and operation of the pest and disease management programme which stalled the march of elm bark beetles and their deadly Dutch elm disease cargo in the area. Losses of elm trees were cut to a bare minimum, while susceptible species of elm were being wiped out elsewhere. Thanks to Rob’s efforts, Brighton and Hove still holds the National Collection of Elm which is internationally recognised as one of the most important Ulmus gene-pool resources.
Half a century ago, 30 million mature elms, including English elms (Ulmus procera), Wheatley elms (Ulmus minor subsp. sarniensis) and smooth-leaved elms (Ulmus minor var. minor), were scythed down by Ophiostoma novo-ulmi, the most aggressive causal pathogen causing Dutch elm disease (DED).
O. novo-ulmi arrived in Britain around 1967 on rock elm (Ulmus thomasii) logs imported from North America and was quickly picked up, carried and transmitted by elm bark beetles (Scolytus spp). The resident and less aggressive DED pathogen was Ophiostoma ulmi, which had been gnawing away at elms on and off for centuries.
The European elm bark beetle (Scolytus multistriatus) and a larger, more aggressive beetle (Scolytus scolytus), homed in on England’s elms, attracted by plumes of a para-pheromone they released. Para-pheromones are volatile chemicals which mimic true insect pheromones, which are communication chemicals essential for normal insect behaviour including the finding of food sources and mating.
Wych elm (Ulmus glabra), Britain’s only true native elm, and a less preferred host for elm bark beetles, succumbed more slowly in contrast to English elm, a preferred host of elm bark beetles. As such, the damage caused was swift and sure. Not total though, because sizeable numbers of mature English and closely related elms survived in a protected pocket on the south coast of England. At the time of my visit in 2012 they numbered 19,000.
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The ‘Brighton Bowl’, shielded on three sides by the South Downs to the north and east and the English Channel to the south became the only safe and sustainable sanctuary for mature English and other closely related elm trees in England.
SOUND AND SUSTAINABLE SANITATION
Geography clearly played a crucial part in saving Brighton’s elms from the worst ravages of DED during the late ’60s and early ’70s. However, the reason why Brighton and Hove’s elms were safe by 2012 was the sanitation programme designed and developed by the council. The base-line strategy was to deal with the resident population of elm bark beetles. Once achieved, dealing with new incursions would become more manageable. Brighton’s policy was not a classic case of direct disease control, but focused on the distribution and movement of the insect vector.
EXPOSED WESTERN FLANK
The soft underbelly in Brighton’s defences against mobile elm bark beetles carrying O. novo-ulmi has always been its weak western flank. This narrow coastal plain, stretching out west through Shoreham and Lancing, in the neighbouring district authority of Adur, is where the council’s sanitation programme was focused.
Beetles fly into Brighton under their own steam up to a distance of 3–4 km but are more commonly blown in over greater distances by the prevailing wind and storms. To the east of Brighton, the South Downs stretch right down to the sea to form a natural tree- and beetle-free barrier not present in the west.
Rob Greenland, senior arboriculturalist and arboricultural manager in the council’s environment department in 2012, had worked for Brighton and Hove City Council ever since the DED epiphytotic (epidemic) kicked off in the late ’60s. He explained how frequent tree inspection and rapid remedial action, whether by pruning single affected branches or felling the tree, under-pinned the city council’s strategy and success. But this clearly would not have been enough, or else why would almost every other local authority lose virtually all their mature elms in the space of 10 years?
PAY-FOR-ALL POLICY
The importance of ensuring all private landowners and householders carry out required work on beetle-infested and DED-infected trees was recognised very early on.
“What would be the point of us spending thousands of pounds pruning or felling our elm trees only to find infected beetles were in the environment, and moving around from infected elm trees on private property?” said Rob. “That’s why we decided to adopt a ‘pay-for-all’ policy.”
This bold and imaginative step clearly worked. Even in the early ’70s, as the DED epidemic picked up speed, Brighton and Hove was only losing hundreds of elm trees per year while other authorities were counting elm tree losses in the thousands. But this strategy did not stop at Brighton and Hove.
Recognising the prevailing westerly wind-borne threat from beetles blown in from the neighbouring Adur District, where the long-term future of elms was already doomed, Brighton and Hove extended this policy to the private sector there.
Each and every year, Brighton and Hove gave Adur District Council £10,000 to pay for private-sector elm tree pruning and felling. In 2011 Adur District Council only used £1,500 of the pot, so the policy was clearly paying dividends and the money well spent.
This pre-emptive strike created a ‘cordon sanitaire’ (a man-made barrier) to the west of city limits, to complement natural barriers on the other three sides and thereby square the circle of protection around the city’s elms.
“In 2011 we lost only 70 trees,” said Rob. “And since we are planting new ones all the time the overall ‘elm biomass’ in Brighton and Hove is actually increasing.”
HANDS-ON SANITATION
Frequent inspection with fast remedial action meant trees were often caught in the very early stages of infection. This allowed the pruning out of the affected limb to save the tree.
“We only use this policy if infection is confined to 10 per cent or less of the crown, because experience shows that over and above this limit it is very difficult to contain infection within the tree,” said Rob. “And we don’t prune elms during the summer months because cut elm wood surfaces release the para-pheromone that attracts Scolytus.”
Inspection information would come from several sources – the environment department’s own staff who recorded suspicious sightings and symptoms during their work, information submitted by interested members of the public and that recorded by a part-time inspector who worked from early June until September.
The inspector was out and about every day and would quickly build up knowledge of hot-spots, especially on the vulnerable western side of the city. He had a number of vantage points on high tower blocks, from which he would survey the scene with binoculars to pin-point the characteristic colouring up and yellowing of foliage on infected trees.
“All the work is done in house,” said Rob, “for speed of response and to meet our target of just four days between diseased tree identification and subsequent treatment by pruning or later felling.”
All except the heaviest logs were chipped at each work site and the chips stored at Stanmer Park for use as mulch around the city. The largest pieces were stored at an EA-licensed elm-disposal site at Waterhall, an area of low elm density on the outskirts, until a sufficient amount of wood was ready for chipping using a large-scale industrial chipper. Woodchips were sent to a biomass-burning power station in Slough.
Elm trunks of sufficient quality were sent to sawmills, and the boards used mainly for making furniture. There was no problem with biosecurity because this took place outside of the ‘cordon sanitaire’ around Brighton and Hove.
ELM TREES IN BRIGHTON AND HOVE
It was now time for a snapshot of the city’s 19,000 elm trees, which proved to be a real treat despite the torrential rain on this mid-autumn day. As I recall, Rob had to attend a previous appointment but despite the weather I received a highly informative guided tour of the city’s elms by council arboriculturalist David Archer.
It was October 11, 2012, and as I recall I had not seen a mature English elm in England since September 1970. Being well into autumn, the first streaks of autumn yellow should have been showing on Brighton’s elms, but this was one of the most topsy-turvy growing seasons on record and the canopies were still uniformly green.
Our first visit was to the locally named ‘Preston Park Twins’, a pair of ancient English elms, probably the biggest in Europe. In spite of their hollow trunks, the trees sported exceptional crowns, their longevity clearly down to the extreme care continually exercised by the council. Age had been estimated at around 400 years, but the arboriculturalists in Brighton hesitated to put such an exact figure on these trees, explaining how their condition made the dendrologist’s calculations more difficult.
Both trees were described as ‘pollarded’ but not in the strictest sense. David Archer explained how the trees required continual thinning and reduction rather than strict pollarding to maintain their interesting canopy structure. Moreover, the hollow condition of the boles (trunks) meant the canopy had to be kept as light as possible, with maximum stem flexibility to let the wind pass through with minimal resistance. Planted alongside each ancient elm was an English elm sapling gifted by the Royal Parks to ensure continuity when these majestic old elms finally expired.
Also in Preston Park were groups of Ulmus minor subsp. sarniensis, called ‘Wheatley’ elm in Brighton but variously ‘Jersey’, ‘Guernsey’ or ‘Southampton’ elm in the wider world. They are straighter than English elm with longer ascending branches to give a distinctly pyramidal and fastigiated-looking crown. These elms were estimated at 140 years old as calculated from a late-1800s photograph taken when they looked to be 15 years old.
Nearby was a Wheatley elm recently felled, though not due to O. novo-ulmi infection. It had succumbed to Rigidoporus ulmarius, a basidiomycete fungus which attacks the elm-tree collar. Higher up the trunk was evidence of darkened brackets of Inonotus hispidus, another basidiomycete fungus relatively uncommon on elm but frequently causing heart rot in ash. The remaining elm stump had been temporarily sculpted into a public seat, but not before complete debarking as a precaution against bark beetle invasion. The standard practice for elm stumps in Brighton and Hove is debarking during the interim period before being finally ground out.
David Archer showed me Rigidoporus ulmarius in the collars of several standing Wheatley elms and how the whitish bracket assumes a characteristically green tinge with age due to a developing layer of algae. “This is a lignin destroyer,” he said, removing a piece of affected wood which was light white and airy, with a feel of balsa wood, having selectively lost its lignin strength and structure.
BRIGHTON PAVILION’S ELMS
On the way to Brighton Pavilion we saw examples of golden English elm (Ulmus procera var. van houttei) and an avenue of golden wych elm (Ulmus glabra lutescens). Mature Wheatley elms along Shirley Drive had recently been reduced, not at the instigation of the council but because residents had complained about how the trees’ root systems were causing problems for their low front garden walls – surely a small price to pay for having such superb rare trees outside of your home. Brighton’s Pavilion Park, with its many magnificent English elms, was our very last port of call.
“This is only a fraction of those which stood here prior to the 1987 hurricane which destroyed between 80 to 85 per cent of elms then spread across the level,” said David. “Many were windblown and others irreparably damaged through roots rocking in the ground.”
Ironically the hurricane saved some explaining by the council, which had already decided there were too many elm trees across the level and that some would have to be removed. In the event, nature intervened and thinned out the stand, but clearly on a scale neither anticipated nor wanted by the council.
On the way back we stopped to admire a weeping wych elm (Ulmus glabra cv. horizontalis) – most appropriate too, because seeing these last surviving mature elm trees with hundreds of years on the clock would be enough to make anyone weep for joy.
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