Is there a right strategy for dealing with ash dieback?

With most of the country’s ash trees coming into leaf in May, tree managers have begun observing how badly ash dieback, now a nationwide problem, is impacting their tree stock and examining the safety implications.

Wrexham County Borough Council senior tree officer Elton Watson says: "Approximately 5% of our urban tree stock is ash and there is a lot along the rural highways, so I think I know what most of our budget will be spent on for the next few years. Obviously this is likely to have a major impact on our attempts to increase canopy cover over the life of our tree strategy."

Caused by the fungus Hymenoscyphus fraxineus, ash dieback is not only lethal to trees by itself, it can leave them open to secondary infections such as honey fungus (Armillaria) that can undermine trees, putting managers under more pressure to keep on top of affected trees, particularly in high-risk locations.

Controversially, earlier this year an ash-removal programme was begun in a high-profile Surrey park. Covering 450ha including 200ha of woodland, the Register of Historic Parks & Gardens-listed Norbury Park near Leatherhead is owned by Surrey County Council but managed on its behalf by the Surrey Wildlife Trust, and forms part of a wider site of special scientific interest. The trust has carried out what it calls "selective felling" of ash trees within 30m of paths and car parks — work it says was approved by Natural England and the Forestry Commission.

But the work has not gone down well with local residents, with one claiming at a council cabinet meeting that "nearly all" the area’s trees had been felled, including any potentially resistant specimens, and damaging lower habitat for other flora and fauna in the process. He suggested the move was prompted in part by seeking revenue from the timber.

Cabinet member for environment and waste Mike Goodman responded that a Forestry Commission site visit had highlighted "significant signs of dieback" and said the felled trees are not being stump treated and will be allowed to coppice, while an ecologist "is checking to ensure the work causes the least impact on the ecology of the site".

He added that the commission’s recent guidance linked ash dieback to honey fungus, and he has "asked woodland managers to revisit their risk assessments and policies" in light of this.

Timber sold

Meanwhile, "most of the timber" is being sold to nearby harvester and marketer Euroforest "to be used in generating power", said Goodman, adding: "Any income from this is being directly used to fund further ash dieback safety works where they are needed across the countryside estate."

Along with the Forestry Commission guidance, the Tree Council, with Fera Science, has also published guidance for tree managers on coping with the disease. Its February 2019 Action Plan Toolkit is intended to help local authorities and other agencies to prepare an ash dieback action plan.

As a first step, it urges tree managers to collect all available data on ash tree populations in their area, such as tree preservation orders, supplemented where necessary with fresh surveying.

Planning for a range of future mortality scenarios can then be used to estimate likely budgetary impacts. One county council put this total figure, based on 75% mortality rate and typical cost of £400 per removal and £15 per tree replacement, at £34.5m, of which nearly £30m comprised felling of privately owned trees adjacent to highways.

"Being reactive to the problem is likely to be more expensive than planning your response through an action plan," the report concludes, adding that the whole exercise can be carried out in a matter of months rather than years. But it admits: "We are early in our understanding of the best approaches for dealing with ash dieback."

Spotting survivors

New research on the impact of ash dieback on the Swedish island of Gotland suggests that simply observing which ash trees appear tolerant may be a better indicator of their likely longer-term survival than known genetic markers for tolerance.

As with mainland Sweden, Gotland’s substantial numbers of ash have been decimated by the pathogen since its arrival in 2001. As described in a yet-to-be-published paper, Swedish and Lithuanian researchers mapped 135 "healthy-looking" ash trees in 2013-14 then revisited them to assess their health in 2017, as well as identifying the presence of a genetic marker for disease tolerance in 50 of the trees along with 10 control trees.

Only one of the mapped trees was found to have crown damage above 10%, while "the presence of dead tops, wilting foliage or cancers was not observed on any of the mapped trees". Meanwhile, 85% of the mapped trees tested showed the tolerance marker, compared with just three of the control trees.

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