It is generally agreed that to pose a risk, a tree has to be not just dangerous (for either mechanical or disease reasons), but also in a location likely to threaten life, limb or property.
Making such an evaluation is no mere managerial exercise. It may have a bearing in court — where, in our litigious culture, more cases are likely to end up, with tree managers liable to face accusations of not having shown due care.
Yet limited budgets mean more than a third of local authorities have no tree strategy either in place or in preparation. And specific risk-management policies are still a rarity, according to parks consultant Sid Sullivan. He gives courses in tree risk assessment for non-specialists on behalf of the Institute of Leisure & Amenity Management.
"Managers who understand the dilemmas are better placed to make the case for more resources," he says.
Authorities that are prepared to invest in inspection regimes will be more able to demonstrate due care, says arboricultural consultant David Lonsdale, author of Principles of Tree Hazard Assessment and Management.
"That doesn’t mean having a system handed down on tablets of stone, but it would mean knowing the age, size, species, condition and location of the tree population," he says. "And an increasing number of local authorities are bringing trees into their overall risk assessment procedures."
But in a world of limited resources, tree managers must prioritise which trees to survey. "A starting point is to decide which are the areas with highest usage," says Lonsdale. "A zoning system of high-, medium- and low-use areas would help you narrow down where you need to bring resources to bear. You could then demonstrate in a court of law that you had apportioned those resources in a sensible way."
This could mean a regime of regular inspection — at least once a year for high-risk areas, less so for medium-risk, and perhaps no regular inspection at all for low-risk areas, he says.
"There also needs to be a means of recording ‘opportunistic’ reports, such as the sudden appearance of fungal fruiting bodies."
In addition to this, evidence of competently trained staff who keep abreast of current literature and undergo continual training "would demonstrate a robust regime", he says.
Fear of legal action, combined with increasingly variable weather and often ageing tree populations, has led many councils to take a safety-first approach to trees, which some in the industry have argued is over-cautious.
"People think of decay as a hazard, but you can have a lot of decay without weakening the tree," says Lonsdale. "There’s a lot of interpretation required — you need to know the biology of the decay and where it’s going from and to."
Rather than attempting to conclude whether individual trees are safe or not, tree managers are increasingly turning to systems that assign a probability to a tree causing harm. This figure is then compared to a predetermined level of acceptable risk. A one-in-10,000 probability is generally considered acceptable by the Health & Safety Executive.
The "probabilistic" tree risk assessment system was developed in the US in the 1990s by International Society of Arboriculture members Nelda Matheny and James Clark. The method is still in wide use internationally. It divides tree risk assessment into three components: the tree’s failure potential, an environment conducive to tree failure, and a target.
Each is rated from one to four and the overall risk is calculated as the sum of these values.
Quantified Tree Risk Assessment (QTRA), devised by tree consultant and Arboricultural Association award winner Mike Ellison, is an elaboration of Matheny and Clark’s approach.
Since being introduced in the UK a year ago, the system has around 130 licensed users. Ellison aims to expand the scheme to the Netherlands next month and to Australia later in the year.
"QTRA doesn’t seek to minimise risk, because that would also minimise the value of trees," says Ellison. "It aims to strike a balance between managing risk at a reasonable level and maximising the value of a tree population."
The risk can be calculated using a manual calculator or the official software. But Ellison warns: "Even the most objective assessment will include an element of subjectivity. You may still have to stand back, review the result and perhaps refine your assessment."
In particular, the QTRA algorithm takes no account of the relative amenity value of the tree. Ellison says: "In terms of sustainably managing a population of trees, it might be more appropriate to take out a low-value tree, especially where there are lots of a single species or similar age."
He believes there would need to be a much stronger case for felling a high-value tree such as Islington’s Barnsbury Beech.
Lonsdale says: "QTRA is very much better than what went before, and Mike has brought us as far as anyone to an objective standard. But there are still some theoretical problems with the term ‘probability’. You’re calculating a value from zero to one, with one being certainty. But could a motorway, which has the highest occupancy, yield a value greater than one? After all, it has many lanes. Also, a human life is notionally valued at £1m. But if the target was a mansion house, and the repair value was greater than £1m, would that also give a value greater than one?"
Easington council tree officer Jim Jones took a one-day QTRA course last year in order to become a "licensed user". He says: "Most local authorities tend to fire-fight when it comes to trees. QTRA should allow us to allow us to manage our tree stock accordingly."
The council has since given the thumbs-up to the system, he says. "I gave a demonstration to our own health and safety ‘guru’, who asked a lot of questions but was more than happy with it. We’re now updating our tree risk management strategy, though it is time consuming and we have to juggle it with usual tree officer work."
Part of the work at Easington is to incorporate data on its tree population into its tree management software, Tree Wise, which includes a QTRA facility.
Tree Wise is supplied by Robin Forestry Surveys of Cumbria. Managing director Simon Stone says the system provides arborists with valuable back-up. "Trees don’t always behave in predictable ways. So all the pressure is on the tree inspector’s subjective assessment. He can’t conclusively say that a tree is or isn’t safe. But he can say that a tree has a one-in-20,000 chance of causing harm. He can then take that to an insurance company, so he has some kind of cover."
The Arboricultural Association welcomes incident reports for inclusion in its national tree accident database. Please call 01794 368717.
• Tomograph Mike Ellison says: "Tomographs have a role in assessing the degree of decay, but it would be inappropriate to use them as a matter of course."
• Hammer "It sounds crude, but it’s still useful to just tap the shell of the wood," says David Lonsdale. "If you can hear that the outer 15cm to 20cm is sound, then the tree is likely to be okay. A hammer is the most sophisticated tool that some inspectors use."
Threats and responses
Tree-related deaths and injuries are more prevalent than many people think, according to Forbes-Laird Arboricultural Consultancy director Julian Forbes-Laird, who collated media and first-hand reports of the subject between 1998 and 2003.
In that period he documented 36 fatalities in England alone — an average of more than seven a year. "Even then, it’s likely I’ve missed a few," he says. "And I haven’t arrived at a figure for injuries, because they’re often not reported, or mentioned incidentally where deaths have also occurred."
Indeed, local authorities insurer Zurich Municipal has rated tree failure the most common cause of death related to local authorities’ resources.
To reduce such dangers, Forbes-Laird devised a risk-assessment system called THREATS (Tree Hazard Rating, Evaluation And Treatment System), which can only be deployed by trained arborists. "It’s not probabilistic — I don’t think there is enough good data for that. It’s better to rely on the judgement of a trained surveyor."
Have you registered with us yet?
Register now to enjoy more articles and free email bulletinsSign up now