Barrell Tree Consultancy managing director Jeremy Barrell, whose practice was last month named Arboriculture Consultant of the Year, has published a briefing note on his website warning that long branches can break from large mature trees with no obvious visible defects after long dry periods in summer.
Barrell’s own casework has included several inquests and legal cases involving branch failures where SBD may have been a factor, though he says it remains "a poorly researched phenomenon". However, it is "well-recognised around the world as a mechanism for branch failure" and so for potential harm, and therefore "it should be considered in the risk assessment process", not just for highways, public parks and botanical gardens, but also showgrounds, theme parks, golf courses and institutional green spaces, he advises.
Several comments from other tree professionals on Barrell’s Facebook post, which first raised the issue, appear to bear out the danger. A fellow consultant wrote: "There’s been quite a few SBD failures in Devon this year but not following heavy rain — seems to be mostly on hot sunny days." One tree officer said: "We had a child's music festival pop up under the canopy of three large horse chestnuts just as the rain started last week. Panic stations."
Another tree consultant posted: "I regularly (annually) inspect a number of fully mature oaks near to roads and property. In lots of cases it would not be realistic to close roads or to advise property owners not to go into their gardens. I would also fear putting warning signs of possible limb failure in these settings as it could cause a certain amount of hysteria."
Risks of failure
But the growing body of anecdotal evidence provides some insights into how the risks of failure and harm might be managed, says Barrell. "All inspectors must be aware of the potential for SBD and factor it into the decision-making process. Many large duty holders already do this in a variety of ways, ranging from pruning to access management."
He suggests: "Duty holders should identify vulnerable trees and be aware of the risk increase in the critical period. In practice, that means keeping people away from vulnerable trees. But the decision should be driven through the normal risk assessment process.
"It is well documented that once SBD from a tree has occurred, it is likely to happen again, so history of failure matters. We also know that it is the big ones that are most likely to go, and the long heavy branches on them, so that is what to look for. I have seen this management approach routinely carried out at many of the top botanical gardens around the world, and it works."
Four years ago Barrell acted for the family of a New Zealand woman, Erena Wilson, who was killed by a falling cedar branch at Royal Botanic Gardens, Kew in 2012. SBD was a suspected cause, but an inquest ruled her death had been accidental.
Indeed, according to a Kew representative: "Through the year we do everything that is reasonably practical to mitigate SBD and provide a safe environment in the gardens for our staff and visitors." The National Trust says: "Our expert rangers and countryside managers employ a robust tree safety management regime to manage any potential risks."
But The Arbor Centre principal consultant David Evans, who has developed his own tree risk assessment methodology, known as "VALID", argues that the risk from SBD is "mind-bogglingly low". Citing data from Middlesex University’s Centre for Decision Analysis & Risk Management, he says that of the 64 deaths from tree failures it has documented over 10 years, "none looks like it could be reasonably caused by SBD", adding: "The overwhelming cause of failure and fatality is severe weather conditions."
Barrell says though that local risk factors matter more than a global risk figure, pointing out: "The risk of being killed by lightning is about half the risk of being killed by a tree, and yet the majority of golf courses have quite onerous bad weather policies."
At the back of tree managers’ minds will be the risk of court action should the worst happen. "The very tricky question a duty holder might have to answer in court is: If other high-profile institutions are managing for it, why didn’t you?" says Barrell.
"It would be extremely risky not to factor it into the normal risk assessment process because it is a known risk. What to do about it is a different matter — it needs intelligent analysis. It would clearly be an over-reaction to fence off all mature trees during the whole of the summer. But at the other extreme, ignoring it may be seen by the courts as not quite enough, especially if other similar institutions are managing it."
Given the apparent peak in failures after the first heavy rain following a long dry spell, "it could be entirely reasonable to react to that window and be extra vigilant during that time, which would only be a day or so", he suggests. "Many institutions close their gardens when winds over a certain speed are forecast. This seems a very similar situation."
Barrell plans to publish a more comprehensive analysis of SBD and its management shortly. He points out: "We have not had weather like this for quite a few years now, so this is a rare opportunity to observe. If there is indeed an increase in incidents after heavy rain, that may allow more proactive future management focused on the time of greatest risk. This is a real-time experiment."
Jeremy Barrell’s own analysis of apparent SBD incidents indicates they are more common:
- In large mature oak, beech, horse chestnut and cedar trees.
- After heavy rainfall following dry periods longer than three-to-four weeks.
- In the early afternoon, often in calm conditions immediately following rainfall.