The warning headlined the first day at the AA Amenity Arboriculture Conference at Warwick last week, repeating the message he gave to the International Society of Arboriculture's annual conference in Sydney last month.
Barrell discussed the pros and cons of quantitative and qualitative methods of risk assessment, with reference to what English courts expect from tree owners.
"How much management is enough is one of the toughest questions to answer and of fundamental importance to duty holders," he said.
"Of specific relevance to tree advisers is whether they adopt a quantitative approach to risk assessment, expressing the level of risk with numerical calculations, or a qualitative approach, where terms such as low, medium and high are preferred."
He warned that the level of risk, however it is expressed, will not be the only matter the courts will consider. "Although the level of risk is important, so is how reasonable it is to carry out remedial works and how much it will cost.
"The courts may well decide that low risks should be actioned if it is reasonable, cost-effective and proportionate to do so. If this turns out to be the case, then risk assessors who have relied solely on the level of risk as a justification for no action may be sitting on a time-bomb just waiting for the next tree failure to trigger.
"Even if a risk is low, if the hazard is obvious and it costs very little to address, duty holders may find the courts expect them to deal with it."