Report suggests large-canopied street trees can exacerbate pollution

Large-canopied street trees can exacerbate air pollution, according to an article in The Times earlier this month. "The effect could be so bad on streets with slow-moving traffic that councils should consider chopping down trees," it added.

Street trees: negative canyon effect highlighted in Swedish research. Image Otto Kristensen
Street trees: negative canyon effect highlighted in Swedish research. Image Otto Kristensen

The piece was based on an online article by Bangor University researcher John Gallagher, who wrote: "Your typical street with avenue trees can prevent pollutants from escaping and air quality can be greatly impacted."

He cited Swedish research published earlier this year that found: "The reduced mixing in trafficked street canyons on adding large trees increases local air pollution."

But London Tree Officers Association (LTOA) committee member and former chair Dave Lofthouse said Gallagher's article "bears very little relation to any actual state of affairs in London".

He explained: "The negative canyon effect is well known, the benefits of tree-lined streets well documented and the presence of such tree-lined canyons in our built environment fairly rare. Tree officers, in London as elsewhere, are aware of the need to take into account all aspects of what trees deliver, good and bad. There is no new debate on this.

"The argument about trees trapping pollution at street level also ignores temperature reductions gained in tree-lined streets and even ignores simultaneous removal of pollution by deposition - bizarrely leaving the impression that trees are responsible for pollution."

A 2012 Woodland Trust pamphlet summarising research on urban air quality and trees concluded: "Avenues of street trees within the worse (sic) polluted street canyons might reduce mixing and dispersion and hence exacerbate air-quality problems at the street level. There may be a balance to be struck and the greatest benefits of street trees may be in the less polluted canyons."

Lofthouse said: "We know that species such as birches may be best at capturing roadside particulates but may not be suitable for a host of other reasons, and that oaks and willows produce VOCs."

He added: "Willows are almost never and oaks seldom planted in streets. But if you want rapid growth or rich biodiversity, these are some of the trees you need."

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