Research from academics at Bangor University has suggested that avenues of trees on urban streets can damage the health of pedestrians by trapping air pollutants beneath their canopy. It warns that the effect could be so bad on streets with slow-moving traffic that councils should consider chopping down trees.
The above was the result of an item in ‘The Conversation’ which bears very little relation to any actual state of affairs in London - click here
A photo shows an avenue of trees, looking very much like a park. Clearly no vehicles are permitted. The avenue is lined with benches. The caption says ‘a tree canopy looks nice but where is the air pollution supposed to go’. What pollution? The fact of trees cleaning the air by deposition of pollution would suggest the opposite in this case – that the presence of trees would be keeping pollution out.
As a matter of fact 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 – although, to be fair, the building bonanza in London is now creating canyons irrespective of the presence of trees. Tree Officers, in London as elsewhere, are aware of the need to take into account all aspects of what trees deliver – good and bad – and have done for years.
There is no new debate on this. The main failing of the contribution in ‘The Conversation’ is the very factor it purports to address – that of looking at one part of a highly complex equation in isolation. Tree Officers have for years dealt with the competing demand for shade and demands for light. We know that species such as the Birches may be the very best at capturing roadside particulates but may not be suitable for a host of other reasons. That Oaks and Willows produce VOCs. Willows are almost never and Oaks seldom planted in streets – but if you want rapid growth or rich bio-diversity, these are some of the trees you need.
The argument about trees trapping pollution at street level also ignores the temperature reductions gained in tree lined streets and strangely even ignores simultaneous removal of pollution by deposition – bizarrely leaving the impression that trees are responsible for pollution.
It is suggested that trees ‘are like walls or cars’ in preventing circulation of air. They are of course nothing of the sort. Almost any vegetation has some porosity and deposition on plant surfaces is known to be many times more ‘probable’ than on manufactured surfaces - concrete, glass and car bodywork.
It’s also true that tree officers in the UK have to consider the weather. Not simply the fact that our urban climate is becoming more and more like an arid Mediterranean one at times but also that the UK is one of the windiest corners of Europe. Practitioners such as our members would dearly love to see the ‘Canyon’ studies applied to conditions in July 2015 – seemingly one of the windiest summer months for years.
It is alleged that it is the job of ‘urban planners and local authorities to ensure trees are not just planted where they look nice’. It might be useful to recognise the fact that between 60 and 70% of our trees are in the private sector with fairly minimal governmental control – a last hint that when discussing the benefits and dis-benefits of trees its best to look at the whole picture and not focus on one little bit. After all, that’s what research is all about!
The role of the LTOA is to enhance the management of the Capital's trees & woodlands. The environmental, social, economic, health and wellbeing benefits of trees are well documented and we would like to ensure that a clearer picture is given which has been well researched and published.
We would draw your attention to a publication by the Woodland Trust and its references - Urban Air Quality Report (April 2012) prepared by Mike Townsend of the Woodland Trust with the assistance of Dr Tom Pugh and Professor Nick Hewitt, both of Lancaster University, and Professor Rob MacKenzie of the University of Birmingham - click here
The above article is the full response given to Horticulture Week and used in their article 'Report suggests large -canopied street trees can exacerbate pollution', 21 August 2015, http://www.hortweek.com/report-suggests-large-canopied-street-trees-exacerbate-pollution/landscape/article/1360654.
Written by Dave Lofthouse on behalf of the London Tree Officers Association
Dip. Arb. (RFS) M.Arbor.A.
Arboricultural Manager, Greenspaces
London Borough of Merton