Comment – So Trees Are Polluters? Huh?
Copyright © Tony Burfield 2002

It was widely reported in the UK National Press at the end of October 20021 that trees are adding to the pollution of the atmosphere – trees were even graded with respect to their allegedly sinful emissions, with Silver Birch trees coming off with a cleaner image that Oak trees apparently. The basis of these curious statement are worth examining – how can something which is deeply green by its very nature be causing these problems?

The source information for the articles, apparently, stems from a team from Lancaster University, lead by Professor Nick Hewitt, who are studying the effects and the interactions of trees with the local atmosphere. Instead or in addition to trees being “the lungs of the world”, the team found that trees produce volatile organic carbon compounds (VOC’s). This information is hardly new2, but it is the interaction of these VOC’s with oxides of nitrogen under the action of sunlight which goes on to produce a nasty cocktail of chemicals (including ozone) which produces photochemical smog which often hangs over our cities in summertime.

According to the press the team surveyed some 32,000 trees in the West Midlands recording species, age, condition, height, trunk diameter, leaf area etc. in order to estimate VOC emission rates. The data was fed into a chemical lower-atmosphere simulating computer-model, and results scrutinised, paying attention to what would happen by varying the species. It was found that more pollution would occur when oaks, poplars and willows were more evident. Finally the conclusion was that planting trees in urban areas does lower pollution, but only if certain species are planted. A policy of discrimination was therefore advised to local councils, planners etc. whereby ash and maple trees should be encouraged, at the expense of poplars, oaks etc.

It seems to me that trees are not the culprits here – they are doing what they have been doing for thousands of years – quietly growing, and adding beauty, shelter and a sense of permanence to the environment. To say therefore that trees are polluters in this situation is a nonsense – the true culprits are motor engines that pump out noxious gases which kill and disable and affect the health of so many of our citizens. Instead therefore of altering the ecology of our local environments to better accommodate the polluting nature of motorcars, wouldn’t the research money that sponsored this study have been better spent on designing or campaigning for a cleaner motor engine?

I happen to think that the oak tree is as much an abstract symbol of what England represents as the Houses of Parliament. Although Guy Fawkes failed in his mission, I would hate to think that the Lancaster University team will succeed in theirs – towns full of silver birch and maple are not so appealing as the present diversity of species which grace our drives, parks and gardens. 


  1. Kate Ravilious “Good copse, bad copse” The Guardian (suppl.) Thurs Oct 31 2002 p10.
  2. Our senses are continually exposed to natural materials - for example - the volatiles from flowers and from leaves etc. Shenck G.O. (1979) Perf Kosm 60, 397.estimated that 438 million tons of monoterpenes evaporate into the air continually from biological materials.