Now I bet you thought that trees were all nice and enviromentally friendly. Not so, some trees can be quite unfriendly.
Some kinds of trees produce stuff known as volatile organic compounds (VOCs) as a byproduct of all that photosynthesising that they do. (Or even biogenic volatile organic compounds, as trees aren't the only things that produce VOCs.) These VOCs fly around the place until they bump into some nitrogen oxides (NOx) and if its nice and warm and a bit sunny the VOC and the NOx happily procreate and produce ozone.
Now the last time I looked we were supposed to have a hole in the ozone layer, which was generally considered to be a 'bad thing', so you might thing that producing a bit more ozone to help plug the gap would be a 'good thing'.
See, ozone high up in the sky, stops ultraviolet rays getting through and giving us all melanomas; good thing. Ozone at ground level, primary constituent of smog; bad thing.
Some bloke called Professor Nick Hewitt from Lancaster University in the United Kingdom has been busy researching into the whole question of trees and air quality and all that, and has been conducting a survey of 32,000 trees in the West Midlands (that's Birmingham basically.) and constructing a computer model that simulates the lower atmosphere of the West Midlands area.
With his computer model he can now test what would happen to air quality if you planted more trees. The basic conclusion is that whilst planting more of some tree species improved air quality, some had quite the opposite effect because they release so much VOC that they actually increase ozone levels.
Good tree - ash, maple, elder, larch, scots pine and silver birch.
Bad tree - oak, poplar and willow.
Start up the chainsaw!.
See Good copse, bad copse-Some trees may not be so green after all by Kate Ravilious
Thursday October 31, 2002, The Guardian;
and Urban Tree Planting as an Aid to Air Pollution Abatement: Cost-Effective Analysis of the West Midlands Case - http://urgent.nerc.ac.uk/Air/Projpages/Ae13.htm