[DoughtyOne]

When Reagan menthioned trees in this manner, he was talking about overall polution in our big cities. (At least I think he was.)

I didn't mean to imply that oxygen was a polutant, but that Reagan's premise was that some trees could compete with humans for the oxygen supply in heavily populated areas.

[seowulf]

Changes in U.S. forests caused by land use practices may have inadvertently worsened ozone pollution, according to a study led by Princeton University scientists. The study examined a class of chemicals that are emitted as unburned fuel from automobile tailpipes and as vapors from industrial chemicals, but also come naturally from tree leaves. These chemicals, known collectively as VOCs, react with other pollutants to form ozone, a bluish, irritating and pungent gas that is a major form of smog in the lower atmosphere.

http://www.scienceblog.com/community/modules.php?name=News&file=article&sid=4211

Trees really do contribute to smog.