From RationalWiki

"Trees cause pollution" is a gloriously stupid concept foisted upon the public in 1981 by U.S. President Ronald Reagan. The full quote of his statement was, "Trees cause more pollution than automobiles do."[1]

Reagan was falsely referencing the photochemical reaction that causes ground level ozone pollution (GLOP), also known as photochemical smog.[2] Smog is created when automobile and power plant emissions are broken down into ozone and other chemicals by strong sunlight. These reactions are amplified by the presence of various volatile organic compounds, or VOCs.[3] During hot weather many tree species release volatile organic compounds into the atmosphere, notably terpenes and isoprenes (pine pitch is a terpene). Other VOCs that contribute to GLOP include gasoline, kerosene, paint, paint thinner and other industrial solvents.

Reagan's statement was disingenuous because volatile organic compounds produced by trees do not cause pollution any more than the sun causes pollution. Reagan might as well have said, "the sun causes more pollution than automobiles do." Photochemical ozone pollution is created when automobile and power plant pollution is broken down by strong sunlight in the presence of any number of volatile organic compounds.

GLOP or photochemical smog is only created in areas that receive large amounts of pollution from automobiles and power plants.[4] Trees are not required to create it. Without the input of large amounts of man made nitrogen oxides by cars and power plants, the VOCs released by forests on a summer day have nothing to react with and they do not create any pollution. In reality, photochemical smog is very harmful to trees.[5]