The polluting trees

Those who grew up in the 1980s might remember how Ronald Reagan was pilloried for this comment: “Trees cause more pollution than automobiles do.”

Well, in the San Joaquin Valley of California, trees cause, to be precise, four times as much pollution as cars. The worst offenders, according to this story, are the cottonwood (which, as anyone who lives near one knows, pollutes in other ways), eucalyptus, sycamore and weeping willow.

Just one question: How is it that natural things can pollute?