The short-lived isoprene leaked from leaves packs a chemical wallop in the lower atmosphere. It reacts with man-made nitrous oxide (NOx), emitted mainly by cars and factories, to create ground-level ozone, otherwise known as smog [see illustration]. While there's thus some truth to Reagan's assertion, scientists say fossil-fuel burning humans are really to blame. "In the eastern U.S., we have enough NOx around that the VOCs end up forming ozone," says Drew Purves, a postdoctoral fellow in the department of ecology and evolutionary biology at Princeton University and
the lead author of the study. "We control every aspect of U.S. forests."