Clones of an Eastern cottonwood ( Populus deltoides) in the Bronx and other city spots grew to double the biomass of clones  　 outside small towns upstate or on Long Island, says Jillian Gregg, now of the Environmental Protection Agency's western-ecology   in Corvallis, Ore. The growth gap comes from  　 damage, she and her New York colleagues report. Ozone chemists have known that  　 may spike skyscraper high in city air, but during a full 24 hours, rural trees actually get a higher cumulative ozone exposure from  　 pollution that blows in and lingers. A series of new experiments now shows that this hang-around ozone is the  　 factor in tree growth, the researchers say in the July 10 Nature. "This study has profound importance in showing us most vividly that rural areas  　 the price for urban pollution," says Stephen P. Long of the University of Illinois at Urbana-Champaign. "This work should be a wake-up call," he adds.