Grassland

The sea of native grassland that once carpeted the United States' Great Plains has steadily been disappearing - replaced at a disconcerting rate by trees and shrubs. | The main tool used to drive out the trees and encourage the grasses is controlled fires, set every few years. | Now, Dirac Twidwell, a range ecologist at the University of Nebraska, says he's figured out why. | Yet these fires have not halted the advance of trees and shrubs. | The problem is not that controlled burns are too destructive, he says. It's that they're not destructive enough.