The sea of native grassland that once carpeted the 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 fire—and controlled burns are set every few years throughout parts of the Plains states to sweep the landscape clean.

Yet these fires have not halted the advance of trees and shrubs. Now, Dirac Twidwell, a range ecologist at the University of Nebraska, says he’s figured out why.

The problem is not that controlled burns are too destructive, he says. It’s that they’re not destructive enough. More extreme fire is needed, he argues, because extreme fire is more effective at killing deeply