I’m not sure what part is confusing you, so I’ll just explain the whole context. Let me know if my explanation helped you, or if I’ve wasted my time: According to the text, forest fires are natural and they eradicate (destroy, eliminate) the underbrush. Since the US Forest Services stops the forest fires, the underbrush is not eradicated. The result is that this underbrush accumulates more than it should, so that if there is a forest fire, the underbrush feeds the fire a lot.