Blood.
We used to control forest fires. We would put out every fire that happened because we didn't want any forest fires. This caused a massive accumulation of brush and overgrown forest. Eventually it became too big to stop it from burning, and so around the beginning of the 20th century we had the most massive forest fires in history. They were so bad that the military was called in to evacuate towns and to fight the fire.
Since then we've learned you have to let small forest fires burn out the overgrowth from time to time, or you will eventually get one so big it becomes a dire threat to everyone in the vicinity.