Why Do Forests Sometimes Burn Without Any Help?

Some trees, like the sequoia and pine, have a special kind of cone that only opens when there's heat. If they get too dry, their cones can burst open on their own, releasing seeds and a lot of flammable material. This is like having a matchstick inside the tree, it just needs a little spark to start a fire. The heat from the first few trees catches other nearby trees, and soon the whole forest becomes a wildfire.