I found this passage from Frontier Doctor to be particularly interesting:

When I came to eastern Oregon in 1905, all of the beautiful pine timber was an open park-like forest, without any underbrush, where game could be seen for a long distance. Each summer there were many forest fires, the vast majority of which were caused by lightning. As there was no underbrush, these fires consumed nothing but the dead pine needles, cones and twigs that had been blown to the ground by the winds. The little blaze, only a few inches high, crept slowly over the ground and cleaned the floor of the forest of all debris, killing the pine beetles on the ground, but did no damage whatever to the green trees. There were a few dead trees scattered through the forest that had been killed by the pine beetles. These dead trees almost invariable took fire and burned up and the beetles with them. It was these annual fires which had existed for centuries that had produced the beautiful open forests free from dangerous underbrush, and killed so many of the pine beetles that they were held in check. The tiny blaze of these fires was not hot enough to injure the pine seed. When the timber was cut off and the sun was allowed to strike the ground, these little pine seeds began to germinate and a new second growth of trees immediately sprang up.

 

No one tried to put these annual fires out, as they were known to be a benefit to the timber. When the big lumber companies began to buy the timber, their representatives in the field saw to it that their holding were burned over every year. If the lightning did not start enough fires, the timber men started more of them.

Nearly one hundred years later, I’ve never known these forests not to be thick with underbrush, and the “normal” forest fire is a raging inferno that destroys everything it touches.

What happened? Ignorance. As usual.