Visited Yosemite several years ago
Park Ranger Said this:
Forest fires are a part of the cycle
We had been preventing them so the brush had built up on the floor, now when there is a fire it gets too hot and can engulf the trees.
If you allow the fires to occur naturally, then the brush gets burned off and the fires don't get hot enough to burn the trees down.
So now we are cleaning up the brush because there is too much buildup because we did not allow the natural burn.
So man created this problem.