What is preventive fire?

Answer 1

When a forest is left alone, there can acumulate a lot of dead wood and plants on the forest floor. The longer it goes without a fire, the more debris builds up and then when there is a fire, there is a lot of fuel to burn. The more fuel the hotter the fire and the more live trees will be destroyed.

The Forest service will periodically set fires within an area of the forest and have a controlled burn. This will be a smaller fire that will not damage the larger trees but will clear out the underbrush.