Black Forest firefighters said a buildup of heat in tree roots is what's keeping reports of hot spots and smoke sightings common.

Lt. Jamal Davis said many trees in the area were already stressed by drought and had little moisture, and the fire worsened the situation by spreading heat through the extensive root system that trees have.

Davis said fire crews will extinguish one hot spot, only to find later that the heat spread deeper into a root or spread to another root.  Some root systems can be 5 feet deep, he said.

"Even though you can't see oxygen and there's really no flames, the roots will still be able to burn underground because they spread so deep and so far," said Davis.  "They retain quite a bit of heat.  Heat can build up to a point where it can cause a flare-up and a new fire if it's close to pine needles or other unburned material."

Firefighters smother the hot spots with dirt and soak them with water, a job that's kept crews extremely busy, with a minimum of 30 smoke reports made every day.  Many residents remain fearful of another fire like the recent blaze that destroyed more than 500 homes.

"But we don't mind," said Davis.  "As long as it prevents another fire.  That's why we want people to call us if they see smoke, no matter how insignificant it might be."