The Spruce Fire in the Yellowstone National Park back country has grown to more than three square miles, a park spokeswoman said.
The park is letting it burn according to its fire management plan because it isn't threatening any structures or roads, and it benefits the health of the forest, Julena Campbell said.
Lightning is believed to have started that fire in the south-central area of the park in an area about one-tenth of an acre last Wednesday, and had grown to more than 2,100 acres as of Sunday, Campbell said.
The fire is burning in a mature lodgepole pine forest, and is about 10 miles west of Fishing Bridge and two miles south of Hayden Valley.
"Even though we are not actively working to contain this fire or to put it out, we certainly are managing it," she said.
That means park officials are tracking its development from the air and lookout points to watch its behavior and rate of spread.
Fire activity has picked up during the afternoons as temperatures rise, relative humidity levels drop, and gusty winds increase. The National Weather Service has forecast a cold front entering Yellowstone with cloudier, cooler and wetter conditions that may include snow, Campbell said.
The fire continues to play its natural role in the ecosystem and crews are managing it for its benefits to park resources, she said.
"It's burning in what we like to call a mosaic pattern, which is pretty typical of fires," Campbell said. "Some areas burn pretty intensely and actually will burn entire trees, and other areas no so much and will skip around and leave areas of green in and amongst the burned areas, which creates a very healthy forest in the long run."
The trees in the area are between 200 and 400 years old, she said.
This fire will clean out the dead and downed wood on the ground, which, if not burned, will create more fuel for larger fires in the future, Campbell said.
It also activates the lodgepole pines' reproductive systems with their serotinous cones, which are glued shut by a sticky resin that protects the seeds, she said. "It has to have very high temperatures like that from a forest fire to actually open the cone and allow the seeds from the lodgepole pine to come out."
The fire also creates a fertilizer for the pines and other plant life, and the burned-away canopy allows for the sun to shine on other growing plants, Campbell said.
Once the fire moves on, park scientists study its effects on wildlife, vegetation, air quality and water quality, she said.
And when it moves on, the park will leave the burned vegetation undisturbed to decompose, Campbell said. "All that will be left in place to do what it would naturally do in the ecosystem."