These pruned branches eventually fall off the tree and accumulate as litter on the ground. As the stand grows older and taller the canopy begins to break up, letting light reach the forest floor and allowing the establishment of an understory layer of shrubs and regenerating trees. This understory layer forms a 'ladder' of fuel that may allow a ground fire to ascend into the forest canopy. This accumulation of fuel on the forest floor and the continuity of fuels between the ground, understory and overstory are factors that predispose older stands to ignition by a lightning strike. Nearly all of Yellowstone's plant communities have burned at one time or another.
Some plant communities ignite and carry fire more readily than others.
Therefore the 'fire regime' of Yellowstone is as varied and complicated as the environments within the park. It must be remembered that any forest will burn provided an ignition during periods of prolonged drought, high temperatures, low relativity humidity or high winds. However each type of forest has certain characteristics that cause wildfires to behave differently. Some trees such as Douglas-fir have very thick bark that insulates the tree against heat. Thick bark ensures that wildfires seldom to kill mature Douglas-fir trees.