Many don't survive the competition, and remain in the stand as standing dead snags which fall over and further contribute to the fuels on the forest floor. 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 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.
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. Other species such as lodgepole pine, whitebark pine, Engelmann spruce and subalpine fir have thin bark but these species have other adaptations to fire.