Fuels may be fine, such as twigs and needles, or heavy, such as logs, branches and whole trees that have blown down in a storm. Fuels may also be living such as an understory layer of tree regeneration or a layer of shrubs. In order for an ignition to take place, the fuels must be dry. On average in Yellowstone, fuels dry out enough to ignite the first wildland fire of the year about the middle of June. Young forests generally do not have the fuel loads that mature and old-growth forests do.
Many young stands grow on old burn sites in which accumulated fuels have burned off.
As the young trees grow they compete for light and other resources. 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. As trees grow they also 'self-prune' old branches that become shaded by new foliage at the top of the canopy. 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.