Vegetation is layered, and a fire behaves differently in each layer. A surface fire burns litter, grass and low shrubs beneath the canopy. A crown fire burns in the tree canopy itself, and reaching it requires a transition: the surface fire must be intense enough, and the gap from the ground to the base of the live canopy small enough, for flames and hot gas to reach foliage that is alive and full of moisture. Ladder fuels, meaning shrubs, saplings, low branches and hanging lichen that bridge the gap, are what make that transition possible at much lower surface intensity than an open-trunked stand would need. Once in the canopy a fire may be passive, torching individual trees, or active, spreading through the canopy as a continuous front far faster than the surface fire below it. The confusion this resolves is treating crowning as simply a bigger surface fire; it is a change of fuel layer with its own spread behaviour. The quantitative criterion for the transition is taught with the spread models later in this path. After this Concept you can identify the conditions under which a surface fire will move into the canopy.
Wildfire fuels are stratified into vertical layers—surface, ladder, and crown fuels—and the quantity and continuity of fuel within and across these layers determine a fire's size, intensity, and its …