Fire requires fuel to burn so where the fuels are reduced, fire intensity will be reduced accordingly. The more continuous the fuel layer, the more rapidly a fire can spread and the greater the potential for high intensity fires. In addition to horizontal continuity, vertical fuel ladders are another concern. Fuel ladders are created by layers of flammable material that allow a fire to move from the ground to the tree canopy. For example, pine needles on the ground can ignite and burn shrubs which in turn ignite tree limbs or leaves.
Fuel ladders are a major cause of high intensity crown fires.
Fuel ladders can often be removed by pruning branches of trees or removing shrubs. A rule of thumb is that, within the defensible space area, vertical separation between fuel layers should be at least three times the height of the lower fuel layer. This could be accomplished by removing the lower tree branches, reducing the height of the shrub, or both. Trees planted in defensible spaces should be far enough apart that a fire cannot travel across tree canopies. This separation is especially important around structures like houses.