Fuel ladder

Fuel ladders are live or dead vegetation that allow a fire to climb up from the forest floor into the tree canopy.

Common ladder fuels include tall grasses, shrubs, and tree branches, both living and dead. To remove the ladder requires pruning low limbs up to a minimum of 8 feet, and potentially as high as 15 feet. How high up depends on how low the branch tips hang, the steepness of slope, and the height and spacing of other nearby vegetation. The desired result is that a low-burning fire could burn to the trunk of a tree, which is of course protected by its bark, without having thinner, more flammable branches, leaves or needles within easy reach of the fire.

Apart from tree limbs, anything that would help that fire move up into the tree canopy is a ladder fuel. This includes shrubs and even tall grass or weeds. It's also worth considering non-vegetation fuel sources such as woodpiles, wooden fenceposts and structures. The point is to have some break in vertical and horizontal continuity so that, if for example a woodpile caught fire, it isn't positioned next to manzanita bushes or directly under trees that could then easily catch fire.