n. The portion of a tree’s limbs, buds, and foliage that is killed from heat during a fire.
Crown kill is the portion of a tree’s crown that is killed by heat transferred by means of either convection or direct flame contact during a fire. It includes not only the foliage, but also buds and limbs, thereby making it impossible for that area of the tree to regenerate or recover from the damage. It is usually expressed either as a percentage of the pre-fire crown volume or as a percentage of pre-fire crown length. If an area of a tree’s crown is killed by fire, the needles not consumed by direct flame contact remain on the tree branch until they eventually weather off by wind, rain, or snow.
For most species, crown scorch and crown kill are the same. However, some species have thick buds (for example, longleaf pine and ponderosa pine) or woody structures around the buds (such as western larch) that help protect the living tissue from heat injury. Other species, such as redwoods and many hardwoods, can also recover from crown scorch by means of epicormic sprouting from dormant buds. In these species, it is possible to have crown scorch without crown kill. For the species with thick buds, this lack of crown kill is often evidenced soon after a fire by a thin area of the crown, usually just below any remaining green foliage. If there is a section of the crown that seems to have lost most of its needles, the foliage in that area was likely scorched and killed, but the buds were not killed. Therefore, it is possible for this section of the crown to recover from the fire; the tree limb is still functional and is able to shed dead needles through abscission. On the other hand, dead limbs cannot abscise needles, so they remain on the limb. This difference between crown scorch and crown kill can be more easily seen immediately after the following spring bud break. Areas of the crown that were killed will remain brown, whereas areas that were scorched but are alive will have tufts of green at the end of limbs where the current needle flush is occurring.