n. The amount of bark from the surface of the tree bole to the cambium layer.
Bark thickness refers to the amount of bark around a tree from the outside surface to the cambium layer and is primarily determined by tree species, age, and size. Bark thickness can also vary around a tree (see below illustration), depending on bark characteristics or past damage (such as that resulting from fire, animals, or mechanical damage). Bark insulates the cambium from heat during a fire. Therefore, trees with thicker bark are more resistant to cambium injury from fire than are trees with thinner bark. Approximately 20 minutes of heat exposure per one square inch of bark is required to kill a tree’s cambium (Ryan 1982a).
Bark thickness is most often measured using a bark thickness gauge. This tool penetrates the bark until the wood interface is reached. The user can then read the bark thickness from the gauge’s scale. Some trees have deeply furrowed bark; therefore, bark thickness will vary depending where on the tree bole it is measured (see below illustration). The ridges of furrowed bark help to protect the underlying cambium from heat injury because the bark is thick. However, in the bark furrows where the bark is very thin, the cambium is very susceptible to heat injury.