The bark of trees, called periderm, consists of different layers crucial in keeping a tree alive. The most outer layer of the periderm, the cork cambium, produces cork that protect trees from bacterial and fungal infections.
As trees mature, the faster growing wood on the inside of the tree pushes against the slower periderm on the outside.
In some trees like the American beech the periderm just stretches and remains relatively smooth when the tree gets older. However, in most trees the periderm breaks apart and becomes inactive, and a new inner layer called the active periderm is formed underneath.
The reaction to the stress caused by the expansion of the inner wood is different for each tree species resulting in different characteristics and structures of the inactive or dead outer bark (rhytidome) and can vary depending on the age of the tree.