Bark is the skin of a tree. It protects the stems, or trunk and branches, of a tree from heat and cold, disease, insects and other animals, disease, fire, and drought.
In maples, young trees and newer growth on older trees have smooth bark (top photo). The trunk of middle aged and older trees is rough, ridged, and wrinkly (bottom photo). Sometimes it’s a darker color too.
Like your skin, hair, and nails, the bark of a tree is made mostly of dead cells. Cells on the surface are always sloughing off, and cells below them move up to take their place. A layer of tissue at the bottom of the bark is constantly cranking out new cells.
As a tree grows, the diameter of its trunk and branches will increase, but the bark does not get thicken very much over the years. The bark of a mature tree is about 2 inches wide.