A tree does not wear one bark for its whole life, and the same species can look quite different at ten years and at a hundred.
How bark changes with age
Why bark cracks
Bark is a protective layer made partly of dead cells. As the trunk grows wider, the outer layers cannot stretch, so they crack, split into plates, or peel. The pattern the cracks make is what gives mature bark its character.
Young bark
Saplings of most species have thin, smooth bark with visible lenticels. Young oaks, ashes and maples are hard to tell by bark. Young cherries and birches show horizontal lenticels from the start, which helps.
Middle age
Furrows begin low on the trunk and spread upwards. Red maple stays smooth for a long time before breaking into dark plates. Shagbark hickory does not show its shaggy plates until the tree is well grown.
Old trees
Ridges deepen and become coarse, plates thicken, and the bark may split into large blocks. Ponderosa pine is a good example: young trees have dark, furrowed bark, and old trees develop broad orange plates. Sugar maple becomes deeply furrowed with long plates that curl at the edges.
Smooth for life
A few trees do not change much. American beech stays smooth, and so does quaking aspen until very old. Look at the whole group of trees in the same stand to judge.
Where the tree grows
Bark thickness and texture also depend on how a tree lives. Trees in open sun often have thicker bark than those in dense shade. Fire-adapted trees such as ponderosa pine and longleaf pine have thick bark to protect the growing layer under it. If you can, check bark on trees of different ages next to each other, which is the quickest way to learn the sequence.