Many people believe there are two layers in a tree’s trunk: the outer bark they can see and the inner ‘wood’ that supports a tree. In fact, there are five layers in a tree trunk and the two layers most people know about are both actually dead.
The outer bark that some people like to carve into and write cliché messages like “Me+Y.L.=Love” is a tree’s protection layer from bugs and insulation from weather. Just inside the outer bark is the inner bark, also known as the phloem. This layer transports nutrients throughout the tree. The phloem soon dies and the older phloem becomes part of the outer bark. The cambium layer is the growing part of a tree’s trunk. You have probably heard that the age of a tree can be determined by a cross section of a tree trunk. This is correct, and the annual rings are formed by this layer. As long as a tree is receiving nutrients, hormones are formed, thus new bark and wood are produced. Sapwood is the new wood that the cambium layer forms. This layer transports water to the tree's leaves. Older layers of sapwood die and turn into the next layer, heartwood. Heartwood is the center of a tree trunk and will not decay or lose its extreme strength as long as the other layers of a tree are intact and functioning.
This is a tree located in the middle of the Student Services Center (down the stairs from Croutons) and shows a tree’s response to pruning. At one time a branch here was pruned. Notice that the clean cut is inside the outer bark. How did this happen? There are actually four layers that formed after the pruning occurred. The first three layers are known as compartmentalization. The first is something like a band aid, plugging cells directly around the wound (yes-pruning trees create an injury!). It protects the tree from further injury from infection and decay. The second layer consists of cells that form each year and act as guards against any infection entering past the first defense layer. The third layer resists lateral spread, using physical and chemical defenses. The final layer is known as a barrier zone. New wood that contains natural fungicides is formed. This wood is harder and stronger due to 1) an altered arrangement of cells, 2) an increase in axial parenchyma, which provides more energy to the injured tissues, and 3) the cells acquire toxins to kill off any invading organisms. This fourth layer is what you see directly in front of the pruned section of this tree.