Trunk | Branches | Roots | Wound Response

Trunk
In order to properly prune a tree or to repair damage to the trunk requires a basic understanding of tree structure. A cross section of a tree trunk reveals it is composed of many layers (Figure 1). Each year a tree essentially grows a new "coat of wood" over the older wood. The outside layer of the tree is dead bark which provides protection from the environment. The inner bark layer is composed of live tissue that transports food downward. Between the bark and wood is the cambium layer which is responsible for increases in tree diameter (by creating annual rings) and responds to injury by producing callus tissue.

The annual rings of wood are composed of large pores that carry water up to the leaves. Each annual ring is essentially a vertical cylinder. The outer 4 to 20 annual rings (referred to as sapwood) are usually alive and light-colored. Wood in the center of a large tree (referred to as heartwood) is composed of dark-colored, dead cells used for storage. Ray cells cut across the annual rings; they distribute food to living cells.

Branches
Branches are attached to the tree trunk by interlocking branch and trunk tissue. A new layer of interlocking tissue is produced each year over the previous layers. A woody branch collar, produced by the trunk, holds the branch base. When branches on the main trunk that have a narrow angle increase in diameter they eventually run out of room to grow. The branch bark becomes surrounded by woody trunk and branch tissue. The bark that becomes overgrown is referred to as included bark (Figure 2). The union is weak and likely to split.

Leaves on every branch must produce enough food to feed itself. Food does not move from roots or other branches to supply a starving branch. Branches unable to support themselves are sealed off. Branches on the interior of a shade tree that do not receive adequate light will die and eventually fall.

Roots
Tree roots develop and survive where there is adequate oxygen and moisture. Most active tree roots are in the top 3 feet of soil; the majority are in the top 12 inches (Figure 3). The more compacted or poorly drained the soil the closer the roots are to the soil surface. Roots grow most of the year, stopping only when soil temperatures are cold. They occur as perennial woody roots and as annual absorbing roots. Woody roots become thicker each year; absorbing roots die but are replaced by new absorbing roots. Annual absorbing roots form shallow, horizontal fans that take-up water and nutrients. A few woody support roots grow downward and outward to anchor the tree in place. Most trees do not have a deep tap root. While a tap root may develop on trees growing in the woods in well-drained soils, they generally do not develop on trees transplanted into the landscape or on trees grown in compacted or poorly drained soil.

Roots normally grow outward to about three times the branch spread. Only 50 percent of the trees root system occurs between the trunk and the dripline. Roots on one side of the tree normally supply the foliage on the same side of the tree. When the roots on one side of the tree are injured the branches on that side of the tree may die back or drop. With some trees, such as maple, the effect may develop anywhere in the tree canopy.

Wall 1 is formed by plugging the vertical vascular system vessels following an injury. It is the weakest wall but can slow the vertical spread of decay. Wall 2 is formed at the outer edge of a growth ring. It is a weak barrier but does offer resistance to inward spread of decay. Each growth ring is subdivided into compartments with a radial wall (Wall 3). It is the strongest of the three walls and provides resistance to lateral spread. It presents a maze of physical obstacles as well as a chemical barrier. Wall 4 is formed by cambium growth after an injury. It is the strongest of all the walls. Internally, it separates the wood present at the time of injury from new wood formed as the tree grows. Externally, callus tissue develops around the injury and should eventually cover it by growing over the dead wood.

Some tree species can activate Walls 1, 2, and 3 very rapidly and maintain them so effectively that the amount of decay is limited. When a tree responds slowly or the walls are weak --- infection can damage a large volume of wood.

A tree branch has a branch bark ridge, often referred to as a branch collar, that separates the branch from the tree trunk (Figure 5). The collar is the swelling located at the base of a branch where the branch meets the trunk. The callus that forms the collar is an area of tissue that contains a chemically protective zone. The natural decay of a dead branch stops when it reaches the collar. When pruning a dead branch, do not create a new wound by cutting into the ring that forms around the dead branch.

Prepared by: Erv Evans, Consumer Horticulturist, NC State University

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