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.
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 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.Tree roots tend to avoid each other when young, but as they grow they may be forced together and from a graft union. These grafts can conduct diseases from one tree to another.
Trees have a natural defense response to wounds and pruning cuts. They form four types of walls to compartmentalizing the area thus preventing the spread of decay organisms (Figure 4). The decay or injury remains but is sealed off and does not increase in size if the walls are stronger than the decay organisms. The storage capacity and function of the injured part is lost forever.
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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