How trees wall off decay: Shigo, CODIT and what extension services say

This page describes what four published sources say about how trees respond to wounds. It does not tell anyone how to cut or treat a tree. The guide on tree biology gives the wider background, and the guide on wound dressings reports what other sources say about covering wounds.

The idea in Shigo's article

Alex L. Shigo, a Forest Service researcher, wrote that animals heal but trees compartmentalize. His article, in a 1985 copy on the Forest Service site, says trees cannot move away from attackers, so over evolution they became highly compartmented, walling off injured and infected wood. It says a tree makes no repairs and instead defends itself against the effects of injury and infection by setting boundaries, while at the same time putting new cells in new positions each year, in effect growing a new tree over the old one.

He and his colleagues named the model CODIT, for Compartmentalization of Decay in Trees. The article describes four walls. Walls 1, 2 and 3 are, in large part, strengthenings of structures already in the wood at the time of injury: wall 1 resists vertical spread, wall 2 inward spread and wall 3 lateral spread. Wall 4 is raised later by the cambium and separates infected wood from the healthy wood forming after the injury. The article calls wall 1 relatively weak, since a tree cannot survive if it plugs all its vertical plumbing, and calls wall 4 quite weak as a structural support but strong as a barrier against microorganisms. It adds that wall 2 is moderately strong and wall 3 the strongest of the first three, and that if wall 3 fails decay can spread like an opening fan, which it names as the cause of hollow trees. Oxford researchers are cited for the finding that cells of the wall 4 barrier are lined with suberin.

Reaction zone and barrier zone

The Forest Service General Technical Report NRS-97, Wood Decay in Living and Dead Trees: A Pictorial Overview by Shortle and Dudzik, June 2012, summarizes Shigo's work with 96 photographs and illustrations. It says a protective layer in sapwood that was already present at wounding is a reaction zone, and one in altered sapwood made after wounding is a barrier zone. It says the two are chemically the same but the barrier zone's structure differs, which makes it more resistant to the spread of infection.

Colorado State and Penn State

Colorado State University Extension's gardening notes describe wood as a series of boxes framed by annual growth rings and ray cells. They say a tree does not replace damaged tissue and that cells in the damaged area change chemically to seal it off, an area called the reaction zone, usually darker wood. Their four walls:

Wall 1: very weak up and down the xylem tubes, with decay moving downward readily, possibly twenty or more feet and into the roots.
Wall 2: rather weak toward older rings at the center.
Wall 3: ray cells, somewhat resistant, which may help suppress spread around the tree.
Wall 4: new rings grown after the injury, highly resistant.

Penn State Extension says the walls made at wounding are mostly chemical and internal, with a physical barrier between wood formed after the wound and older wood. It says wall 1 comes from plugging vessels with tyloses or gums, wall 2 forms in latewood, wall 3 in living ray cells, and wall 4, the barrier zone, from cambium cells, with callus tissue having the strongest resistance. It calls wall 1 the weakest and wall 4 the strongest. It says oak, hickory, sugar maple and redwood are strong compartmentalizers, while silver maple, cottonwood and willow are weak ones that try to outgrow the decay column and may become hollow. It also says sealing wounds with tar or oil-based paints does not stop decay and that oil-based sealants can inhibit closure by damaging callus tissue and woundwood.

Where the sources differ

Wall	Shigo	Colorado
Wall 2	Moderately strong	Rather weak
Wall 3	Strongest of 1 to 3	Somewhat resistant
Wall 4	Strong barrier, weak support	Highly resistant

The pages also use different terms. Colorado State describes the walls partly by the rings and ray cells that frame them, Penn State by the tissues that form each wall, and NRS-97 separates the reaction zone from the barrier zone.