CODIT (Compartmentalization of Decay in Trees)
One of my favorite hobbies is watching old war movies. I find submarine movies particularly interesting. Many a night I had a nightmare about serving on a submarine only to be attacked by some enemy force. Naturally, I’m in that area of the submarine that has been damaged and now we have to make those fateful decisions concerning which doors to seal to prevent water from flooding more compartments. The first time I heard about CODIT, I immediately thought of torpedoes, submarines, people screaming, explosions, you get the picture 😉
Thanks primarily to over 15,000 tree dissections performed by Alex Shigo (Chief Scientist, U.S. Forest Service, 1959-1985), we have a much better understanding of how trees react to injuries. When I’m using the term “injury” I’m referring to wound or opening in which the protective bark layer has been removed (i.e. pruning cuts, openings after branch shedding, collisions with automobiles, lightning strikes, etc). Trees respond to injuries by sealing the wounded area through a process called compartmentalization. You and I respond differently to injuries. For example, a cut on the skin will be filled-in with new skin cells. A similar injury to a tree trunk would result in surrounding cells changing themselves chemically and physically to prevent the spread of decay and new cells would be produced by cells lining the cut area to cover and seal the injured area. In other words… trees seal, we heal. From this day forward, remove the term “heal” from your vocabulary when talking about trees. I’m serious. If I ever catch you saying trees heal I’ll wash your mouth out with tree paint
The compartmentalization process results in the formation of a six-sided compartment, like a box, consisting of four walls since some walls come in pairs. Walls 1 and 3 each consist of two walls, while Walls 2 and 4 each consist of one wall. These walls are made either by existing xylem or recently produced xylem from the vascular cambium. These barriers, or walls, have been numbered 1 through 4 based on their ability to resist the spread of decay, with Wall 1 being the weakest and Wall 4 being the strongest.
Wall 1 is formed by the physical plugging of xylem (tracheids and vessel elements) above and below the injury (two walls). Wall 1 may or may not be present at the time of the injury. Wall prevents the spread of decay upward and downward in the trunk of the tree.
Wall 2 is already present at the gardening time of injury. Wall 2 consists of the annual rings of xylem. Of the two types of cells making up the annual rings, it is the summer wood with its thick cell walls which provides the best resistance to the inward spread of decay.
Wall 3 is also already present at the time of injury. Wall 3 consists of the parenchyma rays located on either side of the injury (two walls). Wall 3 prevents the lateral spread of decay in the tree.
A note about Walls 2 and 3… even though Walls 2 and 3 are already present at the time of the injury, both walls undergo chemical changes which allow them to better resist the spread of decay. These chemical consist mostly of special decay resistant compounds produced by the chemical transformation (oxidation and polymerization) of phenolic compounds in Angiosperms or terpenes in Gymnosperms. And while we’re at it, it’s safe to say that Walls 1 and 4 also may undergo chemical and physical changes for the same reasons.
Wall 4 consists of the new xylem that is produced by the vascular cambium after wounding to prevent the outward spread of decay. If a tree trunk has sustained numerous trunk injuries, it is possible that the original tree diameter becomes hollow with only Wall 4 continually growing to form a cylinder to support the tree. It is true that Wall 4 is the most resistant to the spread of decay among the other 3 walls; however, there are some drawbacks to Wall 4. Wall 4 is structurally weak. The presence of Wall 4 predisposes that portion of the tree to ring shakes and other cracks (radial cracks and closure cracks) or other structural defects [A ring shake is a separation between annual rings… get it? “Ring” shake?]. Wall 4’s new wood is DIY Safety produced separate from the wood it is resting upon so that the new parenchyma rays are not connected to the rays in the older wood, this means it is no longer possible to translocate sugar between the old xylem and the new xylem of Wall 4. Stored starch in the old xylem is stuck and no longer available to the tree and may now be made available to decay causing microorganisms.
How do they rank?
Here are some trees that are considered to be poor compartmentalizers:
Aesculus, Betula, Celtis, Fagus, Persea, Pinus virginiana, Populus, Prunus, Quercus nigra, Q. shumardii, Q. palustris, and Salix.
Here are some trees that are considered to be good compartmentalizers:
Acer rubrum, A. saccharum, Castanea sativa, Juglans, Pinus ridida, Quercus macrocarpa, Q. rubra, Q. virginiana, Robinia pseudoacacia, Ulmus Americana
So what’s more dangerous… internal decay or cracks? Alex Shigo believes that cracks are more hazardous than internal decay. Naturally, I agree with Mr. Shigo!
Harris, R. W., J.R. Clark, & N.P. Matheny. 2004. Arboriculture: Integrated Management of Landscape Trees, Shrubs, and Vines. Fourth Edition. Upper Saddle River, New Jersey: Prentice Hall.