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Free lessonDomain 1: Tree Biology · lesson 4 of 48

CODIT and wood's response to stress

How trees wall off decay after wounding, how wood grows in response to load, and how that shapes management by developmental stage.

Key points

CODITCompartmentalization of decay in trees (Shigo): four walls limit how decay spreads after wounding.
Walls 1–3Reaction zone in existing wood: 1 vertical (weakest), 2 inward, 3 sideways (rays).
Wall 4Barrier zone made by the cambium after wounding (strongest); new wood outside it stays sound.
Response growthWood is added where load is greatest; compression wood in conifers (lower side), tension wood in hardwoods (upper side).
Life stageYoung: structural pruning. Mature: small cuts, defects. Veteran: gradual reduction, keep habitat.

Summary

WallFormed byResists spread
1Plugged vessels/tracheidsUp and down (weakest)
2Latewood of growth ringsInward
3Ray cellsSideways
4Cambium, after wounding (barrier zone)Outward into new wood (strongest)

Quick check

In the CODIT model, which wall is the strongest?
  1. Wall 1, plugging vessels
  2. Wall 2, the growth rings
  3. Wall 3, the rays
  4. Wall 4, the barrier zone
Answer: D. Wall 4 is formed by the cambium after wounding and separates old wood from new.
Which CODIT wall resists the spread of decay sideways around the tree?
  1. Wall 1
  2. Wall 2
  3. Wall 3, formed by ray cells
  4. Wall 4
Answer: C. Rays run radially, so they limit tangential (sideways) spread.
Compression wood forms:
  1. In conifers, on the lower side of a lean
  2. In hardwoods, on the upper side of a lean
  3. In conifers, on the upper side of a lean
  4. In all trees, at the root collar only
Answer: A. Hardwoods form tension wood on the upper side instead.

On the job

  • On a cut log with decay, find the boundary of the old wound and the sound wood outside it.
  • Spot a leaning tree and look for extra growth on the lower or upper side.
  • For three trees on a job, write down their life stage and what that means for your cuts.

Video transcript

Trees don't heal wounds; they seal them off. In this lesson: the four walls of CODIT, how wood grows in response to load, and how both shape your recommendations.

CODIT stands for compartmentalization of decay in trees, the model developed by Alex Shigo. After wounding, the tree forms walls. Wall one plugs the vessels above and below the wound; it's the weakest. Wall two, the latewood of the growth rings, resists spread toward the center. Wall three, the ray cells, resists spread sideways. Wall four, the barrier zone, is new tissue the cambium makes after the wound, and it's the strongest.

The first three walls form a reaction zone in the wood that existed at the time of wounding, and decay can breach them. Wall four separates that old wood from all the wood grown afterward, so a tree can be hollow inside yet add sound new wood outside. That's why a large decay column isn't always a failed tree, and why good cuts at the branch collar matter.

Trees add wood where they carry load. This is response growth. On a leaning conifer, compression wood forms on the lower side, pushing the stem up. On a leaning hardwood, tension wood forms on the upper side, pulling it up. Both are types of reaction wood. Swellings, ribs and buttress flares show you where a tree has been working hardest.

Match your recommendations to the tree's developmental stage. Young trees respond vigorously, so build their structure: a single dominant leader and well-spaced branches. Mature trees need maintenance with small cuts and removal of defects, because large wounds compartmentalize poorly. Over-mature and veteran trees are reduced gradually, if at all, keeping as much living crown and habitat as possible.

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