Trees do not heal the way people or animals heal.

When a tree is wounded, decayed, pruned, cracked, or damaged, it cannot replace injured wood with new living tissue in the same location. Instead, a tree responds by compartmentalizing the damage.

This process is known as CODIT, which stands for:

Compartmentalization of Decay in Trees

The CODIT model was developed through the work of Dr. Alex Shigo, a pioneering tree pathologist whose research changed the way arborists understand tree wounds, decay, pruning, and structural risk.

At the Fort Hunt Tree Observatory, CODIT is one of the key concepts used to interpret real-world tree defects in the field.

When decay enters a tree, the tree does not “heal” the damaged wood.

Instead, it attempts to wall off the affected area and limit the spread of decay into surrounding wood.

This is why some hollow trees can remain standing for many years, while others with less visible damage may become structurally unstable.

The difference often depends on:

– Species
– Wound size
– Location of the defect
– Amount of sound wood remaining
– Root and trunk structure
– Presence of included bark
– Canopy load
– Site exposure
– How successfully the tree has compartmentalized the damage

Dr. Shigo described tree response using four “walls.”

These are not literal walls like concrete or brick. They are biological boundaries formed by the tree to resist decay spread.

Wall 1 helps slow decay from moving up and down through the tree.

This is often the weakest wall, which is why decay columns may extend vertically inside a trunk or stem.

Wall 2 helps slow decay from moving inward toward the center of the tree.

This boundary is formed by growth ring structure.

Wall 3 helps limit decay from spreading around the tree from side to side.

This is often a stronger boundary than Wall 1 or Wall 2.

Wall 4 is the strongest wall.

It forms after the injury as new wood grows outside the damaged area. This is why you may see wound wood, callus tissue, ribs, seams, or rolls forming around an old wound.

Wall 4 separates wood that existed before the injury from wood that formed after the injury.

Understanding CODIT helps explain why tree defects are not all equal.

A tree with visible decay is not automatically unsafe.

A tree with no obvious cavity is not automatically safe.

The question is not simply:

“Is there decay?”

The better question is:

“How has the tree responded to the decay, and how much functional structure remains?”

That is where field observation becomes important.

At Fort Hunt Tree Observatory, tree observations may include:

– Species identification
– Diameter at breast height
– Estimated height
– Root flare condition
– Trunk defects
– Included bark
– Co-dominant stems
– Fruiting bodies
– Cavities
– Wound wood response
– Sounding response with a mallet
– Canopy dieback
– Epicormic growth
– Soil conditions
– Slope and exposure
– Past pruning wounds
– Evidence of decay columns

These observations help connect Shigo’s CODIT model to actual trees growing in Fort Hunt and surrounding neighborhoods.

A silver maple with a large pruning wound may develop decay behind the wound.

Over time, the tree may produce wound wood around the damaged area. If the wound wood is strong and the decay remains contained, the tree may continue to function for years.

However, if decay spreads into a major union, root flare, or load-bearing stem, the risk picture changes.

That is why CODIT is not just a science concept.

It affects real decisions about pruning, monitoring, cabling, removal, and preservation.

Dr. Alex Shigo helped move arboriculture away from the idea that trees “heal” like animals.

His work showed that trees are highly organized living systems that respond to injury through boundaries, barriers, and new growth.

Modern arboriculture, proper pruning, decay assessment, and tree risk evaluation are all influenced by this work.

For the Fort Hunt Tree Observatory, Shigo’s CODIT model provides a foundation for understanding how trees respond to injury in the real world.

Suggested intro text above video:

In this video, Dr. Alex Shigo explains how trees respond to wounds, decay, and internal injury. His work remains one of the foundations of modern arboriculture.
 

The goal of the Fort Hunt Tree Observatory is to document real trees, real defects, and real biological responses over time.

Rather than treating each tree as an isolated jobsite, the Observatory looks at patterns:

– How different species respond to wounds
– How decay presents in mature trees
– How pruning wounds close over time
– How included bark affects structure
– How site conditions influence decline
– How compartmentalization succeeds or fails

This creates a living field record of tree biology in Fort Hunt.

CODIT helps us understand that trees are not passive objects.

They respond.

They defend.

They adapt.

But they also have limits.

By studying those responses in the field, the Fort Hunt Tree Observatory connects the science of Dr. Alex Shigo with the living trees of Fort Hunt, Virginia.