How Trees Seal Wounds Instead of Healing

A cut on your skin heals by replacing damaged tissue. A tree works differently. Once wood is wounded it stays wounded, and the tree survives by building walls around the damage. Understanding this changes how you prune, how you protect trunks and how you read an old scar.

Urban green space, from the I Hug Trees image library

Walling off, not repairing

In the 1970s, the American scientist Alex Shigo described how trees defend themselves against decay. When a branch breaks or a trunk is cut, the tree forms chemical and physical barriers in the wood around the wound. This process is now known as compartmentalisation, or CODIT, short for compartmentalisation of decay in trees.

The barriers limit how far decay can travel up, down and sideways. Over the following seasons, the tree grows new wood outside the wound, which forms the rolled edge of tissue, called callus or wound wood, that you can see around old cuts.

This is why a tree with a large hollow can still be alive, green and standing. The living layer under the bark keeps working while the wood inside slowly rots, and many old trees carry cavities that shelter owls, bats and insects.

Why clean cuts matter

The tree can only seal what is left in the right place. A cut just outside the branch collar lets the tree's natural defence zone do its work. A flush cut that removes the collar, or a long stub, makes the defence harder, and decay can spread into the trunk.

Rough, torn bark from falling branches or from a poorly made cut is also harder to close. Take a few minutes to make clean, correctly placed cuts, and the tree will do the rest.

Timing helps as well. Pruning in the dormant season lets the tree start sealing the wound as soon as growth resumes, and avoiding wet weather cuts the chance of spores landing on a fresh cut.

Do not paint the wound

It may feel caring to cover a wound with paint or tar, but research has found that wound dressings seldom help and may trap moisture. In most cases, arborists leave pruning wounds open and let the tree seal them.

The better protection is prevention. Keep mowers and string trimmers away from the trunk with a mulch ring, guard young trunks against vehicles and avoid nailing or wiring anything to the tree.

Reading old wounds

Mushrooms or shelf-like brackets on a trunk, cracks, cavities and oozing patches show that decay is present. That does not always mean the tree is unsafe. Many old trees carry decay for decades and remain strong, because most of their strength lies in the outer wood.

If a tree beside a path or a house shows such signs, ask a qualified arborist to assess the risk instead of guessing. For a broader look at how trees survive and cooperate underground, read the forest internet, and for why old, large trees are worth keeping in a city, see urban forestry and green living.