What if you still had all the cuts and scrapes from your childhood accidents? How healthy would you be if these injuries to your skin had never healed?

Unlike humans, trees do not heal from damage. Instead, they build walls around the damage and move on.

In the case of trees, when we think of injuries, there are four primary places a tree can be injured — the roots, the stem, the branches and the foliage. Of the four, only the foliage is not woody, so damage to the foliage fits in a different category.

The woody parts of a tree can be damaged in a number of ways. Roots can be damaged either from above or below the soil line. For example, a bulldozer can cut through the soil and damage roots or it can drive over the soil and break and smash roots. The stem can be damaged by cutting or scraping, when something collides with it or when a climber with spikes treats it like a telephone pole. The branches can be injured similarly; cutting and scraping can happen to branches through natural causes when the wind or gravity causes other branches to rub against them. Pruning itself also causes injury to branches.

Each injury to the woody parts of a tree invites decay, which leads to decline and failure in either part of or all of the tree. When injured, which is basically inevitable, what defense system does the tree have to fight decay?

Alex Shigo (1930-2006) developed and popularized the Compartmentalization of Decay in Trees (CODIT) model for understanding how trees compartmentalize damage. The CODIT model states that instead of eliminating the damage, trees build walls around it. Trees build four walls to fight the spread of decay and understanding these walls gives insight not only into the seriousness of injuries in trees, but also gives a window into basic tree biology.

Wall one is a vertical wall. The tree must stop the invasion of decay from using the its own nutrient transport system as a highway to spread up and down the tree. This wall is the tree’s weakest defense. Very often decay can spread several vertical feet from the injury. Wall two, on the other hand, is horizontal. The tree is designed to stop decay from spreading across its rings. Wall three is less obviou but stronger. Trees not only have structures along the vertical and horizontal axes, they also have rays that penetrate the rings. While the rings can help prevent decay from penetrating into the tree, the rays effectively stop decay from moving radially around the it.

The final wall is the new wood produced by the tree in response to the injury. Wound wood, which is the strongest defense against decay, is also the tree’s weakest wood from a structural standpoint.

The moral of the story is, take an active role to keep your trees from injury.

Joshua Arp is an ISA-certified Municipal Specialist, a Clarks Summit Tree Commissioner and an operator of a landscape maintenance business. He can be reached at josarhuap@aol.com.