Tree Decay and Resistograph Testing

Heartwood decay, or heart rot, is the occurrence of decay in living trees. It could be defined as decay that develops primarily in the heartwood or inner wood of living trees, and usually refers to decay that primarily develops in the trunk or stem rather than in the roots or butt of the tree.

Cause / Symptoms
Decay of the heartwood in a tree is caused primarily by the enzymatic activities of microorganisms. Fungi are - for all practical purposes - the only microorganisms that cause the decay of wood. There are other kinds of deterioration caused by insects and UV, but this is not truly ‘decay’ and typically is not as important as decay. Trees have several mechanisms of resistance against decay fungi, with bark being their first line of defense. None of the heart rot fungi will infect trees above ground through intact bark. Fungi are able to invade the heartwood of a tree primarily through bark wounds caused by animal damage, lightning strikes, treefall scars, storm damage, vehicles, carving, hatchet marks, mowers, string trimmers, and improper pruning.

Decay develops and progresses slowly. A tree that becomes infected will have heart rot for the rest of its life, with the condition only worsening as time goes by. The extent of decay is important to know. Hazardous, decayed trees should be removed.

Determining the Extent of Decay
A Certified Arborist can often determine if a tree is a potential hazard by examining the tree and the site. Looking from the ground is not always enough. Getting a view from the crown of the tree may be necessary, requiring a climbing crew or the aid of a aerial lift truck. In some cases decay is apparent, however it is impossible to determine the extent of decay solely by the appearance of the tree. When our Arborists are concerned about the extent of decay in a tree but cannot determine this by visual inspection, we use a tool called a Resistograph.

The Resistograph is a vital tool in performing hazard-tree evaluations. This tool has a tiny drill bit that measures 12 inches long and 1/64th of an inch in diameter. As the tool drills into the tree, the resistance against the bit is measured and displayed on a graph. Through analysis of this data, we can accurately measure the amount of decay in a tree and provide the client a proper hazard evaluation. In the past, trees thought to be hazardous were often removed only to find minimal amounts of decay.

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