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Discoloration and decay

Discoloration in sugar mapleThis column of discoloration in sugar maple started because of the death of the low, large branch. It joins with another column, higher in the tree, that started because of the death of another large branch. Wood formed after the branches died remains free from discoloration. Photo from: Shigo, A.L., & E. Larson.

The information below is intended to help identify the difference between natural colors in wood and discoloration or decay. If a tree’s wood seems to have discoloration and decay, the information here can also help you understand why that might be happening and what to do about it.

Biology

Discoloration and wounding

Discoloration in trees that have naturally-white wood throughout starts because of a tree wound and sometimes by the death of large branches. Wounds expose wood to air, moisture, bacteria and fungi. Discoloration can be caused by bacteria-oxidizing phenolic compounds and by fungi. While there is no proven link between soil chemistry and the size and color of columns of discolored wood, there is a strong link between discolored wood and wounding.

Discoloration and compartmentalization

Discoloration and decay DO NOT move at will throughout a tree as it ages. The compartmentalization of discoloration and decay in trees (referred to as CODIT) is a natural process that starts after a tree is wounded. Compartmentalization is a boundary-setting process that usually limits the spread of discoloration and decay within a tree to the tissue present at the time of wounding.

Signs and Symptoms

Genera with naturally-colored heartwood include cherry, oak and walnut.

Genera with naturally-white wood throughout include ash, aspen, basswood, beech, birch and maple.

HEARTWOOD DISCOLORED WOOD
Triggered by aging Starts due to wounds or the death of large branches
Influenced by genetics Influenced by wound severity, microorganisms. Vigor?
Starts from the center of the tree, goes outward Starts from the outside, goes inward
New tissue affected yearly Tissue formed after wounding is rarely affected
Oldest tissue affected first Youngest tissue affected first
Continuous In columns
Throughout tree Varies
Mineral content lower Mineral content (K, Ca) higher
Usually circular in cross section Any pattern
Microorganisms not necessarily involved Microorganisms involved

Prevention

The most important way to minimize discoloration and decay is to minimize the number and size of wounds on the tree. In northern hardwood stands:

  • understand the link between stand damage and resulting loss in volume;
  • establish a damage assessment procedure; establish a minimal acceptable level of damage (in general, wounds on northern hardwoods that exceed 20 percent of stem circumference are highly likely to have discoloration and decay along with them.)
  • train personnel in directional felling techniques; mark crop trees;
  • establish a reward system for operators who do a good job; operator pride and skill are critical;
  • do not conduct harvest operations during spring breakup;
  • operate on wet sites during the driest season or on frozen ground;
  • extraction trails: lay them out in advance, in a herring bone fashion, slightly wider than the extraction vehicle; make them as straight as possible; log them first; fill in hollows with brush;
  • use buffer or bump trees (cull trees) along trails; remove them at end of harvest;
  • use extraction equipment based on the size of trees being removed, the topography of the site, the spacing of the crop trees and the soil type;
  • maintain a fully stocked stand to minimize the chance of sunscald injury and encourage self-pruning; and
  • favor single-stemmed sprouts located low on the stump. They have the best chance for survival and of being free from discoloration.

Cracks

Tree defects (cracks)Photo from: Shigo, A.L., 1983. USDA Forest Service.

  • When a tree is wounded, dead cambium results. Over time, the wounded area is covered with bark and callus tissue.
  • A ring shake commonly forms where the callus tissue meets the wound.
  • A radial crack or wound crack commonly originates at the ring shake.
  • Differential shrinkage in tissue exposed to low temperatures can expand the wound crack out to the bark surface. This is commonly referred to as a "frost crack."
  • Cracks almost always originate at the site of a wound.
  • The number and size of cracks do not appear to be correlated to the amount of discoloration and decay.

Tapholes

Tapholes in sugar maplePhoto from: Walters, R.S. & A. Shigo, 1978. USDA Forest Service.

  • When a taphole is drilled, the tree undergoes chemical changes. These changes, along with bacteria and fungi, cause discoloration and decay of the wood.
  • Discoloration and decay will be limited--through compartmentalization--to anything from a small area surrounding the taphole to a long vertical column, pointed at both ends and centered at the taphole.
  • Trees less than 12" in diameter should not be tapped.
  • Tapholes should be spaced so that no new hole is drilled closer than six inches horizontally or two feet vertically from the nearest old open taphole.
  • Overtapping can cause columns of discoloration and decay to coalesce and encourage rapid advance of decay.
  • Tapping trees that have poor vigor can slow taphole closure and cause death of cambium around the hole.
Last revised: Wednesday July 15 2015