Understanding Timber Defects from Tree Growth

The question asks about a specific type of defect that occurs in timber as a result of a branch breaking off while the tree is still growing. Understanding these defects is important for evaluating timber quality and its suitability for different uses.

Let's look at the options provided and determine which one fits the description of a defect caused by a broken branch during the tree's growth process.

Analyzing the Options for Timber Defects

• Knot: A knot is essentially the base of a branch that has become embedded in the trunk wood as the tree grows. When a branch is living and growing, it forms a sound knot. If a branch dies or breaks off during the tree's growth, the tree's trunk wood continues to grow around the stub. This results in what is often termed a "dead knot" or "loose knot" if the stub is not well integrated. This definition aligns closely with a defect caused by a broken branch during tree growth.
• Shake: Shake is a separation of wood fibers along the grain. This separation can occur between growth rings (ring shake or cup shake) or across the grain (split shake). It is usually caused by strong winds, frost, or internal stresses in the tree, not specifically by a broken branch.
• Rind gall: Rind gall is a patch of bark that has been damaged and subsequently healed over, sometimes with a swelling or distortion in the wood beneath. It is typically caused by mechanical injury to the bark, not directly by a broken branch breaking off, though it could potentially occur near a past branch injury.
• Burl: A burl is a large, rounded outgrowth on a tree trunk or branch, often with a highly distorted grain pattern. Burls are usually caused by stress, injury, viral or fungal infections, or insect infestations, leading to an abnormal growth response. While injury can be a cause, it's not specifically defined as a defect from a broken branch like a knot is.

Identifying the Defect from a Broken Branch

Based on the analysis, the defect specifically caused by the tree growing around the stub of a broken branch is known as a knot.

When a branch breaks off, the tree's natural growth process involves the new wood layers (annual rings) forming around the remaining part of the branch or the scar. This encased branch stub or its remnant forms a knot in the resulting timber.

Summary of Timber Defects

Therefore, the defect in timber due to a broken branch of the tree during tree growth is a knot.

Revision Table: Common Timber Defects

Additional Information on Timber Defects

Timber defects can significantly impact the strength, durability, and appearance of wood. They are broadly classified into natural defects (like knots, shakes, burls, caused by tree growth or environmental factors) and artificial defects (like checks, splits, warp, caused during seasoning or conversion). Understanding these defects helps in selecting appropriate timber for various construction and woodworking purposes.

Knots are one of the most common defects. Their impact depends on size, type (sound or loose), and location in the timber piece. While small, sound knots might be acceptable or even desired for aesthetic reasons in some applications, large or loose knots can weaken the timber structurally.