Corrugated fibre, knots, bends, gouges - what growth defects are and how they affect wood properties

We like rustic furniture more when we discover a cracked knot, and we particularly appreciate curly-fibered rosewood veneer or poplar bubo poplar tabletops. These are actually tree growth defects, which, not being commonly encountered, are all the more appreciated. The problem is that the defects generally lead to changes in the characteristics of the wood and sometimes to reduced strength. It is good to be aware of them in order to use the wood in areas where the defect is not a nuisance or can be converted into quality, thus avoiding loss of wood mass.

How growth defects occur and what they are

Defect is any deviation from normal growth, structure, composition and appearance of the tree. The reasons for these irregularities are related to the characteristics of the species, growing conditions or accidents during the growing period. Some species are more prone to a defect than others. For example, the occurrence of curly fibers is quite common in palms, while in other species it is the exception.

Growing conditions have a strong impact on tree development. Important are the area where the tree or forest is located, whether it is growing in woodland or singly, extreme weather conditions. A tree growing on a steep slope is more likely to change trunk shape than one growing on flat ground. This is a common defect in larch, a species of resinous tree that likes mountainous areas. Changes in light in a forest that has been heavily cleared can lead to irregularities in the annual rings. If it suddenly receives more light the ring deposited that year will be wider. Very high temperatures, frost before dormancy, dry periods and strong winds all affect the tree's development and can lead to defects.

A short story about dry spells. I have a fir tree planted in front of my house for over 20 years. It has grown hard, city pollution, scarcity of soil and water trying hard. Two years ago the water shortage was even more acute. It was the beginning of the pandemic, and I had made the "office" in the library, at the window in front of which the tree is, so I could watch its growth easily. The tree budded and the young branches began to develop starting at the base. The sap reaching the top was too little for normal development and the top grew small and limp. In a few days, one of the laterally developed branches began to rise, gradually taking the place of the top which became a side branch. It was a wonderful lesson in survival and continuity. In terms of growth defects, however, there is trunk discontinuity in that place, and certainly, inside, the fiber is deflected.

There are also other types of growing injuries that can lead to defects. For example, resin pockets in the interior of pine wood are the result of wounds on the trunk. As the wood mass grows and settles, the resin-protected wounded area is drawn inwards and is revealed when the wood is cut. Clogged bark or knots are also developmental accidents.

The main growth defects are:

• shape defects of the trunk
• defects related to the wood structure
• nodes
• the cracks
• abnormal coloring.

Shape defects

Shape defects refer to the trunk because it is the main exploited part of a tree. In mature trees suitable for harvesting, the trunk accounts for 60% of the total volume in deciduous trees and 85% in resinous trees. A trunk is more valuable the straighter and more uniform it is. Any deviation from an imaginary vertical axis at 90° to the horizontal leads to a decrease in processing yield, loss of wood mass and a decrease in the qualities of wood used in construction. Such trunks are mainly obtained from forests, less so from single elements. Forests must be properly managed, and old trees must be removed to avoid falls that deform other trees by leaning against them.

Curvature is the curved deviation from the trunk axis. There may be a single deviation along the entire length of the trunk or several, in the same plane or in different planes (the trunk appears veined). It is an important defect that lowers the compressive strength of wood used in construction. A pole made from such a log may yield to the compressive forces generated by the construction and should therefore be avoided. The cutting of logs with curvature results in a loss of wood material, and the resulting pieces may warp or crack on drying due to the fibers not being parallel to the axis of the piece. A special case of bending is getting tired, a curvature in one plane at the bottom of the trunk. It is a specific defect lariței.

Taper and lăbărțarea are deviations of the trunk from its cylindrical shape. Taper is the abnormal decrease in trunk diameter from base to tip. A taper above 1% is considered abnormal. The calculation is made by dividing the difference between the base and tip diameter by the length of the stem. The defect decreases the efficiency of the flow or unwind to obtain veneers. Barnacles is the abnormal and sudden thickening of the trunk measured one meter from the base. Pieces cut from this area have a bowed grain and deform on drying.

The trunk normally has a circular cross-section. Ovality is the deviation from this shape and is a defect when it is very pronounced. In this case there are cutting losses and the resulting pieces have inhomogeneous wood due to the different width of the annual rings.

Canelura is also a deviation from the circular shape of the cross-section and represents a veiling of the cross-section. The wavy shape of the circumference is also transmitted to the annual rings. It is a common defect in carpen, but is also found in yew, anin or juniper. The defect poses problems in the process of making cylindrical wood, which is also used in construction for round wooden houses or veneers, and the cutting process produces parts with sloping fibers and low compressive strength.

The rage is the division of the trunk into one or more main branches that develop separately. In that area, the wood has concho-concreted hearts (a structural defect) and a sunken bark, making it irregular in structure.

Structural defects

Structural defects are also growth defects, which are often the consequences of form defects, but also defects which have no exponent on the outside of the tree, being related only to the structural elements of the wood. When used in furniture or decorative objects, most of these defects are transformed into qualities, contributing essentially to the success of the work.

Irregular rings is the most common structural defect and occurs due to different growing conditions from year to year. It is manifested by different width of annual rings seen in cross-section. The greater the differences, the more inhomogeneous the wood and the more difficult to predict behavior.

Curly or crimped fiber is one of the defects sought by wood lovers. It occurs in hardwoods, especially frasin and paltin. It is called a defect because it is an abnormal growth, but it is only an exception, the wood being even superior to ordinary wood, both aesthetically and in terms of properties. The curly spruce used in making musical instruments is such a happy exception.

Inclined fiber is the deviation of the fiber and annual rings from the longitudinal axis. It is the result of trunk defects (tapering, tapering, curvature) or of flow along an inclined plane. Such parts have low compressive strength and are difficult to sand, as the fiber is rough. Inclined-fiber elements should not be used in strength structures for houses.

Twisted fiber is the regular helical deflection of fibers around the trunk axis. It is a regular deflection because the fibers remain parallel to each other. On the tree, the defect can be recognized by cracks in the bark that are inclined in the same direction. It is a low-quality wood that is difficult to work, has low mechanical strength and the pieces warp a lot when drying. Specific especially to single-grown trees.

Tangled fiber is an irregular local deflection of the structural elements of wood. It occurs in so-called gâlme or bube and it's very spectacular. It occurs due to irregular cell development and is considered a kind of cancer, a tumor of the tree. In the gully area, the wood no longer follows the usual structure of the species and is uneven, difficult to work and difficult to sand to obtain a smooth and fine surface. But the unique beauty of the resulting piece far outweighs the effort.

Compression wood appears on the resinous wood and consists in the thickening of the late wood in the annual ring and its dark reddish-brown coloration. The affected wood has a different structure and properties from normal wood because it contains more lignin. Swelling and shrinkage is more pronounced, it is harder and less resistant to tensile strength. It cracks and warps severely on drying. Although it is harder, its use in construction is not recommended because of the marked dimensional variation.

Eccentricity is the lateral deviation of the medulla from the center of the trunk. It is a consequence of ovalization and is also accompanied by unequal annual rings. Building elements have low strength and cut elements deform and crack on drying.

Heart of the heart is the result of tree splitting and occurs in all species. The wood from the splitting zone is inhomogeneous, highly deformed and difficult to process.

Knots and cracks

Nodes are twig debris trapped in the wood and are the most common defect. In the knotty area the wood is difficult to process, has different properties and strengths and sometimes needs to be removed. Softwood has more and smaller knots than hardwood. There are fixed and falling knots, healthy and diseased knots. Subject wood knots has been developed at length in another article which you can find here here.

Cracks are discontinuities in the wood mass and occur in both trees and processed wood. The best-known cracks that occur during tree growth are the galls and the roll.

Frost frostbite or frost crack occurs along the trunk and forms in years when the frost came before the tree entered its dormant period (early frost), when the wood is still full of water. It occurs mainly in hardwoods - oak, nuc, ulm. Gelivura can be open, with bumps on the bark due to scarring growths, or closed, inside the tree.

Rulura is the detachment of woody tissue at the edge of the annual ring. It commonly occurs in molid, brad or chestnut and it is the result of very strong winds that cause the trunk to bend very much and so the rings slide on each other. The defect is favored by the presence of uneven rings.

Abnormal coloring

Abnormal colorations are those that differ from the normal color of the species and may occur locally or throughout the stand. They do not normally lead to significant changes in the strength of the wood. The best known are false heartwood and lunula.

The false peach is a different coloration that appears in the central area of the trunk and is specific to species that do not have a color difference between sapwood and heartwood. It is found in ash (brown heartwood), and in palms (blackish heartwood), plop and you (gray heart), oak (red heart). Such colorations can make a piece of wood unique and highly prized.

Such false duramen is commonly seen in fag and it's called red heart of the beech. It shows a light red to reddish-brown coloration in the central area of the cross-section, with purplish or bluish-gray iridescence. The shape is irregular, it does not follow the annual ring, the coloration is uneven and is delimited by a darker line. With steaming the color becomes more uniform, but differs little from that of beech without red heart. Beech with a red heart impregnates and takes longer to color. It has low elasticity and is therefore more difficult to bend, with the risk of cracking.

Lunura is observed in the cross-section of the trunk and consists in the presence in the heartwood area of annual rings of sapwood color and consistency. It is the result of early frost, the affected wood no longer matures, does not turn into heartwood and remains with the characteristics of sapwood. This area is softer and rots more easily as it is water-hungry. This is a defect specific to oak. Avoid using this wood in outdoor projects.

Bacterial and fungal attack can lead to false coloration, but this usually occurs after the tree is felled and is not considered a growth defect. This is the case with wood blue stain which you can read more about here.

Through the holes and galleries they make, insects also cause defects, but their attack on wood occurs mainly after felling, and is even found in finished objects. Woodlice are such insects and you can find information about them here here. It is also possible that the wood may be attacked in the standing trees, but then we are dealing with diseases that need to be treated in order to avoid total destruction of the forest. One such situation was a few years ago, when chestnut trees of the edible variety were attacked and almost died out. I will return shortly to insects that attack wood, which is a vast and important area.

I hope you find the above information important. If you have any additions, questions or clarifications, please leave them below in the space provided. I will certainly reply.