REACTION WOOD The wood of greatest commercial value comes from the straight main stem of the tree. Wood from branches, apart from not being sufficiently straight, has considerable growth stresses in it due the formation of reaction wood. Reaction wood is formed when the tree is subject to prolonged structural stress, such as leaning trunks or continual exposure to strong winds. It is associated with eccentric growth of wood, with more tissue being formed on the upper side of the leaning stem or branch of hardwoods (called tension wood) and on the lower side of softwoods (called compression wood). In many tropical hardwoods, tension wood is a feature mainly emanating from leaning or crooked stems. Its presence is revealed during sawing by one most characteristic feature, namely the extreme wooliness or furriness of the sawn longitudinal surfaces. In tropical softwoods, compression wood may cause drying problems, but difficulties in conversion are less severe than the tension wood of hardwoods. Spiral Grain The helical orientation of the grain can be caused by environmental factors as persistent strong wind but more often a genetic factor. (Important check for tree breeders). Knots A knot on a tree trunk. Source Qld Forestry ForEd Project 1982-1985 When a tree is very young it is covered with limbs almost, if not entirely, to the ground, but as it grows older some or all of them will eventually die and are either broken off or fall off. Their bases may become overgrown and enclosed by subsequent layers of trunk wood, forming a type of imperfection known as a knot. Subsequent growth of wood may completely conceal the stubs which will however remain as knots. Since in most uses of wood, knots are defects that weaken the timber and interfere with its ease of working and other properties. Knots affect the technical properties of the wood, usually reducing tension strength, but may be exploited for visual effect. Wood knot in vertical section. Source Qld Forestry ForEd Project 1982-1985
14
