What Reaction Wood Is
When a tree grows at an angle — because it has been displaced, is growing on a slope, or a branch is carrying significant load — it produces abnormal wood on one side to counteract the displacement. This is called reaction wood, and it is the tree's mechanical response to gravitational and wind stress. Reaction wood has a different cell structure, different chemical composition, and different mechanical properties from normal wood on the opposite side of the trunk.
There are two types. Compression wood forms in softwoods (pine, spruce, cedar) on the underside of leaning trunks and branches. It is dense, brittle, and shrinks far more along the grain than normal wood when dried. Tension wood forms in hardwoods on the upper side of leaning trunks. It has a gelatinous layer in the inner cell wall and produces woolly, torn surfaces when machined.
How to Identify Reaction Wood
Compression wood in softwoods is typically denser and darker than surrounding normal wood, with wider growth rings on one side of the end grain cross-section. In sawn boards from leaning trees it often appears as a zone of different colour and texture on one face.
Tension wood in hardwoods is harder to identify visually. The clearest indicator is a woolly or hairy surface after planing — the gelatinous cell wall layer tears rather than cutting cleanly. Finish adhesion problems and blotchy stain absorption also point to tension wood surfaces.
Why Reaction Wood Is a Problem
Compression wood shrinks dramatically along the grain during drying — up to ten times the longitudinal shrinkage of normal wood. A board containing both normal and compression wood bows severely as the compression wood side contracts more than the normal side. This bow can develop during drying or after installation when the timber is exposed to any moisture change.
Structurally, compression wood is brittle in tension and fails at lower stress than normal wood, with less warning. In a structural beam, a zone of compression wood on the tension face is a serious weakness. Tension wood causes machining and glue-bonding problems that are difficult to resolve without specialised tooling or surface treatment.
- Compression wood (softwoods): extreme longitudinal shrinkage — severe bow during and after drying
- Compression wood: brittle in tension — fails at lower stress, with less warning
- Tension wood (hardwoods): woolly machined surface — affects finish quality and glue bonding
- Both types: eccentric ring pattern visible on end grain
- Both types: present in any board cut from a leaning or curved log section
Practical Management
For structural timber, boards containing significant reaction wood zones should be downgraded or excluded from applications where the defective zone falls in a critical location. Structural grading rules address this through grain slope limits — reaction wood has severe local grain deviation — and through checks on bow and warp after drying.
For furniture timber, boards with visible reaction wood should be used in short component applications where bow is not significant, or in painted applications where surface quality is less critical. Reaction wood in glued panels can cause cupping and splitting at glue lines — identify and exclude it from high-value panel production.