Reaction Wood

Specialised wood tissue formed mechanically by a tree in response to gravitational stress, trunk lean, or wind exposure to restore its structural equilibrium.

Reaction wood is a highly specialised anatomical tissue developed by trees as a direct mechanical response to physical displacement, such as a lean caused by slope instability, prevailing winds, or asymmetric canopy lighting. Its biological purpose is to generate internal mechanical forces that push or pull the stem back toward a vertical growth alignment. The way this tissue forms represents one of the most fundamental evolutionary splits between tree types.

Broadleaved trees produce tension wood, which develops on the upper side of the leaning trunk or branch. Tension wood fibres contain a high concentration of pure cellulose and suffer from gelatinous layers that physically contract, pulling the stem upright.

Conversely, conifers produce compression wood on the lower side of the lean. Compression wood is characterised by heavily lignified, thick-walled cells that resist crushing forces, effectively pushing the stem back up.

For wood technology and timber processing, reaction wood introduces severe structural challenges. It exhibits abnormal longitudinal shrinkage during drying, leading to warping, twisting, splitting, and structural weakness in processed boards. Recognising reaction wood in standing timber helps foresters assess structural integrity and evaluate the eventual sawlog quality.