Reaction wood is formed in response to mechanical stress, and helps to position newly formed parts of the plant in an optimal position. Reaction wood forms when a woody plant part is subjected to mechanical stress. This stress may be the result of wind exposure, excess of snow, soil movement, etc. The reaction wood appear as asymmetric growth. The cambium in the affected part of the trunk is more active on one side, leading to thicker growth rings. Branches practically always have reaction wood, since they need support to maintain their horizontal or nearly horizontal position. There are two different types of reaction wood, which represent two different approaches to the same problem by these groups of plants:

• In angiosperms reaction wood is called tension wood. Tension wood forms above the affected part of the plant, pulling it up. It is composed almost entirely of cellulose.

• In conifers it is called compression wood. Compression wood forms below the bent part, pushing it up. Compression wood is rich in lignin.

As a rule, reaction wood is undesirable in any commercial application, primarily as its mechanical properties are different. Reaction wood is hard, brittle and it breaks the uniformity of timber. It also responds differently to changes in moisture.