Tension wood is a type of wood which forms in angiosperms in response to environmental stresses. For the tree, the purpose of tension wood is to help the tree stay stable, and to keep the tree upright. This type of wood is not useful for people who work with wood to make flooring, furniture, and other products, because it has an irregular texture. Tension wood can, however, be pulped to make products such as paper and cardboard, in which the structure of the wood is not as critical.
Tension wood is a form of reaction wood. As the name implies, reaction wood forms in reaction to something. Common stresses to trees can include persistent high winds, accumulations of snow, changes in terrain which cause a tree to list, and any kind of stress which puts pressure on the tree. The tree responds by producing reaction wood to stabilize itself.
In angiosperms, the reaction wood forms above the area of the stress. It is rich in cellulose, and helps to pull the tree into position. Tension wood is especially common around branches, as branches require extra support to stay in position. In conifers, reaction wood forms below the area of the stress, pushing the tree up from below with wood which is rich in lignin. The reaction wood is called compression wood in conifers.
There are a number of characteristics which set tension wood apart from the rest of the wood in the tree. One of the key features is that the texture is different, and highly irregular. It tends to be denser, and it creates thicker growth rings. One of the problems with tension wood is that it can be hard to spot casually, and may make a board behave strangely in a saw or during planing. Reaction wood can buckle, split, and fracture apparently at random, making it very frustrating for woodworkers.
This wood also does not absorb paint, stain, and other treatments in the same way that regular wood does. When tension wood is accidentally included in something which is going to be treated, it can look different from the surrounding wood after treatment, being lighter or darker as a result of its slightly different composition.
Mills try to process wood which is obviously reaction wood separately, to avoid releasing it to woodworkers and others who do not like to work with tension wood. However, because it is tricky to spot, it sometimes ends up in a batch of lumber sold for use by people who may puzzle over a randomly capricious board or two: the tension wood will cause the board to warp, absorb moisture differently, and splinter or crack when cut.