Reaction wood is part of a tree that grows in reaction to external stresses. Almost all tree branches have some form of this abnormal wood as a means of resisting the pull of gravity, although it can be more pronounced in branches that have to bear some excessive burden, such as heavy snow. Reaction wood also appears in trees that are leaning, either due to growing on uneven ground or from the strain of strong winds or shifting soil. It is denser than the wood in other portions of the plant, and this can result in problems for woodworkers as it can cause warping and other irregularities that can make the wood very difficult to work with.
There are two types of reaction wood: compression and tension. Compression wood occurs in softwood trees, or conifers. This form of reaction wood forms underneath the pressured area and pushes against the strain that is affecting the tree. It is rich in lignin, which causes it to be very hard and brittle.
Hard woods, also called angiosperms, create tension wood in reaction to stress. Tension wood forms above the pressured area and pulls against the strain. It contains a high amount of cellulose, which can make the wood prone to shrinkage and can prevent the surface of the wood from feeling smooth.
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Though reaction wood is relatively uncommon in pieces of lumber, when a piece of wood does contain some, it can often be difficult to identify. Significant warping can be a sign of reaction wood, as can the presence of small fibers on the surface of the wood. Severe cracking can also indicate that a piece of lumber contains some of the abnormal wood. Not all of these traits will always be readily apparent, however, so quite often woodworkers will only realize that a piece of wood contains reaction wood after the piece has already proven troublesome.
Despite the difficulties in identifying reaction wood, spotting it is important since it can pose many problems. Since its density is different from the wood around it, it will tend to absorb stains differently, which can lead to uneven coloring. Also, it is usually more prone to cracking, especially when under strain from nails or screws. It reacts differently to moisture, which can lead to excessive warping and a general lack of uniformity throughout the wood.