Reaction Wood in Trees

Trees have an amazing ability to find the necessary resources for survival.  Whether it is finding a way to penetrate their roots into a water line or surviving in a crevice in a large rock, trees have been able to overcome challenges to grow and sometimes thrive in harsh conditions.  Trees also have an uncanny ability to find the sunlight they need for photosynthesis to take place. 

Trees are often thought of as being tall and straight and in a forested setting, and most trees do tend to be relatively straight. It is a result of growing in a scenario where there are many trees competing for the same resources. In such crowded conditions, the only available sunlight is often directly overhead, so the trees grow straight up in an attempt to outcompete other trees for the sunlight. 

Sometimes trees will begin the grow crooked if there is a source of sunlight that becomes available that is not directly overhead. This is often evident when a forest is found at the edge of a field, or when a large tree blows over or dies, creating an opening in the forest. A crooked tree could also be the result of wind partially uprooting the tree. When a tree is growing on a lean, it creates wood that has different properties from normal wood. This wood is called reaction wood.

Reaction wood is thought to be a response by the tree to get it back to a vertical orientation and to assist in maintaining the angles of branches. There are several theories about what triggers the creation of this type of wood, including gravity, stress differentials in the stem of the tree, and hormones; however, nothing has been definitively proven. 

There are two types of reaction wood that occur in trees. Conifers create compression wood. The name compression wood is given to this wood because it is found on the underside of the lean. In hardwoods, it is called tension wood and is found on the side of the tree opposite the lean. In both types of reaction wood, the presence of eccentric growth rings (rings showing an off-center pith) is an indication that reaction wood is present.

The properties of reaction wood are quite different than normal wood. In conifers, compression wood causes the tracheids (the main type of cell found in softwoods) to be more oval in shape than normal wood, and there is often space between the cells. The walls of the cells are layered differently in a way that causes the structure to be weaker than normal wood. The walls of the tracheids also have helical checks (or cavities) that weaken the cells.

In hardwood species, tension wood is mainly characterized by a gelatinous layer (G-layer) within the cell wall of the fibers. The G-layer is a sheath of cellulose microfibrils within the cell wall that is loosely attached to and partially separated from the other layers of the cell wall.

How does reaction wood affect the lumber that is sawn from it? One issue is that the longitudinal shrinkage (shrinkage along the length) of a board containing reaction wood is greater than normal. In compression wood, longitudinal shrinkage could be as much as six times more than normal wood. In tension wood, the longitudinal shrinkage could be as high as three to five times higher than normal wood. Those amounts may seem insignificant, but just a .05% increase in longitudinal shrinkage equates to .25 inches for every four feet in length. In normal wood, the longitudinal shrinkage is about .04 inches for every four feet in length. This increase in longitudinal shrinkage causes boards to exhibit an increase in warp, thus making the lumber undesirable or even useless in extreme cases. 

In tension wood, the weak and often separated G-layer in the cell wall causes a fuzzy appearance on the surface of the board. During the sawing process, these fibers are partially torn out rather than being cut clean. Sometimes the fuzziness does not even go away when the board is run through a planer. During the drying process, tension wood has a higher tendency to collapse, which can lead to surfaces that are uneven. Tension wood can also cause an uneven appearance when a board has stain applied to it during the finishing process.

Reaction wood can be a real problem to end users of lumber, but crooked trees can still serve an important purpose in our forests. If the tree is of a desirable species, it could be saved as a future seed source or to provide food and cover for wildlife. Maybe it is a species that is rare in the forest, then again, maybe it would be a suitable candidate for firewood. The decision is up to you and your management goals.