Over the last years ATIBT published 16 data sheets to share some clarifications related to wood science.
The ATIBT has decided to repeat this sharing. Each week, we'll be tackling one of the topics covered in the various data sheets. The news will be accompanied by a number of freely-available bibliographical references to help you delve deeper into these complex subjects.
This Friday, we're talking about reaction wood.
Reaction wood is a chemical and physical modification of wood cells, enabling them to respond to a major imbalance suffered by the tree (sloping ground, unidirectional wind, etc.). Throughout its growth, the tree adjusts its shape to maintain its verticality. This is due to peripheral tensile stresses that compensate for the tree's growth imbalances.
There are two types of reaction wood. Firstly, there's tension wood, seen in hardwoods, and compression wood, seen in softwoods.
Tension wood :
In hardwoods, reaction wood manifests itself by pulling harder on the upper face of the inclined axis than on the opposite face. This highly stressed, pre-stressed wood is defined as tension wood. At the cell level, there are several modifications, including a reduction in the number of vessels, longer and more cellulosic fibers...
Compression wood :
In softwoods, reaction wood manifests itself by pushing harder on the underside of the tree's inclined axis. This highly compressed, prestressed wood is defined as compression wood. At the cell level, there are several modifications, including shorter cells, thicker walls and more lignification...
Consequences of using reaction wood :
The use of reaction wood is complicated by the fact that drying deformations are much greater than in normal wood. Reaction wood can lead to localized breakage and deformation (warping, bending, etc.), but also to collapse, due to very significant dimensional shrinkage. During peeling, for example, the cells of reaction wood crush under the knives, giving a fluffy surface effect.
The formation of reaction wood is a much more complex mechanism, the fruit of numerous physiological mechanisms linked to internal and external factors.
For more detailed information on this subject, please refer to data sheet number 1, as well as these four bibliographical references on reaction wood and, more generally, on tree growth mechanisms.