BDCS Notes - Structure of Wood

Growth Rings: The concentric layers in the stem of exogenous trees (also called tree rings or annual rings). One growth ring is produced per year.

Growth rings are composed of early wood, which is produced by rapid growth during the spring, and latewood from summer growth.

Latewood: dense, dark, and thick-walled cells producing a stronger structure than early wood.

Predominant physical features of wood:

1. Bark

2. Cambium

3. Wood

4. Pith

Bark: The exterior covering of the tree and has an outer and an inner layer. The outer layer is dead and corky and has great variability in thickness, dependent on species and age of tree. The inner bark layer is the growth layer for bark but is not part of the wood section of the tree.

Cambium: Thin layer of cells situated between the wood and the bark and is the location of all wood growth.

The Wood Section of the Tree: Composed of sapwood and heartwood. Sapwood functions as a storehouse for starches and as a pipeline to transport sap. Fast growing species have thick sapwood regions. Heartwood is not a living part of the tree. It is composed of cells that have been physically and chemically altered by mineral deposits. It provides strength for the tree. Since it doesn’t contain sap, it is resistant to decay.

Pith: Central core of the tree. Its size varies with the tree species ranging from barely distinguishable to large and conspicuous. Colors range from blacks to whitish, depending on species and locality. Pith can be solid, porous, chambered or hollow.

Anisotropic Nature of Wood: Wood is anisotropic. This means it has different and unique material properties in each direction. The three axis orientations in wood are longitudinal/parallel to the grain, radial/cross the growth rings, and tangential/tangent to the growth rings. These different orientations affect physical and mechanical properties such as shrinkage, stiffness and strength. The anisotropic behavior of wood is the result of the tubular geometry of the wood cells. Their growth in one direction, and their flexure upon moisture, causes the change in physical and material properties.