Why is teak heartwood weather-resistant while the sapwood is not?
Diametric growth in tree trunks occurs just beneath the bark in a tissue called cambium. Cambium cells constantly divide, forming an external layer that becomes bark and an internal layer that joins the woody tissue of the bole.
The bark not only protects the trunk, but, in its innermost layers, closer to the cambium, also channels the sugar and proteins produced in the leaves to the various other parts of the tree (roots, shoots, flowers, fruits, etc.).
The cell layers added to the woody tissue act initially to channel the water and nutrients absorbed or intercepted by the roots, to the leaves. As time passes new layers are added to the woody tissue and take charge of channeling the sap. The older, inner layers, in turn, start to receive deposits of lignin and other substances that increase the mechanical and biological resistance of the cells and darken their coloration. These modified layers taken together are called heartwood.
The outer rings of cells of the trunk or bole, which are usually of a lighter tone, are called sapwood.
In a healthily growing teak tree, it takes from five to six years for the sapwood to turn into heartwood, which is why long growth cycles are required for the production of wider heartwood boards and planks with wider heartwood.
In natural forests, where there is stiff competition between trees, it can take 200 years or more for teak to reach commercial dimensions.
In traditional Southeast Asian plantations, this period, named rotation, varies from 60 to 120 years.
At Cáceres Florestal, thanks to good soil quality and, above all, the intensive management the trees receive, it has been possible to shorten this period to a mere 30 years.
This is the minimum timeframe for the production of larger quality heartwood pieces. Studies carried out at teak plantations in Nigeria have shown that shorter rotation periods tend to produce woods of lower density, resistance and durability.
Teak heartwood is sought for and valued for its high resistance to fungi and insects, including termites, and is most commonly used in outdoor products constantly exposed to the elements, such as ship, boat and pool deckings as well as garden furniture.
While teak sapwood is equally stable, easy to work with and provides a comparable finish, it is not as resistant to fungi and insects as teak heartwood and its use should therefore be restricted to indoor utilities, such as furniture, panels or even flooring.
