Timber Biology

Sapwood v Heartwood — the secret life of trees

The first thing to understand about timber is some basic tree biology. This will form the basis of everything to do with timber and will help you as you discover some of our other articles.

You may remember from high school that trees are basically big bundles of straws. These straws are used to transport water and nutrients throughout the tree, but also help provide its structural strength.

When trees grow, they add new layers of straws every year or so to the outside of the tree, just under the bark. These layers of growth are visible as rings when you look at the end of a log and they can be used to roughly calculate the age of the tree. Accordingly, the oldest wood is in the centre of the tree and younger wood is found toward the outside of the tree.

In fact, it’s only the very young wood (only a year or two old) that actually does the transportation throughout the tree. We refer to this young wood as “Sapwood” and it is clearly visible as a pale ring of timber just under the bark in the diagrams below.

All of the older rings of straws toward the middle of the tree are used to store all the by-products of photosynthesis like lignin, tannings and resin, and it’s this that forms what we call “Heartwood” or “Truewood” (the darker timber in the photos).

Notice the difference between the relative proportions of Sapwood to Heartwood between Hardwood and Pine? There is obviously much more Sapwood in the pine, likely allowing them to grow so fast.

This distinction between Sapwood and Heartwood is very important to understand, because the physical properties of the two types of timber are very different in two critical ways:

Sapwood is never durable (regardless of the species), but CAN be effectively treated; and

Heartwood can be durable (depending on the species), but CANNOT be effectively treated.

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