Wood Science: Heartwood and Sapwood
When you look at the cross section of a trunk of a typical hardwood tree, it’s divided into three circular regions. The outermost circle is the bark, which encloses the wood of the tree trunk or bole. Lying inside the bark is an area called the sapwood, usually creamy white to yellow in colour. It’s responsible for conducting sap from the roots to the crown of leaves, as well as storing sugars for future use by the tree. Finally, we reach the heartwood, which is darker in colour and surrounds the central pith of the tree. Softwood trees have the same three regions, but they’re sometimes less pronounced.
Young, immature trees are almost all sapwood. These young saplings are concerned only about growth. They’re competing with plants for resources so the maximum flow of nutrients from the roots to the leaves takes precedent as they grow taller. Once they grow taller and thicker, the conversion of the sapwood into heartwood begins.
Zebrawood’s heartwood has a wide range of colours in it, unlike many domestic woods (cherry, maple, walnut, etc.) that we’re familiar with. In the right situation these colours and patterns can be used to accentuate a piece of furniture.
Pale Wood is Less Common
Even, light-coloured woods are often used to provide contrast in a piece with other darker woods, or are used when a less overpowering look is needed. It’s relatively uncommon to find large pieces of lumber that are light and even in colour.
Lots of Contrast
Cherry (left) and ziricote (right) have darker heartwood and paler sapwood. While most larger furniture manufacturers see the sapwood as a defect, many smaller-scale furniture makers see this as an opportunity to create a strong visual.
Include Both Heartwood and Sapwood
Though bubinga heartwood is mostly deep red in colour and the sapwood is a paler beige, the two colours complement each other nicely. You can see the sapwood on the right side of this table top. Including it in the piece of furniture is a personal choice each woodworker can make.
Why is heartwood darker?
The heartwood is created by the deposition of chemicals called extractives into the existing sapwood cell walls. These extractives are responsible for the dark colour of the heart. They also provide protection from fungi and insects. The heartwood becomes the spine of the tree, providing support as it grows in size. There are no cells added to or taken away from the heartwood. Consequently, there is little difference between the sapwood and the heartwood in terms of strength. The structural difference is mostly biochemical due to the added extractives.
There is a lot of variation among trees when it comes to the amount of sapwood and heartwood present. Some trees, such as catalpa, are basically all heartwood with the sapwood only a few growth rings wide. The maples have large areas of sapwood. Eastern white cedar appears to be all sapwood, but its extractives are clear and colourless, making the heartwood invisible to the naked eye.
There is some evidence that the amount of sapwood is influenced by the growth rate of trees. Fast-growing trees tend to have more sapwood as the tree needs the ability to pull as much sap as possible to the leaves. I would expect trees grown on a plantation to have more sapwood than trees grown in the wild as plantation trees grow faster. Heartwood is a different story. It takes time for the heart to develop the rich colours woodworkers appreciate. Older walnut trees will show deeper, darker, more appealing colouring in their heartwood than younger trees.
Dark or light?
Most woodworkers prefer to work with the heartwood of hardwood species. Woods like cherry, mahogany and walnut have long been desired for the colour present in their heartwood. In most species, the heartwood colour is usually uniform with not a lot of variation. Other species can exhibit pigmented figure. In these trees, colour is deposited randomly, resulting in some striking visual effects that mimic clouds, spider webs and zebra stripes. The best examples of pigmented figure are seen in species such as ziricote, Brazilian rosewood, figured redgum and zebrawood.
Of course, there are times when the project you are designing calls for light-coloured sapwood. Choices are usually limited to maple, birch and ash. These are large trees capable of producing decent-sized lumber fit for furniture projects. Other species are usually limited in use by size and hardness. White lumber is hard to produce. It is prone to insect damage and can suffer from fungal staining if not dried properly. Expect to pay a premium for the whitest material.
How about both?
The use of sapwood or heartwood is not an either/or decision. Species where the contrast between the heartwood and the sapwood is sharply demarcated are often used by adventurous woodworkers for visual appeal. This includes such well-known woods as rosewoods, ebonies and species such as padauk. Here, the variation in colour between the heart and sap woods creates turned objects and furniture with unique visual appeal.
On a more practical level, whenever there is colour variation there is potential for waste. If you like to work with cherry and walnut, your waste factor will be higher, because you’ll have to cut away the sapwood when milling the lumber. It’s important to plan your project carefully to avoid waste. Some species, such as beech and walnut, are steamed to even out the colour between the sap and heart woods in order to minimize waste. However, the resulting colour may not meet the needs of a particular project.
In the end, it makes no matter to the tree what colour its heart is; the tree just wants to grow in the most economical way possible. If the heartwood can provide support, buffet strong winds and fight off insects and fungi, that’s all it needs. Likewise, a healthy sapwood will keep its foliage well nourished. It’s a different story for woodworkers. When we look at the varieties of heartwood and sapwood, we see nothing but creative possibilities.