Sapwood is the white or cream colored ring on the outer zone of a tree. Its cells are live cells that conduct water movement throughout a tree.
Heartwood starts to form in trees 14-18 years old. It is the dark colored area in the center of a tree. The size of the heartwood varies by species and growing conditions. The heartwood contains extractives. Extractives are chemically produced from storing sugars and starches. The extractives are what give heartwood its different properties: color, odor, decay resistance, it may be difficult to penetrate with liquids (pressure treating) and to dry, and may have a slightly higher weight. Heartwood and sapwood have the same strength properties even though heartwood might be slightly heavier. Different species produce different extractives. This is why some species have different properties than others.
How does heartwood form?
Even with all of this knowledge about heartwood we still don’t know why or how it forms. There are different theories about this involving lack of oxygen, excess sugars/food production, lack of water to the tree’s interior and physical blockage of passage ways.
Maple and Birch species have false heartwood. This is developed around wounds or injuries that the tree may have. The dark color develops from bacteria. Don’t worry, this bacteria is eliminated during the kiln drying process. Sometimes it can give the wood unique characteristics like dark streaking around bug and worm holes or possibly even holes from tapping Maple for syrup.
Maple, Basswood, Aspen are a few species that are desired for the white or cream like color of the sapwood. Some species like Oak, Walnut, and Cherry are desired for their heartwood. The naturally dark color provides the look that some people want. Sometimes the heartwood is desired for its odor (Cedar) or decay resistance (Cedar, White Oak). Heartwood and sapwood can also be used together to provide a natural contrast of color. This can make a piece of furniture truly one of a kind.