The following was adapted from The Wood Handbook: Wood as an Engineering Material
Look under the bark and you’ll be treated to a dizzying array of growth rings documenting a tree’s growth from sapling to full-fledged tree — but curiously, you’ll also notice something else — two distinct bands of wood.
In both softwoods and hardwoods, the wood in the trunk of the tree is typically divided into two zones, each of which serves an important function distinct from the other. The outer, active portion of the tree in which cells are alive and metabolically active is referred to as sapwood.
A more broadly applied definition is that sapwood is the band of lighter colored wood adjacent to the bark. In contrast, heartwood is the darker colored wood found to the interior of the sapwood.
In the living tree, sapwood is responsible not only for the titular conduction of sap, but also for storage and synthesis of biochemicals, which primarily take the form of starch and lipids. These starch grains are stored in the cells of the sapwood and can be easily seen with a microscope.
The starch content of sapwood can have important ramifications in the wood industry. For example, in the tropical tree ceiba (Ceiba pentandra), an abundance of starch can lead to growth of bacteria that produce ill-smelling compounds that can make the wood commercially unusable.
In the southern yellow pines of the United States, a high starch content encourages growth of another organism, sap-stain fungi, that, though it does not affect the strength (or odor) of the wood, can nonetheless decrease the lumber value for aesthetic reasons.
Living cells of the sapwood are also the agents of heartwood formation. The living cells at the border between heartwood and sapwood are responsible for the formation and deposition of heartwood chemicals, one important step leading to heartwood formation.
The heartwood functions as long-term storage of biochemicals, which vary from species to species. These chemicals are known collectively as extractives. In the past, heartwood was thought to be a disposal site for harmful byproducts of cellular metabolism — a dumping ground for chemicals that, to a greater or lesser degree, would harm the living cells of the tree if not sequestered in a safe place.
We now know that extractives are a normal part of the plant’s system of protecting its wood. Extractives are formed by cells at the heartwood–sapwood boundary and are then exuded through pits into adjacent cells. In this way, dead cells can become occluded or infiltrated with extractives despite the fact that these cells lack the ability to synthesize or accumulate these compounds on their own.
Most importantly, particularly for forest products, extractives lend the tree many of its characteristics, including durability from insect damage, fungi, and other microorganisms. For this reason, researchers at the Forest Products Laboratory (FPL) recommend that if using untreated wood in construction, the extractive-laden heartwood should be used.
FPL researchers continue to work on isolating extractives from durable woods to be used on non-durable counterparts. This could open up many more options for naturally treated, durable wood construction in the near future, and allow naturally durable invasive species to be put to good use as they are cleared from our Nation’s forests.