What Is Heartwood in Trees?

If you look at the cross-section of a stump from a tree that was cut down, there is almost always a portion of the cut surface that appears darker than the rest of the surface. The dark-colored area is called heartwood, and the lighter-colored area is called sapwood.  What causes this change in color in the wood, and how does it affect the wood properties? 

The darker heartwood is always found in a zone around the center of the tree and is surrounded by a lighter-colored zone called sapwood. The size of the two zones varies by tree species and even between trees of the same species. Some species, like black cherry and black walnut, tend to have a large heartwood, while others, such as sugar maple, tend to have a relatively small heartwood. The sapwood portion of the tree is the only place where living cells are found within the tree itself and is where sap is transported up the tree.  All cells in the heartwood are dead and movement of liquids does not occur.

As a young tree grows and cell division takes place in the cambium layer, the walls of the created cells eventually begin to thicken and become lignified.  Lignin is a substance that gives the cell wall rigidity and helps to bind cells together.  As the cell walls thicken, most of the cells die, although some retain their protoplasts and carry on metabolic processes in the sapwood.  These cells are known as parenchyma cells and serve as storage cells within the tree, acting as a reservoir for photosynthates the tree makes.  As these living cells get further away from the outside layer of the tree, they also begin to die.

When the trees are young there is no heartwood in them. Heartwood begins to form between fourteen and eighteen years old, although this can vary among species. The process of how the heartwood forms is a bit of a mystery, and a review of the literature shows that not everyone agrees on the exact process that contributes to the formation of the heartwood.  However, the production of various compounds, broadly called secondary metabolites, is believed to be a contributing factor.  Secondary metabolites are substances that are not directly involved in the normal growth and development of the tree.  These metabolites accumulate in cell walls as well as the lumen (the center part of the cell) and are usually polyphenolic in nature.

Polyphenols that are commonly found in trees include oils, resins, gums, tannins, and aromatic and coloring substances. Some of these secondary metabolites can be extracted from the heartwood through various processes, so they are collectively referred to as extractives.  It is believed that these extractives are formed from metabolites transported by the living parenchyma cells located near the heartwood/sapwood boundary.  Metabolites, unlike secondary metabolites, are products of processes that are directly involved with the normal growth and development of the tree.

Heartwood formation is often associated with a decrease in water content found in the cells.  The columns of water in the tree are believed to be under tension, and eventually, these water columns will form an embolism, or an obstruction, preventing the movement of liquids through that portion of the tree.  These embolisms are more likely to occur the further away from the ground the wood is.  This may be one reason why the roots of trees tend to have very little heartwood in them. 

The basic structure of heartwood and sapwood cells are the same and it is mainly the chemical extractives that differentiate the two types of wood.  It is the extractives that give heartwood several properties that are different from sapwood.

One difference in properties is the darker color associated with the heartwood in most trees. Most polyphenols that are formed by the trees are colorless in regular light, but they can degrade over time. The death of the parenchyma cells that they are found in allows them to diffuse into the wood, where they oxidize and become darker.

Another property that is different between heartwood and sapwood is the ability to resist decay or insect attack. It is known that some species of trees have a much greater resistance to rot. Black locust, which makes great fence posts, is one example of this. The main reason it makes great fence posts is due to its ability to resist fungal attack. Cypress, redwood, and cedar are other examples of trees that have heartwood that is highly resistant to rot. The ability of the heartwood to resist rot depends on the type of extractives that are found in the heartwood. The heartwood of some trees contains high levels of extractives that resist rot, while others have little or none. The sapwood of all tree species is susceptible to rot because it contains no extractives.

Heartwood of certain species may be difficult to penetrate with liquids such as chemicals used for preservative purposes.  This resistance can be caused by several things, including the presence of extractives such as oils, waxes, and gums.  These types of extractives can plug up the tiny openings in the cell lumen and walls, effectively hindering the movement of liquids.  Other types of extractives can block openings in the cell walls from progressive deposition over time.

The distinct odor that some trees exhibit, such as cedar, is usually due to a type of extractive called aromatic extractive compounds.

It is believed that some of these aromatic compounds play a role in defending against certain insects.

Heartwood and sapwood are the same but different. They have the same cell structure but have several different properties that can make one species more suitable for a certain application than another. The chemical compounds found in the heartwood can make certain species more aesthetically pleasing and more valuable, such as black walnut, or it can make them less prone to rot, such as black locust.