Trees possess an intricate internal architecture, far more complex than a simple, uniform column of wood. Within their trunks, distinct layers perform specialized roles, enabling growth and providing stability. This layered composition means that the wood itself is not consistent throughout a tree, with variations in structure and function appearing from the center outwards.
Defining Sapwood and Heartwood
The outermost layer of a tree’s wood is known as sapwood. This younger, lighter-colored wood is physiologically active, primarily responsible for transporting water and dissolved minerals from the roots to the leaves, a process called transpiration. Sapwood contains living cells, including ray parenchyma cells, which also play a role in storing nutrients and water. Conversely, heartwood is the tree’s central core, composed of older, inactive wood that no longer conducts water. Its main function shifts to providing structural support.
How Sapwood Becomes Heartwood
The transformation of sapwood into heartwood is a natural aging process. As new layers of sapwood form each growing season, the inner sapwood cells become inactive and eventually die. During this transition, living ray parenchyma cells in the sapwood undergo metabolic changes, synthesizing and depositing various chemical compounds. These substances, known as extractives, include tannins, resins, and gums, which fill the cell cavities and walls. This accumulation of extractives often results in the heartwood’s characteristic darker color, increased density, and enhanced durability; in some species, a process called tylosis further impedes water flow.
Key Differences and Practical Importance
A notable distinction between sapwood and heartwood is color; sapwood typically ranges from whitish to light tan, while heartwood often displays richer, darker hues like brown, red, or purple due to its extractive content. However, color alone can be misleading, as some species have light-colored heartwood. Moisture content also varies significantly, with sapwood having a considerably higher water and sap content, making it more prone to shrinkage and cracking during drying. Heartwood, containing little to no moisture, is generally more stable.
Durability and decay resistance are major differentiating factors; heartwood is typically more resistant to fungi, insects, and decay because of its chemical deposits, while sapwood is less durable and more susceptible to biological attack. While both contribute to a tree’s strength, heartwood’s increased density and hardness provide a robust core. Furthermore, sapwood is more permeable, allowing liquids to penetrate more easily, whereas heartwood is less permeable.
These differences hold practical importance in human applications of wood. Heartwood is often preferred for construction and furniture making due to its greater stability, durability, and resistance to decay. Conversely, sapwood, with its higher moisture and susceptibility to decay, is generally less favored for applications requiring longevity unless treated with preservatives.