Trees are remarkable living organisms. Their ability to reach towering heights and live for centuries is largely due to their intricate internal structure. The trunk, a tree’s central stem, contains several distinct layers, each playing a specific role in its survival and health.
Identifying the Tree’s Core
This structure evolves with the tree’s age. In a mature tree, the primary middle part is heartwood, a dense, non-living core. At the very center, especially in young trees and new growth, lies a small, soft tissue called the pith. The pith serves as the tree’s initial core, while heartwood develops around it as the tree matures, providing long-term structural support.
Heartwood: The Tree’s Strong Center
Heartwood is the inner core of a tree’s trunk, composed of older, non-living wood that provides structural support. It forms as sapwood, the active outer wood, ages and its cells lose vitality. This transformation involves the accumulation of substances such as resins, gums, and tannins, which often give heartwood a darker color. These compounds also contribute to its natural resistance to decay, fungi, and insects, enhancing the tree’s longevity.
Heartwood formation typically happens during the dormant period in temperate climates. During this transition, cells in the sapwood undergo programmed cell death, and their contents are converted or removed. The resulting heartwood is remarkably strong, capable of supporting immense weight. This dense, durable core is crucial for the tree’s stability, allowing it to withstand environmental pressures as it grows.
Pith: The Early Core
The pith is a soft, spongy tissue found at the very center of a tree’s trunk, branches, and roots, particularly prominent in young trees. It is composed of loosely packed parenchyma cells, storing nutrients like starches and sugars, and transporting water. This central location and composition make the pith important for initial growth and nutrient distribution when the tree is young.
As a tree matures, the pith largely diminishes. The surrounding wood, particularly the heartwood, develops and takes over the primary roles of support and storage. In older trees, the pith may dry out and disintegrate, or it can be replaced by xylem cells. While small, the pith represents the earliest growth of the tree, around which all subsequent woody growth rings form.
Beyond the Middle: Surrounding Layers
Around the central heartwood and pith are other layers that contribute to the tree’s structure and function. Immediately surrounding the heartwood is the sapwood, living cells transporting water and dissolved minerals from the roots to the leaves. This layer is typically lighter in color than the heartwood.
Beyond the sapwood lies the cambium, a thin layer of dividing cells. This growth layer produces new sapwood cells on its inner side and new bark cells on its outer side, increasing the tree’s girth annually. The outermost layers are the inner bark (phloem) and the outer bark. The inner bark transports sugars to other parts of the tree, while the outer bark protects against weather, pests, and disease.