A tree trunk is not a single, solid mass but a complex cylinder of distinct layers, each with a specific function. The innermost region is primarily structural and non-living, providing the enormous strength required to support the tree’s height and canopy against gravity and wind. This central core contrasts sharply with the outer layers, which are actively involved in growth, water transport, and external protection. Understanding the anatomy of the trunk means recognizing the different roles these concentric layers play in the tree’s survival.
The Non-Living Core (Heartwood and Pith)
The physical center of a mature tree is defined by the heartwood, which is the dense, non-functional wood that provides the majority of the trunk’s mechanical strength. Heartwood is essentially composed of dead xylem cells, the same cells that once actively transported water when they were part of the sapwood layer. This transition occurs gradually as the tree ages and the central cells no longer participate in water conduction.
As sapwood converts to heartwood, the tree deposits chemical compounds, such as resins, tannins, and oils, into the cell lumens. These extractives are responsible for the heartwood’s characteristic darker color and its increased resistance to decay, insects, and fungi. These compounds make the heartwood harder and more durable than the surrounding wood. It acts as a stable, permanent pillar supporting the entire structure, even though it contains no living cells or water.
At the very core of the heartwood is a tiny remnant structure called the pith. The pith is the original central tissue formed when the tree was a seedling, composed of soft, spongy parenchyma cells. While active in young trees, the expansion of surrounding wood often compresses or replaces this initial tissue as the tree grows. In a mature trunk, the pith is the oldest part of the tree, remaining as a small spot at the center of the first annual growth ring.
The Active Layer (Sapwood and Xylem)
Surrounding the load-bearing heartwood is the lighter-colored sapwood, also known as alburnum, which represents the active, living portion of the wood. The sapwood’s primary function is the upward transport of water and dissolved minerals from the roots to the leaves, a process carried out by the specialized cells of the xylem. Xylem vessels act as a continuous pipeline, moving water to the crown to support photosynthesis and transpiration.
A tree may have a sapwood band that is only a few rings thick or many rings deep, varying greatly by species and age. The cells within this layer, particularly the ray parenchyma cells that run horizontally, remain alive, storing starch and other energy reserves. This active wood is where the tree’s water-conducting capacity resides, making it susceptible to damage from drought or disease.
As new layers of wood are produced each year, the innermost band of sapwood gradually loses function and dies, undergoing the chemical changes that turn it into heartwood. This constant cycle ensures the tree always maintains a functional layer of water-conducting tissue. The sapwood is the newest wood, situated between the non-functional core and the generative outer layer.
The Outer Protection (Cambium and Bark)
The vascular cambium is a thin, single layer of living, actively dividing cells located just beneath the bark. This layer is the engine of the tree’s secondary growth, which increases the trunk’s girth. The cambium divides to produce new cells: those generated toward the inside differentiate into new xylem (wood), while those generated toward the outside become new phloem (inner bark).
This dual production creates the annual growth rings and causes the tree to thicken over time. The phloem cells, which form the inner bark, transport sugars produced during photosynthesis from the leaves down to the rest of the tree. Phloem is a relatively short-lived tissue, eventually dying and becoming part of the outer bark.
The bark is the tree’s outermost protective shield, composed of the inner phloem and the tough, dead outer bark. This protective layer insulates the tree from extreme temperatures, prevents moisture loss, and defends against physical damage and pests. The continuous production of new cells by the cambium ensures renewal of the tree’s internal transport system and its external defense mechanism.