Tree rings are the visible, chronological record of a tree’s successful growth, seen on its cross-section. These annual markings are not a sign of injury but represent the accumulation of new wood tissue as the tree expands its girth. Understanding the physiological process behind ring formation clarifies that these structures are fundamental to a tree’s survival and strength.
The Anatomy of a Tree Ring
The formation of a tree ring begins in the vascular cambium, a microscopic layer of active cells beneath the bark. This thin layer divides to produce new tissue: phloem (inner bark) toward the outside and xylem (wood) toward the inside. Since the cambium produces significantly more xylem than phloem, the trunk expands primarily inward with wood.
The visible difference between the rings is caused by seasonal changes in cell growth. In the spring, when water is readily available, the cambium produces large, thin-walled cells for water transport; this lighter, wider band is called earlywood. As the growing season progresses, the tree produces smaller, denser cells with thicker walls, known as latewood. The sharp contrast between the dense latewood of one year and the porous earlywood of the next creates the distinct boundary used to count annual rings.
Tree Rings Are Essential Growth, Not Damage
The wood forming the tree rings is structurally and functionally necessary for the tree’s continued existence. The rings are composed of secondary xylem, the primary tissue responsible for conducting water and dissolved minerals from the roots up to the leaves. This continual annual addition of water-transporting and supportive tissue allows the tree to sustain its increasing height and canopy size.
The trunk’s cross-section is functionally divided into two major regions made up of these rings: sapwood and heartwood. Sapwood is the younger, outermost area that remains biologically active, serving as the main pipeline for water transport and food storage. As the tree ages, the innermost layers of sapwood cease to transport water, their living cells die, and they become chemically altered to form heartwood. Heartwood, though technically dead, provides the central, rigid support structure that allows the tree to withstand strong winds and gravity.
What Truly Impacts Tree Health
Threats to a tree’s well-being are external forces that disrupt life-sustaining processes, often by damaging the thin, living layers near the surface. One severe injury is girdling, which involves the complete removal of the bark, cambium, and phloem in a ring around the trunk. Since the phloem transports sugars from the leaves down to the roots, severing this layer starves the root system.
Physical damage from lawnmowers, construction, or rodents can penetrate the bark and expose living tissue to pathogens. Insect infestations, such as borers, target the vascular cambium and phloem, disrupting the flow of water and nutrients. Pathogenic fungi can also compromise health by causing cankers that kill the cambium and phloem, or by inducing decay in the wood. A healthy tree responds to such injuries by compartmentalizing the damage using chemical and physical barriers. The width of the annual rings serves as a barometer of a tree’s health, as a stressed tree will produce noticeably narrower rings.