“You can tell the age of a tree by counting its rings.”

The idea that a tree's age can be determined by counting its rings is a widely held belief, often introduced in early science education or through encountering a tree stump. This intuitive understanding, based on the visible concentric circles within a tree's trunk, correctly points to a fundamental aspect of tree growth. Indeed, the scientific field of dendrochronology, which dates back to observations by Leonardo da Vinci in the 1500s and was later formalized by astronomer Andrew Ellicott Douglass in the early 20th century, is built upon this very principle.

The scientific basis for this method lies in how trees grow. Each year, a tree adds a new layer of wood, or xylem, beneath its bark, causing its trunk to increase in diameter. This annual growth is visible as a distinct ring because of seasonal changes. During spring and early summer, when conditions are favorable with abundant moisture and light, trees grow rapidly, producing large, thin-walled cells that form a lighter, wider band known as earlywood or springwood. As the growing season progresses into late summer and autumn, growth slows, and the tree produces smaller, thicker-walled cells, resulting in a darker, narrower band called latewood or summerwood. Together, these light and dark bands constitute one annual growth ring, accurately reflecting one year of the tree's life. The width of these rings can also provide clues about past environmental conditions, such as rainfall and temperature.

People commonly believe this method because the rings are so visually apparent and the concept is straightforward. However, achieving precise accuracy without specialized knowledge can be challenging. Factors such as unusual weather patterns, like a mid-season drought followed by renewed growth, can sometimes lead to "false rings" within a single year, which might be mistaken for an additional year of growth. Conversely, in very harsh years, a tree might produce an extremely thin or even indistinct ring, making it difficult to count accurately. Professional dendrochronologists overcome these challenges by using specialized tools like increment borers to extract core samples without harming the living tree, and by employing techniques such as "cross-dating," which involves comparing ring patterns across multiple trees in an area to ensure each ring is assigned to its exact calendar year.