Formation of Wood Most of what we call “wood” is actually layers of secondary xylem produced by the vascular cambium. These cells build up year after year, layer on layer. As woody stems grow thicker, the older xylem near the center of the stem no longer conducts water and instead becomes what is known as heartwood. Heartwood usually darkens with age because it accumulates colored deposits. Heartwood is surrounded by sapwood, which is active in fluid transport and is, therefore, usually lighter in color.

Tree Rings In most of the temperate zone, tree growth is seasonal. When growth begins in the spring, the vascular cambium begins to grow rapidly, producing large, light-colored xylem cells with thin cell walls. The result is a light-colored layer of early wood. As the growing season continues, the cells grow less and have thicker cell walls, forming a layer of darker late wood. This alternation of dark and light wood produces what we commonly call tree rings.

Each ring has light wood at one edge and dark wood at the other, making a sharp boundary between rings. Usually, a ring corresponds to a year of growth. By counting the rings in a cross section of a tree, you can estimate its age. The size of the rings may even provide information about weather conditions, such as wet or dry years. Thick rings indicate that weather conditions were favorable for tree growth, whereas thin rings indicate less-favorable conditions.

Table of Contents