Annual rings forms in woody stems through the production of xylem cells, which eventually swell with sap, become immobile and stick together to form rings. This process happens each year in most trees, but does not necessarily begin upon the tree reaching its first birthday. Instead, rings develop later in life as trees age and develop vascular cylinders, which provide the conditions needed for subsequent ring growth.
Annual rings are found in trees with woody stems that contain two types of tissue: primary and secondary. Primary tissues are those that are formed by plants and trees, while secondary tissues are those generated by the tree's or plant's development of cambium. Trees add secondary tissues each year to their pre-existing tissue supplies. The accrual of these tissues is referred to as secondary thickening, which creates the hard, wooden centers in woody stems and creates the visual effects of annual rings. The process of secondary thickening begins in young trees, which have stems comprised of bundles of fluid-carrying tissues. As trees age, cambium layers thicken and merge. Eventually, this merger creates vascular cylinders, which enable the formation of rings in subsequent years. Xylem cells are produced each growing season in these cylinders; eventually, cells clog and solidify, creating the appearance of rings.