(annual ring, growth ring)
A sheath of cells appearing as one of a series of concentric rings in the cross-section of a woody stem. Each ring is usually the result of a single yearly growth flush starting in spring and ceasing in the late summer. The new wood (xylem) cells arise from renewed activity of the vascular cambium. A sharp boundary usually occurs between the rings since cells formed in spring are typically large, thin-walled, and appear pale in colour when compared with the small, thick-walled cells of later summer. In some dicotyledonous species (see dicotyledon) vessel distribution also varies, with most vessels occuring in the early (spring) wood. These ring-porous species contrast with ring-diffuse species in which the vessels are distributed evenly throughout the growth ring. The growth check that causes variable cambial activity may be temperature or water stress or both. In some extreme environments growth may not always be renewed on an annual basis, leading to absent rings (‘missing years’) and false series (multiple rings in the same year). Dating by tree rings is difficult in these circumstances. See also dendrochronology; dendroecology; fire scar; vascular cambium; vessel element; and tracheid.