Growth rings, also known as tree rings or annual rings, are rings that are visible in horizontal cross-sections of the trunks of trees in climates that vary by season or by year. Changing seasonal conditions, particularly temperature and sunlight, lead to differences in tree growth that can be traced by examining growth rings. In areas with annual seasonal cycles, tree rings can be counted to determine a tree's age in years. In areas without particularly distinct seasons, a tree's rings may be indistinct or entirely absent. The scientific method of determining a tree's age based on its rings is referred to as dendrochronology or, more simply, as tree-ring dating.
Changes in secondary growth, or horizontal growth in vascular plants, are responsible for the formation of growth rings in the trunks of trees. Secondary growth occurs from the vascular cambium, part of a tree's vascular tissue. The vascular cambium produces secondary xylem toward the inside of the tree and secondary phloem toward the outside. Xylem is primarily responsible for transporting water throughout the tree and phloem is responsible for carrying nutrients, particularly sugars. This secondary growth occurs differently based on the seasonal conditions; these differences manifest themselves in the form of growth rings.
Each growth ring tends to be made up of two different parts based on the growing conditions. Spring wood, more commonly referred to as early wood, grows quickly in the growing season, usually in the spring or early summer. Summer wood, more commonly referred to as late wood, grows in the mid-summer. Late wood tends to be considerably denser than early wood, and it is generally lighter in color.
Growth rings, particularly annual growth rings, can actually provide valuable records of the climate conditions during a tree's lifetime. Extremely hot and dry years, or overcrowding that limits the tree's ability to receive sunlight, tend to result in smaller rings while temperate conditions with plenty of sunlight and precipitation result in wider rings. Sometimes, multiple rings may form in a single year; this occurs in years when excellent growing conditions alternate with distinctly bad conditions. There are almost no conditions in which a tree will not form a distinct ring during a year; only significant climatic abnormality can lead to such an occurrence.
Many other more specific pieces of information can be taken from growth rings. Basic information about weather conditions can be gained from a simple look at the tree's rings. Further analysis can reveal information about soil acidity, carbon dioxide, and soil nutrition over the course of the tree's life.