Dendrochronology or tree-ring dating is the scientific method of dating based on the analysis of patterns of tree rings, also known as growth rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year.
Secondary growth in the stele results in the formation of secondary xylem and secondary phloem. Due to the addition of these tissues, there is an increase in the girth of the stele.
As secondary growth continues to occur several changes take place in the secondary xylem. In a much older stem the secondary xylem (wood) shows two regions namely duramen and alburnum.
- Duramen, also known as heartwood, represents the centrally located, inactive portion of the secondary xylem. It appears dark in colour due to the accumulation of pigments resulting from oxidation of organic compounds like oils and tannins. It is incapable of conducting water and provides only mechanical support to the trees. It is this part of the secondary xylem that is used as the commercial wood.
- Alburnum, also known as sapwood, represents the peripheral, active portion of the secondary xylem. This portion is still capable of conducting water. Due to its meagre mechanical strength and lack of durability, the sapwood can not serve as commercial wood.
The annual rings of a tree are made each year when a new layer of wood is added to the trunk and branches of the tree. New wood grows from the cambium layer between the old wood and the bark. Since there is more moisture in the spring, the tree’s energy can be directed toward producing large growth cells. As the season moves into summer. the growth slows and finally stops in the fall.
There are two parts to an annual ring – a light portion and a darker portion. The light section is called springwood. This part of the ring is usually widest because the tree does most of its growing then. The darker part, summerwood, is thinner. The tree’s growth slows down, hence a thinner band.
The more optimal the growing season, the greater the distance between rings.