As a tree grows, the wood is laid down in concentric rings. Where a definite growing season exists (temperate climates) the wood laid down early in the growing season ie. spring and early summer has vigorous growth leading to larger cells with thinner walls and hence a lighter colour.
The wood laid down later in the growing season when the leaves are older and sunlight or water less plentiful (latewood) has slower growth, smaller cells, thicker cell walls and, hence, a darker colour.
The growth pattern for each season gives light coloured wood closer Timber Properties
to the centre of the tree and darker coloured wood nearer the outside.
The disparity between these two types of wood gives a ring for each growing season. At least one such ring is laid down each year. In some years, sudden changes in seasons may cause the tree to believe that there are two growing seasons, and two rings will be laid down.
Severe trauma (such as a bushfire) may remove most of the leaves, and the tree will grow new leaves and start a new ring. In these cases, there will be two narrow rings laid down in one year. Growth rings are characteristic of timber grown in temperate regions. Trees grown in tropical regions with excellent conditions for growth all through the year do not show pronounced growth rings.
