Growth rings on chopped logs. The growth rings are exposed by cutting across a tree trunk or branch. Each ring represents a period of growth in which the tree formed new wood. Each year the outer part of a tree trunk, the living part, lays down a new layer. The older layer dries out and dies, forming the hard mass of wood in the core of a trunk, the heart wood. Each ring corresponds to a single year and allows the age of the tree to be calculated. A ring varies in width according to the climate that year.