. Tree growth rings. Researcher using a microscope to count tree rings. This log is from a silver fir tree (Abies alba). Each tree 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. Photographed at the Centre of Alpine Ecology, Trento, Italy. MODEL RELEASED

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