Trees have a natural lifespan, but may live for hundreds and even thousands of years. Once trees reach maturity, which differs by species, growth decelerates and decay begins, eventually resulting in death. The age of a tree can be estimated by the number of growth rings present in a transverse section of wood. In temperate regions, one concentric layer is usually formed during each growth season, but in the tropics the development of the rings is more variable, and thus the age of tropical trees is more difficult to ascertain.