Growth ring
For the entire life of a tree, a year by year record or ring pattern is formed that reveals the climate conditions in which the tree grew.  If a tree grows in a dry place, you can tell its age better than when it grows in a wet place.  Switching in between bad and good conditions can result in several rings forming in one year.  Tree ring growth patterns are affected by the climate, weather, rain, temperature, soil pH, plant nutrition, carbon dioxide concentration, and sunspots.