Most trees begin life as a seed and continue to grow as long as they live, some for hundreds or even thousands of years. The history of a tree, its age, plus events that took place as it grew, can all be revealed by studying its trunk after it is cut down.
A tree grows a new layer of wood every year. These layers show up in a cross-section of a tree’s trunk as rings, called annual rings. Each annual ring represents one year of a tree’s life. Therefore, counting the rings gives us a tree’s age.
But this same cross-section reveals much more. For example, narrow rings at the trunk’s center tells us that when the tree was young, it was probably surrounded by other, larger trees which shaded it from the sun and used up much of the available water.
Small “V” notches in a ring show that during that particular year of the tree’s life, a branch grew from that spot.
Wider and narrower sections of the same ring tell us that the tree bent in one direction (the direction of the wider section), thus causing the cells on that side of the trunk to grow more wood to keep the tree from falling.
As the tree gets older, its rings get wider, indicating that some of the surrounding trees have been cut down and died, giving the tree more sunlight and moisture. Also, the amount of rainfall each year can be determined this way too, a wider ring showing more rainfall, a narrower ring, less.
Scars can also be found on a tree’s rings. They tell the story of a fire or other natural or man-made damage to the tree.
One variety of cottonwood tree, which grows only in the Panama Canal Zone of Central America, has a square trunk and square annual rings!