What Tree Rings Tell Us
1When a tree is cut down, the top of its stump shows a pattern of circles, one inside another. These circles are called tree rings. To a scientist, they are much more than a pretty pattern. Tree rings are like a diary that a tree keeps for its whole life.
2In places with warm summers and cold winters, a tree adds a new layer of wood just under its bark every year. In spring, the tree grows quickly and makes wood that is pale in color. Later in the summer, growth slows down, and the new wood is darker and more tightly packed. Together, one light band and one dark band make a single ring. That is why counting the rings can tell you how old a tree is.
3The rings are not all the same width. In a year with plenty of rain and a long growing season, a tree grows a lot, and its ring is wide. In a dry year, the tree grows only a little, and its ring is thin. By comparing the widths of the rings, scientists can learn what the weather was like long before anyone kept weather records.
4Rings can hold other clues, too. A forest fire that sweeps past a tree can burn a scar into its trunk. If the tree survives, new wood slowly grows over the wound. The dark scar stays hidden inside the trunk, marking the year of the fire like a bookmark in a book. A tree that loses most of its leaves to hungry caterpillars may also grow a very thin ring that year.
5You might think scientists must cut down a tree to read its rings, but they usually do not. Instead, they use a tool called a borer, which is a long, hollow metal tube with a sharp tip. They twist the borer into the trunk and pull out a rod of wood thinner than a pencil. This sample, called a core, shows every ring from the bark to the center. The small hole usually does not harm a healthy tree, and the tree keeps growing.
6Scientists can also match ring patterns from tree to tree. A run of thin rings from several dry years in a row shows up in many trees from the same area, almost like a fingerprint. By lining up these patterns in living trees, old stumps, and wooden beams, scientists have found out when the logs in some very old buildings were cut.
7Some of the oldest living trees on Earth are bristlecone pines, which grow high in the mountains of the western United States. Some of these trees are thousands of years old. Their rings hold a record of thousands of summers, dry spells, and fires, all written in wood.