We now know why these annual rings form and use them for research. In places that have strong seasonal changes, trees usually stop growing part of the year. When the tree starts growing again, the wood looks different, marking the beginning of a new annual ring. When the tree trunk is sliced horizontally, these rings can be seen and counted, giving the tree s age.
Some trees live thousands of years, although most live a few hundred.
If the tree is growing fast, you may be able to see the rings with your eyes, but it is best to use a magnifying glass or a low power microscope. If possible, the wood surface should be sanded smooth, so that small rings can be seen. Researchers use a device called an increment borer to remove a small cylinder of wood without hurting the tree. Using dendrochronology the science of keeping time using annual rings scientists study annual rings to date wooden objects such as buildings, to find out about past environments, and to answer other questions. Because annual rings respond to climate during the growing season hot and dry yields skinny rings, while cool and wet makes fat rings they form identifiable patterns of wide and narrow rings in long series.