Reading the rings of a tree

The next time you see a tree stump, count the rings and notice how they're shaped. Trees don't talk much, but they tell you a lot.

Steve Leavitt is the associate director of the Laboratory of Tree Ring Research at the University of Arizona. He says the rings are loaded with lots of information, such as what the climate has been like since the tree started growing.

"That could be for example temperature, precipitation, relative humidity, those are the dominate kinds of environmental factors that could be reconstructed from tree rings. Hydrology – stream flow – is actually another one. Because under certain circumstances the trees may be limited more by water," says Leavitt. "When you have good years of precipitation you not only can reconstruct precipitation, but also stream flow."

Large tree rings indicate sufficient moisture and small rings indicate a drought, or maybe even an insect infestation.

Where you live depends how easy it is to decipher this information. In temperate climates such as the Midwest, the tree lays down a wood layer as it's growing, and stops in winter, which creates the visible rings. In some climates, such as where it's warm year-round, they often aren't as well defined.

Leavitt says the rings also tell you the age of the tree, which is very useful in dating historical structures that contain wood.

"If there's what we call a chronology against which the patterns of rings in that sample can be matched, we can assign an absolute age to every one of those rings that are in the sample, and we can determine what the outside ring is," he says. "So, we can determine in fact what year the wood was cut in order to construct whatever that archeological structure is."

Each ring usually equals one year. But Leavitt says in a stressful environment, a tree may not lay down rings in a given year.