Tree Rings

Trees are extraordinary plants! They can grow from a seed weighing only fractions of an ounce, to a structure hundreds of feet tall, weighing tons and living to be centuries old.

When you look at a cross section of a tree, you'll see a distinct pattern of rings. Some rings may be close together; others may be far apart. Each ring is a layer of wood produced during the tree's growing season.

As the tree begins to grow in the spring, a one-cell-thick layer - the cambium (kam-bee-uhm) - begins dividing at a feverish pace. New tissue is created both inside and outside the cambium, adding to the diameter of the tree. The cells produced on the outside of the cambium become part of the phloem (flow-uhm). The job of the phloem is to carry food produced in the leaves to the branches, trunk and roots. Some of the phloem dies each year and becomes part of the outer bark.

Most of the cells generated by the cambium collect on the inside of the cambium as xylem (zeye-lem). Xylem cells account for most of the diameter growth in a tree each year. Xylem transports water and nutrients from the roots to the leaves.

In the spring, when the tree is growing fast, the xylem cells are large with thin walls. This lighter-colored part of the ring is called springwood or earlywood. In August and early September, the growth rate slows and eventually stops. As growth slows, the walls of the xylem cells become thicker and appear darker than the earlywood. This area of the annual ring is called summerwood or latewood. Together, the earlywood and latewood form one annual ring.

In most trees growing in northern and temperate climates, one annual ring is added each year. Tropical trees may have more than one growth ring per year depending on the rainy and dry seasons of the region. Most temperate trees show a distinct line between the previous year's latewood and this year's earlywood. This difference is easy to see in conifers, such as pine and spruce, and in hardwoods, such as oak, ash and elm.

The width of each year's annual ring varies, depending on weather and other environmental factors. Drought causes slower growth with narrow rings, while abundant rainfall will increase growth, producing wider rings. Droughts usually do not last longer than two or three years so a long series of narrow rings suggests crowding from neighboring trees. Years of poor growth followed immediately by good growth probably means the tree got more room to grow because its neighbors were cut or died.

A growth pattern of narrow rings on one side with wide rings on the other shows the results of crowding on the side of the tree where the rings are narrow. Forest fires can also leave scars. By counting the annual rings between fire scars, scientists can determine how many fires there were during the life of the tree. Floods and insects also mark tree rings.

Scientists learn about past weather patterns and forest fires by studying the annual rings of trees. The science of studying the past by looking at tree rings is called dendrochronology. One research technique used by dendrochronologists is cross dating. It compares the growth rings from one tree to another tree and matches the ring patterns of the years when both trees lived.

How cross dating works:

A scientist first takes a core sample from a living tree. The core sample is removed by drilling into the center of the tree with a special tool called an increment borer. The core is about the size of a soda straw. Annual rings show as lines on the core. Scientists count these lines to learn the tree's age and when it began growing.

Next, they find an older tree to compare with the younger tree. In cross dating, scientists often use stumps, logs and wooden beams in old buildings.

Dendrochronologists compare the inner (oldest) rings of the core sample with the outer (youngest) rings of the stump or log to find a section where the ring patterns match. Since the age of the younger tree is known, dates can now be assigned to the older tree by counting backward. By finding still older trees, and overlapping with increasingly older trees, scientists have discovered cycles of drought from over 10,000 years ago.