Applications of Tree-Ring Dating

Applications of Tree-Ring Dating

The dating of tree rings, by itself, is not much use unless the technique can be applied to answer various questions in the earth and ecological sciences. For example, what can we learn from past climate by looking at the varying widths of the annual rings? What can we learn about past glacial activity by looking at the growth patterns of trees that exist near glaciers?

At the right is an example of a partial cross section taken from a living ponderosa pine tree from the Chiricahua Mountains in southeastern Arizona. The right side of the sample shows indications that this tree was burned by several forest fires. By extracting a small section from the tree, dendrochronologists can date the exact year in which each of these fires occurred (marked by arrows on the sample). Land management agencies, such as the U.S. Forest Service and the National Park Service, can therefore use this information when developing fire management plans. By knowing how often fires occurred in the past, agencies can plan how to manage fires in the future.

But forests fires are not just limited to the western United States... on the right is a cross section from a pine tree that grew in the Sierra Madre in northern Sonora, Mexico. The arrows, once again, point out the scars left in the tree rings by the low intensity fires.

Such low intensity, surface fires were actually beneficial to the forests, not just to those in Mexico, but all over the world. During recent decades, ecologists have learned that forest fires were a pervasive phenomenon in practically all forests of the world, even the rainforests. Humans have severely disrupted the natural pattern of fire across the landscape, especially during the last 100 years. Therefore, if forests are to be returned to their more "natural" state, fire will have to be reintroduced. The consequences of not doing so could lead to more high intensity, catastrophic crown fires that we often see on the news.

When we compare the two samples side by side, we see an interesting situation. Fires have been occurring regularly in the forests of northern Mexico throughout the 20th century. In the sample taken from the Arizona forests, there has been a greater attempt at fire control, causing practically all 20th century fires to be suppressed. Note the large curl of growth on the sample to the right of the image. This is called the "Smokey Bear Effect" because no fire scars are present during the last 100 years or so.

Changes in tree growth rates as seen in their tree rings also supply clues to forest ecologists about possible disturbances that may have affected the forests. The image on the right shows a closeup of tree rings for a shortleaf pine (Pinus echinata) tree growing in Clemson, South Carolina. Growth is from the left to the right.

The abrupt decline in tree growth rates, marked by the narrower rings in the center of the image, suggests some event occurred during the life of the tree that killed or damaged many of the pine needles, reducing its ability to grow. Note how some rings appear to "pinch out" or disappear - these are known as "locally absent rings." The disturbance event could be related to storm dmage caused by high winds or perhaps some insect defoliator may have attacked the tree as well.

Finally, tree rings can also supply forest ecologists with information about wildlife. The cross section on the right is a red oak sampled at the Adrian J. Rogers Memorial Woodlot in Ashland, Wisconsin. The scar at the top of the sample was made by a porcupine that was feeding on the inner cambium, the softer tissue just inside the bark. Using tree-ring crossdating methods, ecologists can determine the exact year in which the porcupine damaged the tree.