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02 January 2004

Science Notebook: Getting Started in Dendrochronology

Forrest M. Mims III, Editor

In countries where Christmas is celebrated, millions of conical conifers are disposed of in dumps, incinerated or ground into mulch shortly after the arrival of the new year. For a brief few weeks, these trees were festooned with lights and ornaments. Now they are destined for oblivion.

The bottom of a typical Christmas tree trunk, in this case a Douglas fir, looks like this prior to sawing and sanding. Click image to enlarge.

But wait, some of these trees may have important scientific value if their species is known and their origin can be traced. Some of these trees may have preserved a record of rainfall and sunlight in their annual growth rings.

Christmas tree dealers can often provide the origin of the trees they sell. They usually know the species of their trees. While discussing this with Sheldon Greaves, he suggested that Christmas tree farms would be an excellent source of many freshly cut stumps.

If you can find trees with established pedigrees, use a hand or power saw to slice a disk several centimeters thick from the trunk. Be sure your cut is above any support hole drilled in the center of the trunk. Discard this disk and saw a second one. This will provide you with two clean sides to study.

Allow the wood to dry thoroughly for a week or more. Then rub the disk over a sheet of rough (60 to 100) grit sanding paper to smooth the ridges left by the saw blade. Follow by rubbing the disk over 200 and then 400 grit paper. For best results, place the sanding paper on a slightly flexible surface, such as a thin sheet of corrugated cardboard.

You will now have a "tree cookie" with well-defined growth rings, a characteristic of conifers. Depending on the size of the tree, the disk will contain from 5 to 15 or more rings.

Study the rings by measuring their width and color. A10-power loupe is very helpful for this. You can use a camera or digitizer to photograph the rings. Moisten the wood to enhance contrast. Compare your measurements with records from the nearest weather station. Also look for data from the United States Department of Agriculture (USDA) UV-B Monitoring and Research Program Network (http://uvb.nrel.colostate.edu/) and the Aeronet Sun photometer network (http://aeronet.gsfc.nasa.gov/). Compare the rings from the same species and different species grown in the same region.

For everything you ever wanted to know about tree rings, visit www.google.com and enter "tree rings" and "dendrochronology."