TREE RINGS AS RECORDS OF THE PAST

Marcia Bjornerud
Geology Department
Miami University
Oxford, Ohio 45056

Level: Grades 1-8, depending on level at which discussion is pitched.

Estimated time required: 45 minutes or less

Anticipated learning outcomes

• Students discover how trees preserve a record of times in the past

Background

Year by year, trees grow in diameter by producing new wood in the layer just beneath their bark. In most species, a year's growth is easily recognized because wood produced during the first part of the growing season (early wood) is generally lighter in color than the denser wood produced as the season comes to an end (late wood). The width of a given ring reflects the tree's growth rate in a particular year. Since growth rate depends largely on precipitation (or the lack of it) during the growing season, the widths of tree rings can be used to reconstruct rainfall patterns in the past, and in particular to identify periods of drought. Most trees (like people) grow fastest when they are young, and this must also be taken into consideration in interpreting ring widths. It should be noted that tree ring analysis does not generally involve cutting trees down; instead, a special auger is used to extract a slender core for study. The tree is unharmed by this.

Materials

• Sections from a recently cut tree, ideally one that the children might have known.
• Hand lenses, if available
• Local precipitation records for the period of the life of the tree (Grades 5 and up).

Procedure

1. Observe and describe the difference between early and late wood. Use hand lenses.
2. Make a guess about the age of the tree, then determine the actual age by counting the rings.
3. Identify rings that represent the years the child, the child's parents, and the child's grandparents were born, or years of historical importance. (Remember that the youngest rings are on the outside).
4. Have older students identify the narrowest rings, then check precipitation records to see if these coincide with years of low rainfall.

Results and Discussion

The key idea to be conveyed is that trees preserve records of the past that allow us to 'see' backwards into times before we were born. The oldest living trees, the bristlecone pines in eastern California, are four to five thousand years old and thus have very long 'memories' of past times. After the exercise, students might be asked to think of other things that provide records of the past (These don't need to be geological in nature -- even scars on knees and arms are a kind of historical record!) To geologists, important archives of the past include the shapes of landscapes, layers of glacial ice (which contain ancient air bubbles), and of course rocks, which outlast everything else.

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