Source: "Logs of Straw: Dendochronology," U.S. Geological Survey, 2002. Adapted with permission.
Dendrochronologists use tree rings to go back in time to learn more about past climate. Using straws to recreate tree rings, you can learn how dendrochronologists work. Construct a 50-year climatic history on a three-meter time line.
In groups of four, examine the set of straws that your teacher has prepared for you (by copying the Core Sample template onto the straws). Your samples have come from:
Measure the tree rings in the photograph on the back of the poster. Assume the tree was cut in 1992. How old is the tree? Can you determine good and bad years for growth?
Find and map the locations of some of the oldest known trees in your neighborhood. Sketch what you think a core from one of these trees might look like.
Contact your local forestry service or science museum and obtain some actual cross sections of trees that have been cut in your area. Use the techniques applied during this activity to "read the tree." If a tree has been cut in your neighborhood recently, look at the tree rings on the stump or ask if you can keep a small piece of the trunk.
Create some simulated core straws of your own for another group to analyze and report about.
The following illustration shows how samples 1 and 2 can be aligned. Have the students align all four samples so that the patterns match, and determine the years when each tree was cut and when it began to grow. Have them count all the rings from the oldest samples as they are aligned with the younger samples to determine the total amount of time represented by the rings. Count aligned rings that appear on several samples only once.
The charts should be completed as follows:
The total time covered by the tree rings is 50 years, from 1942 to 1992.
The answers to some of the questions in the activities will depend on the individual class-for example, when they were born or when buildings in their area were built. In looking at the climate record as revealed in the tree rings, notice that there is a significant period of poor growing conditions in each of the four decades covered by the tree samples. This pattern, which can be graphed, is the type of pattern scientists might look for when studying climate change.
Angier, Natalie, Warming?-Tree rings say not yet: New York Times, Tuesday,
December 1, 1992, p. C-1, C-4.
U.S. Geological Survey, 1991, Tree rings-timekeepers of the past: Reston, Virginia, USGS, 15 p.