Classroom Activity: Dendrochronology -- Log of Straws

| Introduction | Objective | Concept | Time | Materials | Preparation | Procedure |
| Evaluative Questions | Extensions |

Introduction:

To determine if the climatic changes occurring now are part of the Earth's normal pattern or are induced by human activity, scientists rely on the history of climatic changes as revealed by tree rings, ice cores, pollen samples and the fossil record. The evidence collected so far suggests that climatic changes are simply a natural occurrence on Earth. The extent to which human activity affects the way the global climate is changing is now only beginning to be understood.

Trees are some of nature's most telling timekeepers. Their growth rings record years of abundant water (thick rings), droughts (thin rings) and even earthquakes and lightning strikes. Tree growth depends on local conditions, which include the availability of water. Because the amount of water in the environment varies from year to year, scientists use tree-ring patterns to reconstruct regional patterns of drought and climatic change. This field of study is known as dendrochronology.

To study tree rings, dendrochronologists often use straw-sized core samples extracted from trees. In this student activity, drinking straws will be used to simulate tree-ring core samples. Students will work in cooperative learning groups to reconstruct a 50-year climatic history and record this chronology on a three-yard time line designed to highlight significant personal, cultural and scientific events.

Objective:

Students learn how to interpret tree rings to determine basic changes in past climate.

Concept:

Characteristics of tree rings reveal local climate changes of the past.

Time:

45-60 minutes, plus further class sessions for research if necessary.

Materials:

For each team of four students: one set of white straws marked with the ring patterns in the four samples indicated in the Ring Pattern Diagram (markings can be made with a permanent black marker on paper or plastic straws), one three-yard strip of adding-machine tape, colored pencils, colored markers, reference materials such as almanacs that provide students with the dates of significant cultural and scientific events over the last 50 years.

Preparation:

Prepare one set of four straws for each group, marked similarly to the set shown in the diagram.

Procedure:

1) Have students imagine that the straws represent core samples from the following trees:

Sample 1: From a living tree, July 1992, Pinetown Forest.

Sample 2: From a tree stump in the Pinetown Christmas Tree Farm.

Sample 3: From a log found near the main trail in Pinetown Forest.

Sample 4: From a barn beam removed from Pinetown Hollow.

2) Have students determine the age of each tree (how many years it was living) by counting the rings. Students record their answers in the first column in a chart like the one below.

	Age of Tree	Year Tree Was Cut	Year Growth Began
Sample 1
Sample 2
Sample 3
Sample 4

3) Have students look for patterns in the rings. Once a ring pattern has been discovered, line up all the samples. Because they know that Sample 1 was cut in 1992, they can match the patterns of all the other samples and determine what year the other trees were cut or cored, and also when they began to grow. Have students record this information in the chart.

4) Now have students make a time line. Beginning at the left end of the strip of adding-machine tape, have them mark each year from the earliest year identified on the tree-ring samples through 1992. Space the years according to the ring spaces on the imaginary core sample (they should be larger, but proportionately the same).

5) After the years are recorded on the strip, identify good growing years (thicker rings) and bad growing years (thinner rings) for the trees in Pinetown. Have students identify other events that happened during this time period such as their birthday, presidential elections, important scientific discoveries or record-setting sports achievements. Students should then create a timeline of these events. If time allows, they can illustrate the time line with drawings, photographs or newspaper clippings and articles corresponding to the years.

Evaluative Questions:

Which ring on each tree represents your birth year?

Which buildings in your area were built during the lifetime of these trees?

What kind of growing seasons existed in these years in Pinetown?

In which years did droughts occur in Pinetown?

Is there a pattern to the droughts?

Which years had the highest rainfalls?

Is there a pattern or cycle to these climate changes?

What other conditions besides climate could affect tree growth?

If the climate of the Earth is warming, what would future tree rings be expected to look like?

Extensions:

• Tune in to the Wild Wings: Heading South broadcast on November 22, 1996 and see students and scientists apply the same skills to interpret past climate changes from tree rings in New Mexico. Watch closely for any new techniques. Does your class agree with the interpretations presented?
• Have students locate the biggest tree stump they can find in their neighborhood. They can study its tree rings and make a climatic and event time line for their community. See which student can travel back in time the furthest.

Adapted with permission from Global Change: Time and Cycles, The U.S. Geological Survey, United States Department of the Interior, Reston, VA.