Signs of Change:
Studying Tree Rings

In a Nutshell:

In this very hands-on lesson, students will learn about dendrochronology (the study of tree rings to answer ecological questions about the recent past) and come up with conclusions as to what possible climatic conditions might affect tree growth in their region. Students determine the average age of the trees in their schoolyard, investigate any years of poor growth, and draw conclusions about the reasons for the years of poor growth.

Goal:

To understand the relationship between climate and tree growth.

Background Learning:

Students should be familiar with the basic science of climate change and its anticipated impacts as reviewed in:

• Intermediate Backgrounder #1: Climate Change Basics: What’s the Big Deal?
• Intermediate Backgrounder #2: Climate Change Impacts: A Changing World?

Teachers should be familiar with the basic science of climate change and its anticipated impacts as reviewed in:

• Intermediate Backgrounder #1: Climate Change Basics: What’s the Big Deal?
• Intermediate Backgrounder #2: Climate Change Impacts: A Changing World?
• High School Backgrounder #5: How do we Know?

Teachers should also be able to help students identify differences in tree rings (and their significance) as outlined in the Teacher Handout: Dendrochronology: Investigating the Recent Past (attached to this lesson).

Learning Outcomes:

Click on the icon for your territory to review the learning outcomes that are addressed by this lesson:

Introduction to Lesson Plan:

The understanding and awareness of historical climate change is steadily growing as people around the world gather information from glacier ice, trees, lake bottoms and oral traditions.

While scientists use glacier ice and muck from the bottom of lakes and oceans to determine that the climate was like thousands of years ago, trees can tell about the more recent past.

Here in the north, close study of the growth rings of a tree can reveal a hundred years or more of climate history. Tree rings can tell us stories about relative temperatures, precipitation and growing seasons, and about extraordinary events, such as fires, that may have affected tree growth.

Before this lesson, go over the basics of climate change and the potential impacts outlined in Intermediate Backgrounders #1 and #2. It is also necessary to teach your students a little bit about dendrochronology and it’s importance to the study of climate change. A brief overview has been provided below and you may refer to High School Backgrounder #5: How do we know? for more information. Reading the teacher’s handout on dendrochronology that is attached to this lesson will help you to complete the lesson successfully and help the students identify the differences in the tree rings that they might possibly encounter.

It’s recommended that you review with your students the process you’ve chosen for acquiring and studying a tree sample (options described below).

Download and make copies of the Student Handout: Dendrochronolog Sheet attached to this lesson. Prepare the necessary materials.

Overview of Dendrochronology:

Dendrochronology is the study of tree rings to answer ecological questions about the recent past:

dendron = tree, chronos = time, logos = the study of

From the study of tree rings, students can determine the approximate age of their trees, as well as the climatic conditions each of their trees might have faced over its lifetime. Every year, trees produce a new ring of wood underneath their bark. The width of a ring tells us about the climate in the past because the growth rate depends largely on precipitation and temperature during the growing season. Trees grow more during wetter years with good temperatures (wider rings) and less during colder, drier years (narrow rings). It is important that students learn that tree rings are an important source of confirmatory data, at least over the near term (the life span of the tree being studied).

Two ways to get a sample of the tree rings:

1. Students can find dead standing trees, stumps, or cut wood from their woodpile. You may choose to cut cross sections of tree trunks using a band saw or chain saw (depending on the size of the trunk). This is the most destructive way, but tree disks are the best samples since incomplete rings can be detected, narrow rings can sometimes be seen better somewhere else around the stem, and patches of rot can be avoided when dating the disk. (Note: Tree sections can be used year after year. If students mark them with pencil, they can be erased or sanded clean.)
2. Core samples can be taken from trees with an increment borer. Step by step instructions are included in the activity section below. You can obtain an increment borer from your local Forestry office (from Bob Sharp if you are in the Yukon), or purchase one on-line from Forestry Mall, in B.C. at: http://www.irl.bc.ca/Forestry%20Supplies/increment_borer.htm

Important note: If you use the increment borer, you need to make, beforehand, a small grooved piece of wood to hold the core sample so that you can sand it down to examine the rings.


core sample holder

The following resources were drawn upon for the development of this lesson plan:

“Creating a Climate for Change Teacher Resource, 2001,” developed by individuals from Alberta Environment, Southern Alberta Environmental Group, USC Canada, Helen Schuler Coulee Centre, and the University of Lethbridge.

“Dendrochronology: Investigating the Recent Past,” by Chris Marion; February 1998

Photographs of the process of taking a core sample, and the core sample holder compliments of:

Brian C. McCarthy,
Dept. Env. & Plant Biology, Ohio University
Athens, Ohio, USA
http://www.plantbio.ohiou.edu/epb/instruct/ecology/dendro.htm

Activity:

Before you move on to the steps of the activity, review the climate change information referenced above and introduce students to an overview of dendrochronology. Encourage students to speculate about what they might be able to learn about climate change from tree rings.

Steps 1–8 are for acquiring core samples with an increment borer. If you are using tree cross sections for this lesson, skip ahead and start at step 9.

1. Prepare the pieces of grooved wood to hold the cores before going out to get them (see picture of core sample holder, above).
2. If you have an assistant to stay with the rest of the class, take your students out into the schoolyard area four at a time to use the increment borer to get a core sample. Otherwise, you could get them involved in an outdoor activity while you take them aside a few at a time. Select a tree that you wish to get a core sample from. Identify the species. If you are working in a forested area or park-like setting, consider keeping students busy with a nature scavenger hunt or identifying tree species.

   
   The increment borer: handle, bit, and extractor

3. Place the end (bit) of the increment borer against the bark of the trunk about 50 cm up from the ground. Hold the bit just behind the threads and apply as much body pressure as possible. Slowly turn the handle until the threads are fully engaged. You want to core on a slight up angle so that, later, water and fluids will drain out, not into the tree. This reduces the negative effect of coring on the tree. Note: Plugging the tree is not recommended.
4. Once the threads are engaged, step back and turn the handle clockwise. When you’ve reached the right depth, if the extractor is not already inserted in the increment bit, back the bit out one full turn, and then insert the extractor into the hollow shaft. Note: place a slight up-pressure on the back of the extractor to ensure the leading tip stays under your sample. Insert the extractor to its full length (depending upon the species, this may require that you apply some pressure with the heel of your hand). Do not drive the extractor in with a hammer or other implement. If the extractor is inserted, pull back on the handle and pull out the shaft of the borer.

   

5. Place the core sample into a straw to transport it back to the class in one piece. Note: It’s important to only take one or two cores from any one tree.
6. On returning to the room, glue the core into a piece of grooved wood.
7. Sand the core down with a palm sander (can also be done by hand using a piece of sandpaper) to make a flat surface so that it is easier to see the rings.
8. Count the number of rings, beginning at the outside of the core sample; use a magnifying glass or dissecting microscope for easier recognition of the tree rings.
   
   Note: Some students may have difficulty counting the rings and if there is a rotten section, their core sample may fall apart when it is taken out of the borer. Careful cross dating of the samples collected this way is necessary as very narrow rings or incomplete rings may be missed since the cores represent only a small portion of the trees’ cross-section.
9. Use a pencil mark to mark every 10th year (decade).
10. Calculate the age of your tree.
11. Using the Student Handout: Dendrochronolog Sheet (see Handouts), start counting at the outer most ring. This ring represents the harvest year of the tree if your students are working with a cross section and the present year if they are working with a core. If the ring is narrow, draw a long line on the sheet, and if the ring is wide draw a short line on the sheet. These longer lines represent the poorest growth years.
12. Students can check each other’s core samples.
13. Once all the samples have been checked and the information recorded on the sheets, compare the results with the entire class.
14. Make two bar graphs: one graph showing the number of trees cored (on the y axis) and the decade they were planted (on the x axis), and the other graph showing the number of trees cored (on the y axis) and the years of poor growth (on the x axis). Check out the web page for the author’s class to see their graphs and pictures of the students at work in the field and the classroom: http://www.yesnet.yk.ca/schools/jackhulland/classes/colberg/climate_change/trees/dendro.html
15. Correlate the students’ findings with climate records from your local weather office (they can tell you what the weather was like in certain years), or with information from the following web site that will give both daily and average records: http://www.climate.weatheroffice.ec.gc.ca/
    Welcome_e.html
    
    Find out from your local fire office, or possibly from the Internet, when fires came through in your region – were they in the years of poor growth? Have students ask their parents what they remember about unusually dry winters with little snow or very wet summers. Correlate this oral history information with student findings. Follow up these exercises with a discussion about the class’ findings.

Handouts:

Click on the icon for the complete set of handouts that support this lesson:

Teacher Handout: Dendrochronology: Investigating the Recent Past
Student Handout: Dendrochronolog Sheet

Student Web-Exchange

Students can post their findings on the student exchange portion of the web site and encourage other schools to try the activity and then compare their findings (the age of the trees at their school, what they learned about the history of the area (climate, fires, etc). Click on the icon for information on how to post material.

Evaluation:

Evaluation will vary depending on the objectives of the learning outcomes. Student tasks for evaluation may include:

1. Have students show you their core sample and explain how they have determined the age of the tree it came from. (You should see the pencil marks to show each decade counted. By counting their marks, they can calculate the age of the tree to the nearest decade.)
2. Have students identify a ring of poor growth and explain why it is from a year of poor growth. (Narrow rings indicate years of poor growth.)
3. Have students suggest possible reasons for this year of poor growth. (Dry and cold summers, late cold falls, and fires can be possible reasons for poor growth rings – see attached “Dendrochronology: Investigating the Recent Past.”)

Enrichment Ideas:

English Language Arts:

Tree-ography: Have students write a story about their tree, either imagining that they experienced change with it, or telling about changes from the tree’s perspective. Changes that have affected the tree could include climate (wet, dry, warm, cold), the building of the new school, the tree’s arrival from a nursery, the student’s involvement or connection with the tree, fires, or other events apparent from the sample. Students could also write about how they think climate change may have affected their tree.

Science:

Investigate: Students can discover other ways that scientists learn about past climates and monitor today’s changing climate by visiting the following website designed for kids, grade 4–9:

http://www.epa.gov/globalwarming/kids/detectives.html

Teachers can also refer to High School Backgrounder #5: How do we know? for information about ice cores, sediment samples, and so on.

About the Author:

My name is Nancy Colberg, and I teach grade 4 at Jack Hulland. Elementary School in Whitehorse, Yukon. I have a wonderful class of 27 active students, who are very much interested in doing experiments, writing and performing plays, and working on our class website.

Whitehorse is a city of about 30 000, fluctuating up and down according to the economic situation of the day. It’s a beautiful community, with lots of nature right outside our backdoors. In winter you can cross country ski and snowshoe while in summer biking and hiking are popular.

I received a BSc in chemistry and biology from the University of Alberta way back in 1970, went overseas to work with CUSO in Sarawak, East Malaysia, for two an a half years, obtained a teacher¹s certificate from U of Manitoba in 1974 and then taught in Snow Lake, Manitoba, a northern mining community, for three years. I needed a change from teaching, so I moved up to the Yukon in 1977 where I lived in the small village of Teslin, with its population of 500 people. I returned to teaching there in 1987, and moved into Whitehorse in 1991 where I I began teaching at Jack Hulland School.

Many people who have lived in the Yukon their entire lives or since the 40’s and 50’s have stories to tell about the cold winters here: How in the 50’s it would get to -50°C and -60°C, and you could push your car to one side with your finger because the road was so slick with the cold, or how people would have to thaw their car engines by directing heat from a fire through a stove pipe shoved under the car. Even since the 70’s it doesn’t seem to be as cold in the winter. Then we often had two or three weeks of –40°C temperatures. Now that seems rare.

I am very much interested in learning, along with my students, how we in our northern communities can make a positive difference with regard to climate change.