Trees: Recorders of Climate Change

Students are introduced to tree rings by examining a cross-section of a tree, also known as a “tree cookie.” They discover how tree age can be determined by studying the rings and how ring thickness can be used to deduce times of optimal growing conditions. Next, they investigate simulated (paper printout) tree rings applying the scientific method to explore how climatic conditions varied during the Little Ice Age.

Learning Objectives

  • Students will identify seasonal and annual growth in a cross-section of a tree.
  • Students will understand that thickness of a tree ring is affected by environmental conditions.
  • Students will understand that evidence of past climates is recorded in series of tree rings.
  • Students will learn to interpret past climate conditions from tree ring thickness.
  • Students will collect and analyze tree ring data, testing a hypothesis and drawing conclusions.

Materials

Simulated Tree Cores for the "Trees: Recorders of Climate Change Activity"

Preparation

  • Prepare or order tree cookies.
    • To prepare: cut a cross section through a tree trunk for each student (0.5 to 1” thick and 4 to 6” diameter is ideal). Contact a local tree trimming service and ask for cuttings.
    • Or, order tree cross sections from a classroom supply company (allow several weeks for delivery).
  • Print copies of the Simulated Tree Cores PDF and cut them apart. (You may wish to provide several copies of each simulated tree core.) The dates will need to be kept with each core.
  • Place simulated tree cores at stations around the room with signs that indicate the time range.
  • Print one Student Page for each student.

Directions

Part A:

This is an image showing the cross section of a tree trunk with visible light and dark tree rings.

A simple, yet elegant picture of tree rings from a very young balsam fir (Abies balsamea).
Credit: Henri Grissino-Meye

  1. Pass out tree cookies and magnifying glasses (if using) to each student. Familiarize students with their tree cookie cross section.
  2. Explain that trees produce rings as they grow each spring and summer.
  3. Ask students to describe the tree ring colors. Is there a pattern to the light and dark rings?
  4. (Show slide 2: Tree Rings) Explain that wood grown during the first part of the growing season is light in color and wood grown late in the growing season is darker. A light and a dark band together are the total growth for one year. Explain to students that the study of the ages of tree rings is called dendrochronology ("dendron" is from the Greek for “tree,” and “chronology” is the study of a time sequence).
  5. Ask students how old their tree was when it was cut. (Students should count the light/dark couplets of rings to estimate age.)
  6. Ask students which rings they think are youngest. Which are the oldest? (There may be multiple hypotheses suggested by your students. If so, discuss the likelihood of each hypothesis, leading students to understand that the outer ring is, in fact, the youngest.)
  7. If the time that the tree was cut is known, have each student count backward to find the ring that represents the student's year of birth.
  8. Explain to students that past climates can be interpreted based on how the tree rings formed. This is called dendroclimatology.
  9. Ask students if all the rings are the same thickness. They will likely notice that some rings are thicker than others. This can be for many reasons, but mainly the variations in ring thickness relate to growing conditions. In high latitude areas, tree growth is limited by the length of the growing season, which is controlled by temperature. In other environments, tree growth is greatly controlled by water availability.
  10. Tree ring thickness can vary from year to year. Review with students that thick rings indicate a"good" growing season, and narrow rings indicate a shorter or drier growing season. Ask students to identify which year had the best growing season.
  11. Tell students that there are two ways to study tree rings. Scientists can use cross sections, but this is typically only done if the tree has died because the process kills the tree. Instead, scientists usually take cores from living trees to study the rings. When a tree is cored, a small cylinder of wood is pulled out, smaller than the diameter of a drinking straw. Coring does not harm the tree. The rings can be studied from the cylindrical core. (Show slide 3: Tree Coring)

Part B:

  1. Show students the simulated tree rings that they will use for this activity. They are made to look like tree cores (except they are flat). The simulated tree rings combine the light and dark growth bands so that each interval along the simulated tree ring core indicates a year’s growth.
  2. Explain that the simulated tree ring samples are based on data from trees from high northern latitudes where the length of the growing season controls ring thickness. Since the length of the growing season is based on the temperature, thick rings will form when the climate is warmer and thin rings will form when the climate is cooler. The simulated rings cover a period from AD 1402 to 1960.
  3. Pass out a copy of the Student Page and a ruler to each student. Familiarize students with the directions before beginning. Students will make a hypothesis (or do this as a class), collect data, and make interpretations.
  4. Instruct students to develop a hypothesis about the climate over the total time interval. Explain that the class will test the hypothesis by collecting data from tree rings. After studying the tree ring data, they will either accept the hypothesis or reject the hypothesis.
  5. Data collection: Instruct students to visit each station and measure the total thickness of the tree rings for each time interval in millimeters (model this procedure for students), recording the thickness in the appropriate place in the data table on the Student Page.
  6. Data analysis: Instruct students to divide their measurement for each time interval by the number of years in each interval to get average ring thickness for each time interval. Record this in the table.
  7. Interpretation: Have students answer the thought questions on their student page and discuss answers as a group.
  8. In summary, describe that paleoclimatologists use a variety of “proxies” to interpret past climate. These proxies indicate the relative temperature but are an indirect record of it. Ask students if they think that the tree rings are a source of direct or indirect evidence of past climates.

 

Background

Dendrochronology: The study of the growth of tree rings.

Dendroclimatology: The study of the relationship between climate and tree growth with the objective of reconstructing past climates.

The growth layers of trees, called rings, preserve an interesting record of environmental conditions over the lifespan of the tree. They record evidence of environmental events such as floods, droughts, insect attacks, forest fires, lightning strikes, and even earthquakes. Many consecutive tree rings also record longer-term and more subtle changes in climate over time.

Each year, new wood grows on the outside of the tree trunk, just under the bark. A year’s growth is called a tree ring. Each tree ring is made of a band of light-colored wood produced early in the growing season (spring and early summer) and a dark-colored band produced late in the season (late summer and early fall). Counting the rings of a tree will determine its age.

Scientists seldom cut down a tree to analyze its rings. Instead, core samples are extracted using a borer that’s screwed into the tree and pulled out, bringing with it a cylinder of wood about 4 millimeters in diameter. The hole in the tree is then sealed to prevent disease (see images of this process on the Tree Coring slide).

Tree rings are an example of climate proxy data, providing indirect evidence of past climates. Scientists can use tree ring patterns to reconstruct regional patterns of climatic change. The amount of tree growth depends upon various local environmental conditions. At high latitudes, the amount of tree growth is mostly controlled by temperature. Trees grow thicker rings when the growing season is longer and thinner rings when the growing season is shorter. The length of the growing season is related to the climate, namely the temperature. If the rings are a consistent thickness throughout the tree, the climate likely did not vary over the lifespan of the tree.

The simulated tree rings used in the second part of this activity were developed based on the results of a study by Briffa et al. (2001) in which the ring patterns of 387 trees from northern latitudes were used to interpret temperature variations over the past 600 years. Since the thickness of the simulated tree rings is based on the results from many trees, it shows an average trend. Typically, climatologists require at least 30 years of data to establish understanding of climate. To understand changes in climate requires even more data. Generally, dendroclimatologists use large databases of tree ring data to compare the records of many trees, and interpret when, where, and how quickly climates have changed.

This activity is included in theCLEAN Climate and Energy Resource Collection, a peer-reviewed collection of activities, curricula, videos, and other tools for teaching.

Extensions

Take a field trip to take core samples from local trees with a core tool and then analyze the tree rings.

Sources of Tree Ring Cross Sections

Related Resources

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