7-12, Unit Two, Activity 2: "It Was a Very Good Year"

Students learn to reconstruct history through the careful examination of tree-rings, and then construct a tree-ring representation of their own life history.

Grades: 6 – 12
Time: 2 – 3 hours
Subjects: History, biology math, art, language arts, geography, intrapersonal skills


  • Copies of lodgepole, ponderosa, and whitebark pine tree-ring photos and explanations (ppt) from the U.S. Forest Service Fire Lab FireWorks Curriculum.
  • Real tree cookies and color photos of tree cookies available from Glacier's FireWorks Educational Trunk. Contact the Education Specialist.
  • Drawing paper and colored pencils / markers.
  • (Optional) Whitebark pine seedling available from the USFS Hungry Horse District nursery (call to make arrangements for pick-up).
  • LCD projector or Smart Board to show National Oceanic & Atmospheric Administration (NOAA) slideshow of how tree rings are used to understand climate change.

Whitebark pines are proving to be very valuable to scientists doing work in dendrochronology, the dating of past events through the study of tree ring growth. (Dendro- = trees, chronology = happenings through time.) Paleoclimatology, on the other hand, includes dendrochronology and is the study of past climate for the time prior to instrumental weather instruments. Paleoclimatologists use clues from natural "proxy" sources such as tree rings, ice cores, corals, and ocean and lake sediments to understand natural climate variability. The National Oceanic & Atmospheric Administration (NOAA) website has current research information in all aspects of paleoclimatology, including tree rings. View a slide show from the website about how tree rings are used to understand past changes in climate.

Some whitebark pines in Waterton-Glacier International Peace Park are over 1,200 years old, which means they have seen history unfold since before medieval times, when knights and castles were in Europe, 700 years before Native Americans had even seen a horse.

In the 1100’s, the world was in the “Little Ice Age”, a time of very cool summers and cold winters. Some historical climatologists believe that the sun actually reduced its output of radiation for a hundred years or so. Historians recorded that cool time, and tree rings from whitebark pines did the same, right here in the Crown of the Continent Ecosystem.

Recently, Scientists in the Waterton-Glacier International Peace Park have been examining the rapid shrinking of alpine glaciers and correlating that shrinking to global climate change. The 1990’s had 8 of the 10 warmest years in recorded climate history, and the first year of the new millennium is proving to be at least as warm. Glaciers form by the accumulation of winter snows which do not melt completely, and therefore have “growth rings” of layered ice, just like a tree. In warmer years, a tree such as the whitebark pine will have a longer growing season, and a larger tree ring. Whitebarks are showing just what the glaciers show, that recent years have been warm, indeed. An added bonus to the climatologists are a few whitebark pine snags and stumps which melt out of the lower edges of glaciers. Some were buried hundreds of years ago, and were very old when they fell into the clutches of the ice. By adding the growth rings of the pine snag to the layers of the glacier, these scientists come up with an accurate picture of the climate back thousands of years.

The difficult questions that arise from climate study are: (1) Are trends short-term variations or indicators of significant long-term change?; and (2) If the climate is changing, then why?

In Glacier Park, the number of glaciers, has declined from 150 in the year 1850 to less than 25 in 2007. Sperry Glacier, for example, has shrunk from 970 acres (392.5 hectares0 to just over 200 acres (90 hectares). It is clear that the Waterton Glacier International Peace park is warming. Two thousand, five hundred scientists, most of them Nobel Prize winners, geographers or climatologists, signed a statement in 2000 that recent global warming is probably due to the effects humans have had on the amount of hydrocarbons in the atmosphere – that is, the burning of fossil fuels such as coal, fuel oils and gasoline. Much smaller alternative groups, mostly economists and political scientists, have voiced objections, saying that global warming either is a myth or that it is not necessarily due to human influences on the world climate. In 2007, the Inter-governmental Panel on Climate Change says that with over 90% certainty, humans are contributing to global warming.

Climate has fluctuated between ice ages and extra-warm times in the last few million years. The average person does not know that it only takes an average yearly temperature drop of 2 or 3 degrees Celsius (6 or7 degrees Fahrenheit) to put the world into another ice age. Glaciers completely covered the Northern Rocky Mountains during those times, and sea level dropped about 300 feet because water was being stored on land (in the form of ice). The glaciers were a mile thick over the Flathead and Mission Valleys!

Most climatologists estimate that, if world temperatures rose by that amount, sea level would rise a corresponding amount. Florida is mostly 10 to 30 feet above sea level now, and most coastal cities aren’t much higher. If the world were significantly warmer than today, climate would be changed everywhere, and weather patterns would be difficult to predict. There could be, for instance, far more typhoons and hurricanes than today.

Predicting the future is always complicated. Most predictions, no matter what they are about, start with the records of the past and factor-in present trends. It is guesswork, and it is very seldom accurate. The glaciers in the Waterton-Glacier International Peace Park can help answer some questions: Is global warming happening? (Yes). In geologic history, is it unusual? (No). Will it keep warming? (No one knows, but current models indicate, yes).

1. Introduce this activity by giving the students some basic information about dendrochronology techniques. You may want to use a projector to show the slide show from the NOAA website or use the real tree cookies and "Tree Stories" role play to illustrate how tree rings form. Alternatively, you can use the whiteboard to illustrate rings growing while you read this narrative:

Each year a tree adds a layer of wood to its trunk and branches, creating the annual rings seen in a cross-section. New wood grows from the cambium layer between the old wood and the bark. In spring, when it is wet, the tree produces large cells and later, in the fall, the cell sizes produced are much smaller. In general, the width of the cells indicates the amount of available water for growth. The thickness of the cell walls is controlled by temperature and the length of the growing season (show diagram of tree ring from NOAA website).

Rings can be counted to see the age of a tree, but the size of a ring, the width of the cells and the density of the cell walls combine to tell a more detailed story about the life of that tree. Fire scars can tell us exactly which year a fire occurred, far back into history. Different trees from the same area, even those that are dead and buried in glaciers or under glacial deposits can be cross-dated to paint a picture thousands of years into the past! (more information on this is in the "Tree Stories" activity from the Fireworks Curriculum)

Cross-dating is simply an overlap of the identical sections of tree growth to match good years and bad years. These years can be matched to historical records of climate from other sources. This cross-dating has been very valuable in aging archeological sites. Bristlecone pines, both dead ones and live ones, have stretched climate records 9,000 years into the past (even though the oldest live trees are less than 5000 years old). Radioactive dating can be done on wood from trees which are well preserved and both dating methods can confirm the other.

1. The photos you have of a lodgepole, ponderosa, and whitebark pine from the will be your experimental “tree cookie”.

2. Assuming the tree was alive and just cut this year, how old is it? Why is it harder to tell on the Whitebark Pine tree cookie? (The tree rings are very close together because the growing season is so short and the tree grows very slowly).

3. What were some wet years you can identify?

4. The darkness of the rings shows the relative quality of the growing season, either the warmth or the length of the season. Were there some good years you can identify?

5. Is there any trend you can see in the past few decades?

6. Is a “good year” for the tree necessarily a good sign for other living things? Why?

7. Comparing whitebark pine's growth with the other species, why do you think the whitebark pine community is struggling with impacts from white pine blister rust and reduced fire? (The other species also have had reduced fire and diseases/beetles to contend with).

Optional: Students make "life history rings." (This can bring up some "serious" memories for children with troubled pasts. Discuss the idea with the school counselor when deciding whether to do this with your students).

1. As a homework assignment, have the students work with their parents/guardians to construct a tree ring representation of their own lives. They can use colored markers and make it an art project if you wish.

2. Wide and/or dense rings should represent years that stand out in their lives. Narrow rings can represent years they would like to forget. Parents/guardians can help to assess the quality of the early years. (Students who were adopted may have some vague rings from early childhood.)

3. Have students make their personal “tree cookie” large enough to write highlights and lowlights inside the ring.

4. Next day, have students share their personal histories, taking care to preserve whatever private happenings they may not wish to disclose.

How do you think historians decide which parts of history are important? Do you think that their own personal history influences their decisions? If you were 1,200 years old, like a whitebark pine, what would you think about recent history (since Europeans immigrated to this continent)? Do you think your own tree cookie will have different sized rings in the future? Will things beyond your control or your own decisions have more influence on your future rings?

Variations and Extensions:
Students can make tree-rings of their family history, local history, state or national history, history of the Waterton-Glacier International Peace Park – literally hundreds of possibilities… You may also look at real tree cookies and compare growth conditions, different species of trees or recent climate.

This activity works well as a transition to the Fireworks educational trunk.

A unit on Climate Change works well as an extension.

"Life history rings" and answers to questions are assessments.

Last updated: April 17, 2019

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