Analyzing Tree Ring Data Sequences

Students align "tree ring cores" of different ages to build up a long timeline of past climate data.

Learning Objectives

  • Students will understand that data about past climates and climate change can be gathered from sources other than weather data, and that some of these data sources span time periods long before human weather observations began.
  • Students will observe the direct correlation between climate and annual tree growth patterns.
  • Students will learn how longer climate chronologies can be built up by combining tree ring samples from multiple trees as long as the growth periods of the different trees overlap somewhat.

Materials

  • For each group: 
    • Transparent tape 
    • one set of tree ring samples  (See preparation section below) 
Four Simulated Tree Ring Core Samples

 

Preparation

  • Print copies of the simulated tree ring samples, which has two sets of simulated tree cores on each page.

    Cut apart a set of four simulated tree ring cores for each student group. Print on cardstock if you want to make sturdier cores. For a more realistic experience, you can slide the paper strips into clear plastic straws to make the cores look more authentic.

    Introduction

     

Tree rings showing light, early wood and dark, late wood rings plus the central pith

  • Tell students that, each year a tree grows, their trunks grow bigger around. The new growth that makes the trunk larger is called a tree ring.

    Each tree ring has two parts. In the spring and early summer each year, trees normally grow quickly, creating light-colored wood in a wide ring. This wide, light part of the ring is called "early wood." In the late summer and fall, trees normally grow more slowly, producing dark-colored wood in a narrow ring. The dark, narrow part of the ring is called "late wood". A full year's growth includes both a light, early wood ring and a dark, late wood ring.

    Some years a tree grows more than it does in other years. If a year is very dry, the tree hardly grows at all. If there is plenty of water in a different year, the tree will grow more than in a dry year. Other factors, like the temperature, can also affect how much the tree grows.

    If you count the number of rings of a tree, you can accurately tell the age of the tree. If you measure the width or thickness of the tree rings for each year, you can tell whether each growing season for trees in that region was good or poor.

    To see the tree rings without cutting down the tree, scientists use a tool to extract a long, thin sample - the shape of a straw.

Tree ring core and tree boring tool

  • Directions

    1. Orient students to the simulated tree cores. The samples came from the following sources:

    • Core Sample 1: From a living tree from the Pinetown Forest in the fall of 2018
    • CoreSample 2: From a tree from the Pinetown Christmas Tree Farm a few years ago
    • CoreSample 3: From a log found near the main trail in Pinetown Forest
    • CoreSample 4: From a barn beam removed from Pinetown Hollow

    2. Tell students that they will count the annual growth rings in each sample to determine how many years each tree lived. Then, they will line up the core samples next to each other to build up a longer history of climate based on the combined tree ring patterns. Finally, they will analyze the combined climate record to see what you can determine about past conditions in Pinetown.

    3. Instruct students to not count the pith when counting tree rings. In the simulated tree ring cores, each annual ring has a wide, white area that represents early wood, followed by a thin, black line that represents late wood. Finally, the last ring in each core is followed by bark. Don't count the bark when counting tree rings. This example (below) shows what a simulated tree ring core, including the pith and the bark, for a tree that lived for five years would look like.

    Example of counting rings on a core from a 5-year-old tree, showing the pith, early wood and late wood

    4. Have students make a table with headings like the one below in their science notebooks. (By the end of the activity, it will be filled with the information below.)

    Core Sample Age of Tree Year Cut or Cored Year Growth Began
    1 30 2018 1989
    2 27 2015 1989
    3 38 2013 1976
    4 27 1995 1969

    5. Ask students to count the number of tree rings in each core sample. That number is the age of the tree in years. Have students record the age of each tree in their table.

    6. Have students determine the year that the tree that core sample 1 was taken. Tell them that core sample 1 was taken from a tree in the Pinetown Forest in the fall of 2018. Have students count backward along the rings in core sample 1 to determine the year the tree started growing. Tell them to write the year in the first row of the Year Growth Began column of the table.

    7. Next, have students align samples 1 and 2. Tell them to place core sample 1 and core sample 2 next to each other. Describe how these two trees lived at slightly different times, but most of their lives overlapped. Have students slide the two samples back and forth until their tree ring patterns line up.

    8. Have students use the known dates for core sample 1 to determine the dates for sample 2. Matching ring patterns show that both trees began growing in 1989. Sample 2 was cut or cored 3 years earlier than sample 1, so its last ring is from 2015. Have students determine start and end dates for sample 2 by comparing with sample 1, then record these values in their table.

    9. Tell students to use the same technique to align core sample 3 with sample 2. Have them place sample 3 next to sample 2 and align the ring patterns. As before, have students determine the year that sample 3 began growing and the year it was cut or cored, and record those values in their table.

    10. Ask students to repeat this process with core sample 4. Have them write the Year Cut or Cored and the Year Growth Began in their table.

    The diagram below shows the appearance of the four simulated core samples when they are properly aligned (click to enlarge).

    Tree ring cores properly aligned with the years 1990, 2000, 2010, and 2018 indicated.

    Assessment

    • Check the values students have entered into their tables. If you spot any errors, ask students to double-check their work by counting the rings again. The inner pith and outer bark layers might cause confusion for some students; check the Background & Extensions section of this activity for more about the pith.
    • Ask students to point to the tree ring (or rings, if they appear on more than one sample) that matches the year they were born.
    • Ask students whether the weather in Pinetown the year they were born was good for tree growth (wide ring) or not very good (narrow ring).
    • Suppose that poor tree growth around Pinetown was mainly caused by drought. Ask students to identify years that were probably drought years, which correspond to narrow tree rings.
      • Drought years include: 1969-1971, 1975-1979, 1983-1986, 1992-1995, 2003-2005, 2011-2013

Tree rings showing light, early wood and dark, late wood rings plus the central pith

  • Ask students whether Pinetown had more years of drought or plentiful rainfall. To reply, they need to count the total number of thin rings (drought years) across all of the samples and compare that with a count of wide rings (rainy years). The cutoff for what constitutes a sufficiently narrow ring to indicate drought, and likewise sufficiently wide rings to indicate rainy years, is subjective. This is a good discussion point to consider with your students. By our count, there are more narrow rings (roughly 20) than wide rings (closer to 10), so Pinetown has been experiencing more dry years than rainy ones over the past five decades. Your results will vary, depending on what you consider an "average" width ring and how much narrower or wider a ring must be to indicate drought and plentiful rain, respectively.

  • Background: Tree Rings & Dendrochronology

    Trees contain some of nature's most accurate evidence of the past. Their growth layers, appearing as rings in the cross section of the tree trunk, record evidence of floods, droughts, insect attacks, lightning strikes, and even earthquakes.

    In this activity, students discover how scientists "decode" tree rings to learn about the past. Scientists combine records of tree ring patterns from many trees to create longer chronologies of climate history.

    Each year, a tree grows. Trees grow taller over the years, but their trunks also grow "fatter" or bigger around each year. The new growth that makes the trunk larger is called a tree ring.

    Each tree ring has two parts. In the spring and early summer each year, trees normally grow quickly, creating light-colored wood in a wide ring. This wide, light part of the ring is called "early wood." In the late summer and fall, trees normally grow more slowly, producing dark-colored wood in a narrow ring. The dark, narrow part of the ring is called "late wood". A full year's growth includes both a light, early wood ring and a dark, late wood ring.

    The pith is a special type of plant tissue found at the center of tree trunks. Scientists don't count the pith when counting tree rings. In the simulated tree ring cores used in this activity, each annual ring has a wide, white area that represents early wood, followed by a thin, black line that represents late wood. Finally, the last ring in each core is followed by bark. Students shouldn't count the bark when counting tree rings. This example (left) shows what a simulated tree ring core, including the pith and the bark, for a tree that lived for five years would look like.

    Some years a tree grows more than it does in other years. If a year is very dry, the tree hardly grows at all. If there is plenty of water in a different year, the tree will grow more than in a dry year. Other factors, like the temperature, can also affect how much the tree grows. If you count the number of rings of a tree, you can accurately tell the age of the tree. If you measure the width or thickness of the tree rings for each year, you can tell whether each growing season for trees in that region was good or poor.

    Scientist can use tree ring patterns to learn about past droughts and other aspects of climate. If a region is suffering from drought and is very dry for several years, the trees in that region will grow slowly and have very narrow rings. The science of studying tree rings to see what they tell us about past climates is called dendrochronology.

    Scientists who study tree rings, dendrochronologists, almost never cut down trees to analyze their rings. Instead, they use a tool called a borer to extract a long, thin sample - similar in size to a pencil or a straw - from a living tree without harming the tree. Scientists screw the borer into the tree and pull out a sample of wood that is about 4 millimeters (less than a quarter-inch) in diameter, then seal the hole to prevent disease.

    Tree ring core and tree boring tool

    Dendrochronologists combine the data from different trees to create a longer history than they could make from any single tree. For example, imagine two trees that each lived for 100 years in the same location. If tree A started growing in 1830 and died in 1930, and tree B started growing in 1910 and died in 2010, we could combine their tree ring data to build a history of climate from 1830 to 2010. This would allow us to create a history of climate spanning 180 years, far longer than the record from a single 100-year-old tree.

    In this activity, students align simulated tree ring core samples to build up a longer climate chronology for the area around the fictional village of Pinetown.

    Most actual tree ring studies are done in either cold or dry regions where trees are barely able to survive and grow very slowly. In places where trees grow slowly because of cold temperatures, such as sub-polar latitudes and in mountains just below the tree line, different growth rates are most strongly associated with slight changes in temperature. In such locations, tree growth rings that form during warm years are much wider than the usual rings formed in typically cool years. Variations in moisture levels are not as important of a factor in tree growth rates in cool climates, so the tree ring records primarily record temperature data in those places. On the other hand, trees that grow in arid regions are more strongly influenced in their growth rates by slight changes in precipitation from year to year, while changes in temperature would have a lesser impact on the variations in tree ring widths. By looking at tree ring patterns in different environments, scientists can learn about both temperature and precipitation patterns in the past somewhat independently.

    In practice, dendrochronologists combine patterns from many trees in a region and average those together to estimate the climate history in that region. Two trees that grow within a few meters of each other could still experience substantially different microclimates, influencing their growth rates and, thus, ring patterns. One tree might be shaded by other nearby trees, or be in a slight depression that collects water when it rains. The soil might be richer or poorer in one area. Scientists collect data from many trees in a region to average out these microclimate-induced fluctuations.

    As modeled in this activity, scientists also combine records from trees that lived at different times to build up longer climate chronologies. As long as the trees have some overlap in their lifespans, it may be possible to match up their ring patterns to produce an extended history. Especially in arid regions, it may take a VERY long time for the trunk of a dead tree to decompose. In some cases, dendrochronologists can extract samples from trees that have been dead for centuries, which they then align with patterns from old, but still living, trees. Wood can also be preserved in human construction and crafts, and samples from various wooden objects have been used to extend tree ring chronologies. Examples include beams from old buildings and wood from log cabins, planks and beams from old wooden ships, and even samples of wood from old wooden instruments such as violins. In some places, forests have been flooded when new lakes formed, or trees fell over into peat bogs, preserving the wood for many generations. Some climate chronologies created by combining tree ring records span many centuries of time.

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    Credits

    This activity was a part of Project LEARN at UCAR, was modified with permission from Global Change: Time and Cycles (U.S. Geological Survey), and was revised in 2018 by Randy Russell.