Part 2 - Coring Trees and Observing Ring Patterns

Now that you have considered the nature of your research site, and how environmental conditions affect tree ring growth, it's time to roll up your sleeves and get to work!

Trees typically grow one ring per year. They start growing in the spring (the cells are light tan in color, known as early woodis the light colored portion of a tree ring produced in the spring) and, as the growing season ends in the fall, the cell walls thicken (the dark band or late woodis the darker part of an annual tree ring produced in the summer season) and eventually stop growing in the winter causing a very distinct ring. The ring pattern that forms over the entire life of the tree reveals the climatic conditions in which the tree grew. At sites like Humpty Dumpty talus slope, enough moisture and a long growing season would result in a wide ring. A dry year might result in a very narrow ring. At sites where trees are more sensitive to temperature (e.g. at treeline on a mountain top or in the borealof the north or northern regions forests of northern Alaska and Canada) wide and narrow rings represent warmer and colder years, respectively. Because scientists study climate sensitive sites, where climate is the main driver of tree growth, trees from the same site will tend to grow the same pattern of rings and reflect the climate conditions.

Tree-ring scientists take core samples from trees to learn about climate change of the past. Watch dendrochronologist Nicole Davi describe how to core a tree.

WATCH: Nicole Davi cores a tulip tree with assistant Augie. Credit: Jacob Tanenbaum (MP4 Video 268.2MB Mar28 16)

People often ask if coring a tree hurts the tree. The core samples that scientists take from trees are quite small (less than the diameter of a pencil) and non-destructive to the tree. The tree will heal itself fairly quickly, similar to when a spout is removed after tapping a tree for maple syrup. If there is concern for a particular group of trees, scientists can dip their tree borers into alcohol to be sure they are not spreading any diseases from tree to tree. If forest managers are concerned for a particular forest, they can deny scientists permission to core trees when they apply for a permit.

In this activity, you will analyze a series of five cores from the Humpty Dumpty talus slope site. What clues can you uncover about the climate of this region over the past 100 years?

Instructions

1. Below are images of five core samples taken from trees on the talus slope. The cores were collected in 2002, so the last complete ring is the year 2001 and the 2002 ring is incomplete. Note that the bark of the tree is on the right side of the image. Since each ring represents one year of growth, you could count backwards one ring at a time to find the age of the tree when it was cut down. But to make this a simpler process, scientists place one mark per decade (1990, 1980, etc), two marks per 50 years (1950, 1850, etc) and three marks at the century mark (2000, 1900, etc). The marks (dark pencil dots) are shown in the image below:

2. Download this EXCEL spreadsheet (Excel 2007 (.xlsx) 34kB May19 17).

3. Data analysis example: Look at the first column in the spreadsheet. For the first core sample (HDU01A), "N" have been marked for rings that are narrow and "W" has been marked for years that have wide rings. These data are determined by comparing each ring to each nearest neighbors.

4. Your goal in this exercise is to figure out what the most consistently narrow, or consistently wide, rings are in the remaining four cores.

Zoom in to the most recent twenty years of the second core sample (HDU03A). This means you need to scroll all the way to the right. You will be looking for the narrowest and widest ring(s) in this time period. To assist you in the data gathering process, look at the example image above. In this sequence, the 1980s ring is a dot on the right, 1970 is marked with a dot in the center and the dot on the left is 1960. Look for the narrowest ring or rings in the series. If you determined that 1965 was the narrowest ring, a year of drought, you were correct! Now look for the widest ring of rings. Because 1974 and '75 are equally wide, they both should be considered the widest years in the sequence. After analyzing a 20-year segment, go on to the next period of 20 years (scroll left) and select the narrowest and widest ring in that series. Record all your data on your Excel data sheet.

Sample HDU01A

Sample HDU03A

Sample HDU06A

Sample HDU10B

Sample HDU12A

Stop and Think

  • 2.1 Compare your data for the five cores. Do the years marked "N" and "W" in your cores fall at approximately the same years or are they different?

    • 5. Look for marker years.

    Marker years are the years that stand out for being consistently narrow (dry) or consistently wide (wet) across all or most of the samples. Why are these years consistently wide or narrow? Because these trees are all growing in a moisture stressed site, and climate is the main factor driving their growth. If you look at many cores, as tree-ring scientists do, these marker years start to become more and more evident, and they help to guide scientists as they mark calendar years to the core samples.

    6. Re-examine your five core samples on your Excel worksheet and look for marker years.

    Stop and Think

    2.2 What marker years did you find in the samples? In other words, which years consistently stand out for being very narrow or wide compared to their neighbors?

    2.3 Share your results with your classmates. Decide as a group the four most prominent marker years to study in the next part of the lab.