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These "skeleton plots" help scientists learn the patterns of each tree core and then mark the cores correctly. Photo credit: N. Davi.

Provenance: nicole davi, William Paterson University of New Jersey
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Part 2: Use a Skeleton Plot to Discover the Age of a Pueblo Ruin

In the introduction, you learned that in order to connect a tree-ring record from historical wood to that of living trees, Douglass and his team would have had to find additional historical wood that existed around 1000 A. D. In 1929, Douglass and his team unearthed an interesting beam at a ruin in Show Low, Arizona. This piece of wood, shown below being wrapped by Douglass's assistant, turned out to be the key that made it possible to tie the historical record to the present day and assign actual calendar dates to historical pueblos across the US Southwest. It was the missing link. It allowed Douglas to develop a tree-ring chronology for the region. A tree-ring chronology is a series of annual values derived from the ring-width measurements of 20 or more trees of the same species at a single site. To create a tree-ring chronology, cores from the sampled trees at each site are cross dated a technique that ensures each individual tree ring is assigned its exact year of formation by matching patterns of wide and narrow rings between cores from the same tree, and between trees from different locations. This is accomplished by matching patterns of wide and narrow rings between cores from the same tree, and between trees from different locations.

To see this process in action, watch historical footage of A.E. Douglas at work cross-dating samples. Right click on the link to open the video in a new window.

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brief history of dendrochronology Jeff Lockwood Download Brief History of Dendrochronology (MP4 Video 15.9MB) Details

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A. E. Douglass's assistant excavating Sample HH-39 (charred piece of wood and wrapped in white and blue rope). Courtesy of Dr. Paul Creasemen/LTRR

You might imagine that to learn anything in science you need to collect reams of data and use mathematics, statistics and graphs to derive meaningful results. In some cases, however, useful information can also be obtained using qualitative means. In this activity, you will use a qualitative methodology to create a skeleton plotis a simple graphical representation of the ring width pattern in tree ring sample tree ring core (which you'll learn about below) from a scanned image of a sample of Western Juniper. You'll then use the plot you created to compare to one created by a scientists to date the pueblo. You will be doing exactly what Neil Judd and pioneering dendrochronologist A.E. Douglass did after collecting this piece of a supporting beam from a kiva in Pueblo Bonito in 1922.

Instructions

1. Obtain a piece of millimeter graph paper from your instructor.

2. Building skeleton plots is a useful strategy for accurately marking calendar dates on each ring on your tree-ring sample. Through skeleton plotting, scientists are able to compare ring patterns of different samples and assign an exact calendar date to each ring. Right click on the link for the Tree Ring Dating video for an explanation of how to make a skeleton plot and compare your plot to a master tree-ring chronology, which is composed of many tree-ring samples.

3. Make a Skeleton Plot for Pueblo Bonito

You will be creating a roughly 60-year skeleton plot for a section of the sample below. You must zoom in significantly to be able to see each ring.

4. Look at the image of the tree core sample below. Note that the bark of the tree would have been on the right side of the image, although no bark is visible. Had there been bark, you would be able to count backwards from the bark one ring at a time to find the age of the tree Zoom in so you can see a series of dots. To make analysis a simpler process, scientists place one mark per decade--ten tree rings--(1990, 1980, etc), two marks per 50 years (1950, 1850, etc). and three marks at the century mark (2000, 1900, etc). Select a section of the core that contains 6 decades of rings to study. Do not choose the very first interval from the center of the core to the first dot. The first series of rings is difficult to analyze. As shown in the video linked to above, each mark on the x-axis of your graph paper will be equal to one ring, or one year of growth. Each vertical line on your graph paper will correspond to one ring of the sample. The various heights of the lines will be drawn for those lines which are narrow, with the narrowest line in a sequence given the longest vertical line. "Normal rings" are left undrawn.

The specimen is a juniper cross-section from the kiva in room 162 at Chaco Canyon. It was collected by Neil Judd and A.E. Douglass in 1922. The ring at 954 AD is missing. Image provided by Dr. Pearce Paul Creasman.

5. Look at the sequence of rings you have chosen and decide what the width of an average growth ring is. You will be ignoring all of those rings and will focus on the narrowest rings. Remember that these data are determined by comparing each ring to each nearest neighbors.

6. Go through your tree-ring sequence and mark the narrow rings on your skeleton plot with a pencil. When you get to a single, or a sequence of, narrower rings, mark a tall line for the narrowest and proportionally shorter line for the others in the same grouping. See the example shown below.

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Each square on the x-axis represents on year. The yellow bars that appear on three of the graph's lines represent the narrow rings in this small sample. The narrower the annual ring, the the longer the line you draw. Credit: PBS

7. Dating the timber sample

In this part of the activity, you'll compare your skeleton plot to one produced by Richard Warren and his colleagues in 1971 to determine the age of the wood sample. They used the same technique dendrochronologists used to derive the actual dates during which the ancestral pueblo people lived in Pueblo Bonito. The task is to cross date, or match, your set of plot lines by sliding them along the lines shown in the Warren plot until the majority of your lines match. Right click on the Warren skeleton plot (Acrobat (PDF) 3.4MB Sep8 16) link to view in a new window.

8. Adjust the size of the Warren skeleton plot on your screen so it matches the same scale (1 mm) as your graph paper.

9. Fold your graph paper so that the top of your vertical bars show at the top of the page. Slide your 60-year skeleton plot along the Warren skeleton plot until you find most of your lines matching the standard plot. Record the beginning and ending dates of your skeleton plot.

10. Now, compare your skeleton plots with your classmates. This is an important step. If your plot does not match that of Warren, or your classmates who evaluated the same time period, then it is possible that you have made an error in your skeleton plot. It is easy to miss a ring or add an extra ring inadvertently when you are first learning this process. If you are having trouble matching your skeleton plot patterns with Warren's, go back and check you skeleton plot compared to your wood sample and see if you missed anything.

Skeleton plotting helps you assign exact calendar dates to a tree-ring sample, and at the same time, it helps you catch any mistakes! If you have made a mistake, your skeleton plot won't line up exactly with the master plot.