|| ES 331/767 Exercise II|
James S. Aber
This lab exercise is based on tree-ring samples from the Pine Ridge of northwestern Nebraska--see Fig. II-1. The samples were taken from ponderosa pine (Pinus ponderosa) in the summer of 1980. These trees grow on well-drained slopes. Precipitation is normally less than 20 inches per year; winter snowfall is most important for recharging soil moisture. Cores were taken about ½ m above ground level.
||Ponderosa pine growing on the Pine Ridge in northwestern Nebraska. Tree-ring core samples were collected from this vicinity during the growing season (summer) of 1980. Photo date 7/80; © J.S. Aber.|
The core samples are mounted on a wood backing, then planed and sanded to create a flat surface. Two core samples are presented for this exercise; each is ~31 cm long (excluding bark). Both were obtained from tall, straight, large pine trees growing on steep slopes of the Pine Ridge.
|Tree-ring core sample #1. Download full-sized image via FTP (tree_ring1.jpg). |
|Tree-ring core sample #2. Download full-sized image via FTP (tree_ring2.jpg). |
Note: images on the webpage are not suitable for completing the exercise.
The relatively dark portion of each core is the "heartwood" of the tree, and both cores pass nearly through the growth center of the two trees. Bark on the left side marks the outer surface of the trees. The first band of light-colored wood inside the bark is the early growth of 1980.
An individual tree ring consists of two portions--an inner thick, light band of early wood grown in spring and summer, and an outer thin, dark band of late wood grown in autumn. Assume the light band just below the bark is the early wood of 1980. Use Adobe Photoshop (or similar software) to display the full-sized core images.
Hint: enlarge the images, so you can see each ring clearly. It is recommended that you mark each tenth ring with a small dot or asterisk (*) to expedite counting.
- Select specimen #1. What is the date of the innermost ring?
- Approximately what year did the tree begin growing (year of germination)?
- What period of growth is represented by the heartwood?
- Can you see any overall trend for increasing or decreasing ring width with age? If so, offer an explanation.
- Identify periods of unusually good or poor growing conditions.
- Do rings on either side of the core match exactly? If not, give an explanation.
- Now examine specimen #2 in the same way. What is the date of its innermost ring?
- Compare specimens #1 and #2. Do they show the same growth patterns? Do they exhibit anomalous growth for the same years? Explain your answer.
- Stout, T.M. et al. 1971. Guidebook to the Late Pliocene and Early Pleistocene
of Nebraska. Nebraska Geological Survey, 109 p.
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ES 331/767 © by J.S. Aber (2006).