Tree Rings - How We Use Them
Notes by J. Zimmerman, Ph.D.
Forest and Tree Home.
How can you use tree rings?
What do Dendrochronologists do?
Tree-ring science and
Dendrochronologists -- What Do They Do?
- Read the patterns of wide and narrow rings in wood from different
trees. They match patterns from different trees to identify the year that a
tree produced a particular ring. This process of matching across specimens is
- Estimate archaeological ages. Andrew Ellicott Douglas was the
first dendrochronologist. He became famous for his studies of Pueblo ruins,
which he analyzed by cross-dating, the tool that he developed as his "key to
- Predict future drought, flood, and other conditions. A. E.
Douglas proposed that tree rings would be allow us to measure past climate
and, potentially, to predict drought and flood over a period of years, which
would provide economic advantage.
- What tree rings are.
- The longest continuous tree-ring chronology in the world is a series of
oak sequences that overlap for almost 10,000 years (100 centuries). This
almost reaches the end of the last Ice Age, about 11,500 years ago. Bernd
Becker and his colleagues developed this in Germany.
- In the USA, tree rings have provided dates back over 13 centuries.
- Researchers can determine the age of a living tree by using an increment
borer to take a careful core sample. A core of the width of a pencil can be
extracted with the hollow borer.
- In the tropics, trees can grow year-round. Therefore, annual rings do not
grow in most tropical trees.
- Why tree
What is Dendrochronology?
The science of studying the past by looking at
tree rings is called dendrochronology.
Here are examples:
- The year that a building was constructed can be estimated if wood
used in its construction has bark still intact. The tree ring next to the bark
is the the ring created in the last growth-year of the tree. Tree ring dates
from different sections should cluster within a short period. This would
indicate that the trees were cut specifically for the construction, rather
than being miscellaneous sections of re-used wood.
- Reconstruction of past climate variations, such as precipitation.
In North America, climate changes may have triggered certain migrations in
cultures before the European arrival.
- Not only frequency of fire, but also intensity of fire
can be estimated. For example, if a tree had a growth spurt after a fire, the
An intense fire usually burns off the foliage of a
tree. If the tree survives, growth rate is reduced for years until foliage
regrows, restoring the photosynthesis of sufficient nutrients.
- Removed competition
- Was not fierce enough to damage this tree permanently.
- Reconstruction of past insect infestations. Insects are an
integral component of forest ecosystems. They can cause mass tree mortality,
leading to increased wildfire hazard. Or they can devour leaves, reducing
- Reconstruction of past glacial activity.
- Reconstruction of past volcanic events.
- Reconstruction of how much rain fell in the past. The width of an
tree ring usually depends on how much rain fell during the year the ring was
- Accurately measure the widths of all rings.
- Calibrate the tree-ring widths with current tree rings and known local
- From the widths of the old rings, calculate how much rain fell annually
before rainfall records were kept.
- Assessment of the "age structure" (distribution of tree ages) of
a forest. Forest managers need this.
- Climate and wildfire predictions. Based on the past found from
tree rings, it is becoming possible to predict future climate behavior over a
period of years, and to predict seasonal conditions that lead to severe
Forest and Tree Home,
© 2002-2013 by J. Zimmerman.
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