Tree Rings - How We Use Them

Notes by J. Zimmerman, Ph.D.

How can you use tree rings? What do Dendrochronologists do? Tree-ring science and tree-ring trivia.

Dendrochronologists -- What Do They Do?


  1. 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 "cross-dating."

  2. 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 prehistoric chronology."

  3. 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.

Tree Ring Trivia

  1. What tree rings are.

  2. 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.

  3. In the USA, tree rings have provided dates back over 13 centuries.

  4. 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.

  5. In the tropics, trees can grow year-round. Therefore, annual rings do not grow in most tropical trees.

  6. Why tree rings vary.

What is Dendrochronology?

The science of studying the past by looking at tree rings is called dendrochronology.

Here are examples:

  1. 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.

  2. Reconstruction of past climate variations, such as precipitation. In North America, climate changes may have triggered certain migrations in cultures before the European arrival.

  3. 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 fire probably: 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.

  4. 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 growth.

  5. Reconstruction of past glacial activity.

  6. Reconstruction of past volcanic events.

  7. 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 formed. Therefore:

  8. Assessment of the "age structure" (distribution of tree ages) of a forest. Forest managers need this.

  9. 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 wildfire.

See also these pages