The Science of Tree Rings

What is Dendrochronology?

Dendrochronology is the study of tree ring growth.  Trees that live in temperate climates which experience seasonal changes in temperature and moisture levels often produce one ring of growth per year.  The growth rings can be counted to help determine the age of the tree.  They can also be analyzed to determine data about weather patterns that the tree experienced in previous growing seasons.  Dendrochronology has been used to determine that the bristlecone pines of eastern California are the oldest known living trees on Earth.  They have been determined to be over 4700 years old.

Tree rings are formed during the yearly “growing season,” the exact timing of which is dependent on the geographical location of the tree along with the type of ecosystem that the tree is growing in.  As the growing season comes to an end the growth rate of the tree slows down.  The cells produced are smaller with thicker walls relative to previous growth. This creates the contrast between the light-colored rings and dark-colored borders between them. Many trees only grow during certain seasons, and remain dormant during winter months. Because tropical climates do not experience the temperature and weather changes necessary to slow tree growth, creating the darker rings, dating tropical trees by dendrochronology is difficult.

In order to obtain samples of tree rings a coring tool is used.  The coring tool is a long hollow tube that is screwed perpendicularly into the trunk of the tree until it reaches the center of the tree.  The tool is then removed from the tree along with a long slender rod of the trees tissue.  The core sample will show the growth rings that have occurred throughout the life of the tree.

 

The Uses of Dendrochronology.

Dendrochronology is useful for the study of present-day trees and forests, but can also be utilized in several additional fields.  Tree rings can also be used to provide information about past climates. Trees generally grow faster in years with more rainfall; thus, the rings corresponding to “wet” years will be thicker than those corresponding to “dry” years. This data can be used in a variety of ways. Scientists studying current climates and ecosystems often use dendrochronology to examine the effects recent weather patterns have on vegetation growth. However, in older trees, both fossilized and still living, dendrochronology can be used to determine past weather patterns.

This data in turn is used in the study of climate change. Through examining the rings of old or preserved trees, scientists can infer past climate cycles and then compare this data to current effects of climate. This provides information that could be essential in predicting effects of today’s climate change, especially those focused on vegetation and ecosystems. This is especially useful in that evidence of past climate patterns can be otherwise difficult to uncover, but dendrochronology-based climate studies are far from perfect. Because tree rings are only created during specific seasons, the data given by them creates an incomplete representation. Additionally, tree ring growth can be affected by factors outside of weather. In the absence of, or in conjunction with, other sources of information, dendrochronology remains an important tool in the study of climate change.

Archaeologists have also used dendrochronology to determine the relative dates of wooden artifacts, and can in some cases it can be used to approximate the age of other findings in related areas.  On June 6, 2016 tree ring coring was used on two persimmon trees (Diospyros virginiana ♀) located on the Carrigan Lot of the Lincoln Home National Historic Site, located in Springfield Illinois.  The purpose of the project was to determine the age of the trees to establish if the trees were present when the Lincoln family occupied the home.  The cores obtained from the trees contained between 103 to 116 rings (see below).  Adding this number to the estimated age (4 – 6 years) of the tree at the height the core was taken indicates that one of the trees is likely to have germinated somewhere between 1897 – 1899.  The other tree is believed to have germinated between 1894 – 1919.  These results indicate that the trees were not alive when the Lincoln family occupied the home.  Dendrochronology provides researchers with information that can be used in a variety of different ways.

Additional Information:

Image Credit:

  • By Larry McElhiney (Created this image on Honeymoon in Santa Cruz, CA) [CC BY-SA 2.5 (http://creativecommons.org/licenses/by-sa/2.5)], via Wikimedia Commons
  • Coring tool: Kayla Matthews – used by permission
  • Tree core sample: Core sample prepared by Glen Freimuth PhD . Photo by Guy Sternberg – used by permission

Article by Dave Cox and Kayla Windelspecht


updated on Sept 20th to correct name of the historic site and provide photo credits

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