Tree Ring Research - August 29, 2007
Jeff Schalau, Associate Agent, Agriculture & Natural Resources, Arizona Cooperative Extension, Yavapai County

Just because your local area may have had some summer rain over the last month, it does not mean that drought is over. As the co-chair of the Yavapai County Local Drought Impact Group, it is my job to remind people that periodic drought is part of Arizona’s climate history and we are still in the midst of an extended drought period. One of the methods used to look at prehistoric climate patterns and drought is through the study of tree rings. The science of tree ring study is called dendrochronology. This is also the science used to date buildings inhabited by prehistoric cultures.

Each year, a live tree or woody shrub grows a new layer of wood on the trunk, branches, and roots. The region where bark meets the wood contains a thin layer of live cells called the cambium. The cambium gives rise to cells that will become wood (xylem) to the inside of the tree and inner bark (phloem) to the outside. This growth of the cambium is why trees and woody shrubs increase in trunk diameter as they grow older.

The portion that produces new wood grows vigorously in the spring and early summer producing lighter colored material we call early wood. As the growing season progresses and growth slows, wood is usually produced more slowly making it darker in color creating what is called late wood. The yearly pattern of early wood then latewood produces the characteristic tree rings that we commonly observe in wood. The combined width of early wood and late wood is called and annual ring. The outward growth of the inner bark does not have distinct annual rings because the growth of the wood causes these cells to be crushed over time. This phenomenon also gives each tree species its characteristic bark pattern.

The thickness of the annual ring is correlated to environmental conditions experienced by that tree during that growing season. The tree ring integrates the effects of soil properties, available water and nutrients, light, temperature, and competition from neighboring plants. Over the life of a tree, soil and site properties are not likely to change, so climate is a primary factor reflected in the relative tree ring widths. Wet years produce more growth creating relatively wider ring widths. Dry years produce less growth and narrower rings.

Dedrochronologists collect samples of living and dead wood from localized areas to reconstruct climate and history of human occupation. The live samples provide data about recent history which are crossdated with dead samples from tree stumps, down logs, and wood samples from structures and protected environments such as caves. Charcoal samples can also be used. It takes many samples to minimize localized “background noise” and reconstruct the climate history for that region. Forest fires also leave scars on these samples which allow scientists to reconstruct prehistoric fire patterns across landscapes and recurrence intervals.

Dendrochronologists have reconstructed 1,000 years of climate history for the southwestern United States to reveal past periods of drought. In Yavapai County, the driest ten year period was between 1662 and 1671 which only received 39.7% of the historic average over that 1,000 year period. The driest single year was 1773 when it appeared that no precipitation occurred. Going back even further, samples taken from bristlecone pines near Bishop, California have allowed scientists to reconstruct paleoclimate going back more than 2,000 years. Other data are also used to reconstruct our paleoclimate. These include lake sediments and pollen grains that become trapped in these sediments.

The drought of the 1930’s that caused what we call the Dust Bowl helped us recognize that drought can have severe and lasting effects on people and ecosystems. Subsequent droughts of the 1950’s and now, our current drought, continue to challenge our ideas about living sustainability in an arid environment. Thankfully, trees and the associated science of dendrochronology continue improve our understanding of climate, fire, and southwestern ecosystems. See links below for additional information and tree ring photos.

The University of Arizona Cooperative Extension has publications and information on gardening and pest control. If you have other gardening questions, call the Master Gardener line in the Cottonwood office at 646-9113 ext. 14 or E-mail us at and be sure to include your address and phone number.

Climate/Dendrochronology Links
University of Arizona Laboratory of Tree Ring Research Web Site

Dr. Henri D. Grissino-Mayer's Ultimate Tree Ring Photo Gallery

Climate Assessment for the Southwest

US Geological Survey Arizona Water Science Center

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Arizona Cooperative Extension
Yavapai County
840 Rodeo Dr. #C
Prescott, AZ 86305
(928) 445-6590
Last Updated: August 23, 2007
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