Teaching About Tree Rings

The stories recorded by tree rings connect natural history and human history to provide rich, place-based education for students of all ages. Trees living in the arid ecosystems of the Southwest are particularly sensitive to changes in moisture levels which become reflected in the pattern of growth rings. By understanding the history books within trees, scientists gather information about two of the most critical natural resource issues facing the Southwest; water supplies and wildfire. 

What is dendrochronology?

The relationship between tree rings and precipitation was fully illuminated by A.E. Douglass, an astronomer working near Flagstaff, Arizona who was trying to match the pattern of tree growth to sun spot intervals. As he searched for evidence of solar intensity recorded by tree rings, he realized that the trees were recording rainfall patterns with wet years forming wider growth rings. One revelation led to another and Douglass realized that the distinct pattern created by climatic conditions could be used to crossdate logs used in ancient pueblo construction. The overlap between the growth of living trees, dead trees, and trees within ancient dwellings bridges back in time and crossdating identifies growth anomalies within certain trees. In 1937, Douglass founded the world’s first tree-ring laboratory at the University of Arizona and became the father of dendrochronology or tree ring science. 

Do dendrochronologists cut down trees to study them? 

Dendrochronologists can study tree rings without cutting trees down by using an increment borer. The tool consists of a hollow metal straw with a serrated tip on one end and a handle on the opposite end. The tip is placed against the trunk partway up and aimed towards the center or pith of the tree. The handle is twisted clockwise to drill the metal tube into the trunk. Once the tube extends at least halfway into the trunk, the metal spoon is inserted and the borer is turned once counter-clockwise to break the core off inside the tree. The core is then extracted by gently removing the spoon from the metal shaft. The borer is twisted counterclockwise and removed. The hole will never fill up but the tree stays healthy by sealing the damage internally. The core is placed within a drinking straw to maintain the correct sequence of growth and protect the core from damage. Back in the laboratory, each core is carefully sanded and inspected under a microscope. Analysis of the tree cores provides the age of the tree and a reconstruction of the climatic conditions while the tree was growing.

 

 

 

 

 

 

 

 

 

 

 

 

 

Not all forest stories can be told with tree cores. Only trees that survive but are injured in a fire become authors of fire history. These fire-scarred trees are less frequent in the forest and a bit hard to find by the untrained eye. Fire ecologists, however, have a special knack for spotting these fire storytellers. After the tree is scarred the first time, each subsequent fire leaves a mark that corresponds with the timing of each fire. Because the story only gets written at the site of injury, scientists must cut a cross-section of wood through the fire scar rather than coring the tree. By studying a series of trees across the landscape, scientists can reconstruct the history of fire at the site. 

What stories do trees tell?

In the example reconstruction from Monument Canyon, each horizontal line and label (e.g. GP 101) represents a single tree. The vertical lines are fire scars that correspond with the timeline along the x axis. An abundance of vertical lines within any given year indicates that fire was widespread across the sampled landscape. Conversely, the lack of fire scars within the green area clearly shows the lack of fire since 1900. Based on fire histories from throughout the region, scientists know that fire was much more common in most southwestern forests than it has been for the last century or so. Some people attribute this shift to Smokey Bear’s effective messaging but the change was actually initiated by the influx of massive herds of goats, sheep, and cattle that arrived with the railroads in the later part of the 19th century. By chowing down the grass and small shrubs, the hungry livestock effectively removed the kindling that helped spread fire across the landscape. When land managers got serious about suppressing fire just after the turn of the century, the fate of the forests was sealed. The green block on the Monument Canyon fire history represents a period of accumulation where trees were growing without the recycling process of fire. An analogy would be a dumpster filling with trash that never gets hauled away. 

Based on the stories from fire-scarred trees and the widespread survival of non-scarred trees through frequent fire periods, scientists know that most of the historic fires that occurred prior to 1900 were of low intensity. Small flames burned through the underbrush and lapped at the scars of previously injured trees but most trees had little to show for the events. Frequent, low intensity fire renewed the forest, recycled nutrients, and maintained the balance between trees and available water. Once fire became infrequent, the system became skewed and the forest outgrew the available resources. The overgrowth currently presents a tremendous liability as temperatures warm and water supplies become more limited. Forest conditions now support high severity fire that torches the canopy and kills lots of trees. In many cases, high severity fire may shift an ecosystem away from the tree species that once existed on the site. To reduce the threat of catastrophic, high severity fire and restore the balance of the forest, land managers are working to reduce tree density and return low intensity fire to southwestern forests. This effort requires communities to acknowledge the natural role of fire, accept the presence of smoke, and take responsibility for reducing risk to private property. Tree ring stories can help land managers and residents alike understand how to live with fire.

 

 

Books

 As An Oak Tree Grows

Recommended for K-3 students, “As an Oak Tree Grows” depicts 200 years of history from the perspective of a magnificent oak tree. Students can track the date on each page as the town transforms over time. The included poster relates tree rings to various technological advances such as the invention of the telephone. It is an engaging story with beautiful illustrations that describes the life cycle of a tree and the tree’s relationship to its surrounding environment.


The Tree Rings' Tale

“The Tree Rings’ Tale” provides a dendrochronology primer for students in 4th through 8th grade. John Fleck uses a series of real-world stories and scientists to bring the subject alive. Each chapter concludes with a scientific inquiry such as maintaining a journal and using a rain gauge. This book provides interdisciplinary content across social studies, environmental science, and water resources.


The Charcoal Forest

“The Charcoal Forest” discusses the benefits of fire by describing the adaptations of twenty species of plants and animals. The book discusses fire intensity and assigns each species a habitat of fire dependent, fire adapted, or disturbance oriented. The beautiful illustrations and simple language are recommended for preschool through 3rd grade students but older students can also benefit from the introduction to fire adaptations. The book focuses on the northern Rockies but several of the species extend into the Southwest.


Our Tree Named Steve

“Our Tree Named Steve” does not discuss tree rings or fire but it is a fun story that helps kids think about the trees in their lives. Steve the tree is spared when the family home is constructed and the tree becomes part of the family over time. The publisher recommends the story for preschool through kindergarten students.

 

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10 December 2016
10 December 2016

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