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 lessons within trees, researchers learn about two of the most critical natural resource issues facing the Southwest; water supplies and wildfire. 

What is dendrochronology?

Courtesy of University of Arizona

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 beams 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 to study 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 drill 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 with wood but the tree stays healthy by sealing the damage internally with thick resin. 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, in the Jemez Mountains of New Mexico, 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, enter Smokey Bear,  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 trees, 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. 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.



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Wildfire Network works in pursuit of community wildfire resiliency by working with young adults and communities to promote firefighter safety, job development and youth mentorship. Through wildland fire and land management training, we employ and mentor at-risk youth in forest health, wildfire mitigation and safe firefighting techniques. We provide assistance to communities with wildfire risk reduction and property stewardship.

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