Ancient Forest Society 2025

What Can We Learn From Giant Sequoia Tree Rings?

by Taylor Sheriff

Giant sequoias (Sequoiadendron giganteum) are renowned for their remarkable longevity, with some individuals living for over 3,000 years. While the title of longest-lived tree species goes to the Bristlecone pine (Pinus longaeva), giant sequoias are among some of the oldest living organisms on Earth. You might be wondering, how do we even know how old these trees are? The answer is, by looking at their growth rings!

Trees grow larger around their stems by forming new layers of wood. In seasonal climates, one new layer - a tree ring - will form each year.  So one year of growth equals one tree ring. The size of these rings are highly dependent on the surrounding environment; water availability is often the main driver of growth but other factors like extreme heat or cold, tree competition, and fire can have an effect. A ring is usually wider if the tree experienced optimal growing conditions, especially moisture, in a particular year and can be very narrow (or even invisible) in years with less optimal conditions (such as during severe drought).

Perhaps as you have walked through a forest, you’ve noticed a pattern of rings in a tree stump. By counting these individual rings, you could determine the relative age of that tree when it died. But a tree with thousands of rings? That can get complicated!

Discoveries into the oldest-living trees have been attributed to the field of dendrochronology, the scientific study of tree rings. Scientists use a technique called “cross-dating” to accurately determine the exact year a tree ring was formed. In this process, ring patterns of one tree are compared to patterns in other trees of the same species. These patterns are matched to assign specific dates to specific rings. Since new rings are formed toward the outside of the stem, the oldest rings will be found in the center and will continue to get newer as you move out towards the bark.

In the early days of dendrochronology, the only way to count tree rings was to either observe stumps or chop down a tree and obtain a “tree cookie.” A cookie is a cross-sectional slice of the trunk and looks like a flat disk. Scientists today prefer to use less destructive methods, often utilizing an auger-like instrument called an “increment borer.” With this instrument, you can bore into the stem of a tree and then pull out a pencil sized, cylindrical piece of wood containing the tree rings.

Giant sequoia serves as a cornerstone for dendrochronology due to its impressive longevity and size. These ancient trees possess well-preserved growth rings that provide invaluable records for climate and ecological change. It is for this reason that A.E. Douglass, dubbed the “father of dendrochronology,” collected his first giant sequoia samples in 1915 from what is now the Sequoia National Forest. Douglass first undertook his sequoia study in an effort to better date wooden beams found in an ancient Pueblo ruin in the Southwestern U.S. Wait - wooden beams?! Yes, in fact dating tree rings has been a very important tool in the field of archaeology. Tree rings have been analyzed in old wooden artifacts and structures to help scientists understand more about ancient civilizations and peoples. From Douglass’ giant sequoia collection, he created a 3,220 year-long record of growth rings dating back to 1305 BC. Unfortunately, he was unsuccessful in crossdating the wooden beams with the sequoia samples because they occurred in such different environments. As you might imagine, forests of the Sierra Nevada mountains and the deserts of the Southwestern U.S. are not very similar. Nevertheless, Douglass’s work helped establish the core principles of the field of dendrochronology.

Remember how tree rings can be bigger or smaller based on how much moisture the tree received? Trees are also responsive to other climatic stressors, such as temperature, wind, snowpack, and big events like hurricanes. By analyzing the year-to-year variation of growth rings, scientists can reconstruct what our climate was like in the distant past. These reconstructions can put our modern climate trends into perspective, and help predict how the climate may change in the future and how trees may respond to those changes.

Reconstructing the history of wildfire on the landscape is also a big application of dendrochronology. These reconstructions can help determine the frequency, intensity, and timing of past fires on the landscape. This information is very useful for forest managers because it helps them make informed decisions about best practices for forest health and restoration. Wildfire was once a common and natural occurrence on our public forests, but at the start of the 20th century it became a general policy to suppress all fires. This policy of fire suppression increased the density of our forests and the amount of dead wood on the forest floor, creating conditions more conducive to severe fire.

Many forest types, such as giant sequoia, depend on mixed-severity fires to remove competitors and facilitate seedling regeneration. Mature trees are left largely unharmed and actually record evidence of the fire in their rings, which scientists call “fire scars” and are used to reconstruct fire histories. The bark of giant sequoias is remarkably tough and resistant to significant damage from fire. However, fire can penetrate the bark and scar the living tissue. As the tree grows, new layers of wood can form over the scar and leave a rich record of fire histories documented in the tree. Repeated fires over the same scar can cause a “catface.” Perhaps you have noticed these blackened, triangular scars at the base of some giant sequoias. Often, they can extend many feet up the trunk of older trees!

Tree rings serve as natural records of environmental change and contain a wealth of information. They are invaluable to helping us understand more about the natural world. No species has been more important in this effort than giant sequoia. They serve as living archives and can help us understand the impacts of modern climate change by comparing past and present growth patterns. Ancient Forest Society is utilizing this valuable resource and have undertaken a dendrochronology project of our own. This project aims to reconstruct the growth patterns of giant sequoias during the 2012-2016 drought to compare with previously documented droughts, and to evaluate whether tree drought responses in the tree ring record correlate with remotely-sensed estimates of canopy water content observed in 2015-2017.

From 2012-2016, California faced one of the most severe droughts in recent history. As a result, unprecedented foliage dieback was observed in giant sequoias and dozens have been killed by the combined effect of severe fire damage, drought stress, and native bark beetle attack. Mortality from bark beetles has so far been minimal, but land managers are concerned this could only be the beginning in our changing climate. Giant sequoias are further threatened by increasingly severe wildfires, which have killed more than a fifth of all mature sequoias since 2015. By analyzing sequoia growth patterns, we can better identify which trees are more vulnerable to water stress and damage from drought. This work will be valuable to land managers as they strive to increase sequoia resilience to severe wildfire and bark beetle attack.

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Celebrating International Day of Charity

International Day of Charity is observed each year on September 5th to raise awareness and mobilize people around the world to participate in volunteer and philanthropic work. Ancient Forest Society is dedicated to the understanding, protection, and restoration of ancient trees and forests. Through scientific research, preservation initiatives, and immersive educational experiences we promote the ecological, cultural, and climate benefits of forests worldwide.

In celebration of this day, we invite you to join us in tackling the immense and important work of honoring old trees and ancient forests by supporting Ancient Forest Society and becoming a member of our family.

Joining Ancient Forest Society’s membership is an opportunity to participate in our work protecting and conserving these amazing trees and forests. As a member, you become a monthly donor to help support our research, conservation, outreach and education, and volunteer programs. Membership also comes with discounts on our events and merchandise.

Ancient Forest Society is a 501(c)(3) non-profit organization making your contributions a tax-deductible donation.

Thank You to The Bushnell Family!

Ancient Forest Society extends our deepest gratitude to Tim and Mark Bushnell for a generous new grant from the John and Ann Philanthropic Fund to support our work.

Over the past several years Tim Bushnell has been a dedicated supporter of Ancient Forest Society. As a tree climber himself, he understands the work required to accomplish our research and conservation initiatives. We are so grateful for his and Mark’s generosity!

Congratulations Jeremy Collins!

In 2024, award-winning artist and storyteller Jeremy Collins joined Ancient Forest Society in Calaveras Big Trees State Park to help us with our ongoing bark beetle research. As a seasoned rock climber, he was eager to add the largest trees on the planet to his list of climbing experiences. It was a wonderful opportunity to share the grandeur of the sequoias with him and it was an honor for Ancient Forest Society to be featured in his new book, Eventually a Sequoia.

Eventually a Sequoia is available now HERE. There is a special edition copy of this book available too - you can find it HERE. Learn more about Jeremy and his work at JerCollins.com

Congratulations Jeremy!