Standing the heat: the changing role of fire in giant sequoias forests

by Allison Kidder

California’s fire season is well underway, with more than a dozen fires in the Sierra Nevada having been ignited by lightning during the past few weeks alone. 

Fires in California are expected during summer and fall when vegetation dries out during summers with little to no rain. For thousands of years California’s forests experienced relatively frequent mixed-severity burns, ignited by Native Americans and lightning. These mixed-severity fires burn hot in small patches and at lower severity in larger areas, removing understory plants and small trees, creating a mosaic pattern on the landscape.

Giant sequoia trees (Sequoiadendron giganteum) evolved to live with fire and have several adaptations that help them survive the flames. They have thick, spongy bark that insulates the wood and precious cambium layer located immediately beneath the bark from heat injury. The thick bark also stores water that can prevent embers from taking hold.

And have you ever noticed how the trunks of giant sequoias have no branches for dozens of feet up? That is because a giant sequoia self-prunes its lower branches as the tree ages, helping to prevent ground fire move up into its canopy.

Each summer giant sequoia seeds rain down onto the forest floor from a combination of cones dying and and releasing their seeds, Douglas squirrels harvesting and eating the outside of the cones and Long-horned beetles laying larvae in the peduncle of the cones resulting in them drying and opening up. However, giant sequoias have semi-serotinous cones, meaning that a larger number of seeds can be released during certain conditions. Heat from mixed-intensity fires can dry out and open large numbers of cones, resulting in a large pulse of seeds being released as just the right time when the forest floor is primed for giant sequoia regeneration. The seeds fall onto recently-burned areas that were just cleared of vegetation and leaf litter from fire and contain a nutrient-rich layer of ash on top of the soil, perfect for germination. Small patches of high-severity fire are critical for giant sequoia regeneration.

But these adaptations aren’t enough to survive in the face of today’s catastrophic fires, persistent drought, and increased susceptibility to bark beetles.

Unprecedented Fire Activity 

Climate change has permanently changed the millennium-old relationship of giant sequoia trees with fire. Recent research suggests that fires in the western United States are becoming larger and more extreme with climate change (Iglesias et al, 2022). Higher temperatures means earlier snowmelt and more evapotranspiration from vegetation and the soil. The steady loss of water over years can gradually lower water tables to levels too deep for the roots of giant sequoias to reach, resulting in trees that are drought stressed.

During the late 19th century and early 20th century, foresters and land managers within the United States Forest Service suppressed fire throughout the Sierra Nevada Mountain range. Decades of altered fire regimes promoted growth of a dense understory and mid-story brush and trees that would normally have been removed by relatively frequent mixed-intensity fires. Drought and extreme heat dry out the dense understory vegetation, fueling higher-severity fires.

Historically, giant sequoias with a trunk diameter of four feet or greater easily survived wildfires. But since 2017 a large number of these larger giant sequoias have been killed in wildfires. In 2020-2021 alone, researchers estimated that 13-19% of the world’s larger giant sequoias were killed by wildfires (Shive et al 2021).

Here are the approximate sizes and giant sequoia mortality figures from some of the more notable recent fires, according to the studies provided below.

In addition to direct tree mortality from fire, land managers and researchers have become increasingly concerned about low natural post-fire recovery of giant sequoia forests. In some groves, most of the adult seed-bearing sequoia trees were killed. A recent study by Soderberg et al 2024 have also observed that some fires were so intense that seeds burned right along with the adult trees.

Seeds that did manage to germinate after the high-intensity fires in 2020 and 2021 were smacked with a double whammy of notable summer heat and continued severe drought conditions (Stephenson et al 2024). Seedling mortality was high, leaving large areas of the forest floor devoid of the next generation.

Ancient Forest Society Takes Action

Researchers and land managers agree on the importance to take action. Rewilding efforts, field research, and prescribed fire are all part of the many-pronged strategy land managers are deploying.

Ancient Forest Society is taking a leading role in all of these. In 2023 they started a seed collection program in partnership with One Tree Planted and State and National Parks to collect giant sequoia cones from high in the canopy. They are continuing this work in 2024 with funding from the National Park Service. These cones contain seeds that can be grown into seedlings and planted where seedling mortality was high due to extreme climatic conditions.

Giant sequoia trees weakened by ongoing drought and extreme heat are more susceptible to bark beetle attacks. Healthy trees are normally able to fend off these native beetles but when trees are weakened they succumb and die. Ancient Forest Society is continuing its intensive monitoring of bark beetle activity and studying its impact on tree health throughout the range of these trees.

Ancient Forest Society is researching the ecophysiology of giant sequoia trees located within prescribed fire units in wetter and dryer areas. They are collecting measurements and samples before, during, and after prescribed burns. These data will help land managers identify which trees are most vulnerable to fire impacts.

And lastly, Ancient Forest Society continues its work with the Giant Sequoia Lands Coalition, which has planted more than 500,000 giant sequoia seedlings in areas that experienced high-severity fires and have inadequate natural seedling regeneration.

Ancient Forest Society’s work in all of these areas, now and in the future, directly ensures giant sequoia trees and their relationship with natural processes like fire will persist for future generations.

Our contributing writer, Allison Kidder, has been a Board Member of Ancient Forest Society since 2023. She has a PhD in plant ecophysiology from UC Berkeley and has been a docent and Managing Editor at the East Bay Regional Parks Botanic Garden since 2017.

References:

Iglesias, V., J. K. Balch, and W. R. Travis. 2022. U.S. Fires became larger, more frequent, and more widespread in the 2000s. Science Advances Vol 8, Issue 11. DOI: 10.1126/sciadv.abc0020

Shive, K., C. Brigham, A. C. Caprio, P. Hardwick. 2021. 2021 Fire Season Impacts to Giant Sequoias. (Accessed July 30, 2024: https://www.nps.gov/articles/000/2021-fire-season-impacts-to-giant-sequoias.htm)

Soderberg, D. N, A. J. Das, N. L. Stephenson, M. D. Meyer, C. A. Brigham, J. Flickinger. 2024. Assessing giant sequoia mortality and regeneration following high-severity wildfire. Ecosphere Vol 15, Issue 3. DOI: 10.1002/ecs2.4789

Stephenson, N., C. Brigham. 2021. Preliminary Estimates of Sequoia Mortality in the 2020 Castle Fire. (Accessed July 30, 2024: https://www.nps.gov/articles/000/preliminary-estimates-of-sequoia-mortality-in-the-2020-castle-fire.htm)

Stephenson, N., A. C. Caprio, D. N. Soderberg, A. J. Das, E. L. Lopez, A. P. Williams. 2024. Post-fire reference densities for giant sequoia seedlings in a new era of high-severity wildfires. Forest Ecology and Management Vol 562, Issue 15. DOI: 10.1016/j. foreco.2024.121916.

Volunteers make it happen

As Ancient Forest Society makes their way through the field season, we are thrilled to report that we have engaged 54 unique volunteers to join us in the field so far. Our volunteers are a valuable part of the work we accomplish throughout the season and to see so many great people come together for our common goals is heartwarming.

We truly feel that everyone who is drawn to honor and support the old trees and ancient forest is part of the society and watching how this group of tree loving people is growing is so important. We have found that these majestic trees hold no borders on this earth, bringing people together from various places around the world. We are so thrilled to have support from volunteers traveling from Italy, the UK, Germany, Puerto Rico and all over the United States to help accomplish our work in these beautiful trees.

Being surrounded by giant sequoias can shrink our perception of the world when we all realize how valued and special these trees are to each of us.

Having this many people to report as volunteers at the midway point of this season makes us feel proud.

If you would like to volunteer with Ancient Forest Society in the future just complete the volunteer application form (link is on this page).

Merchandise coming soon!

We are thrilled to announce that Ancient Forest Society will soon be launching a new place on our website to shop for the coolest new Ancient Forest Society merchandise. There’s no better way to show that you are a part of the society than to wear a cool hat, shirt or beanie! We are accepting pre-orders and expect to begin deliveries before the end of summer.

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