TULARE COUNTY – Even when wildfires are put out, the damage might be far from done. Thanks to a pair of giant sequoia regeneration studies, researchers with the U.S. Geological Survey’s Western Ecological Research Center have found that some areas affected by recent mega-fires may not have enough seedlings to grow the next generation of trees. Extreme wildfires have killed up to 20% of the world’s mature giant sequoias since 2015, a majority of which perished from three wildfires in 2020 and 2021.
The recent mega-fires burned at a size and severity more extreme than the historic norm, and these studies show that without intervention, some giant sequoia groves may experience a long-term or permanent loss of acreage as seed trees have died, and the number of new seedlings is exceptionally low.
“What we used to call high-severity fire does not compare to the unprecedented scale and severity of the wildfires we’ve experienced in recent years in the Sierra Nevada,” said Nathan Stephenson, Ph.D., scientist emeritus at the U.S. Geological Survey’s Western Ecological Research Center. “Giant sequoias are the largest trees in the world. They’re fire-adapted and rely on fire to reproduce. But recent fires have killed thousands of mature trees and, in some cases, their seeds too.”
The U.S. Geological Survey’s Western Ecological Research Center published the study “Post-fire reference densities for giant sequoia seedlings in a new era of high-severity wildfires” in the June 2024 issue of the peer-reviewed journal Forest Ecology and Management. In it, Dr. Stephenson and co-authors developed some conservative estimates of the natural amount of sequoia seedlings per area that would be considered adequate for forest regeneration, along with the number of seedlings per area that would be needed to expect sufficient tree survival to adulthood.
The data from the study will serve as a reference that land managers can use to interpret the effects of wildfires on sequoia reproduction and assess – on a case-by-case basis – whether they need to replant sequoias to maintain current populations and groves or not.
Consistent with past studies, this research also documented an initial post-fire burst of giant sequoia seedlings, which was then followed by a significant decline due to high seedling mortality rates, low subsequent germination and low establishment of new seedlings in subsequent years. For giant sequoias, seedling establishment is generally limited to the first two summers following a fire. The results are based on post-fire records from Sequoia and Kings Canyon National Parks, spanning 48 years (1969–2016) in 42 sites in eight giant sequoia groves that burned in 26 different fires. The authors then quantified the difference between the climate that prevailed between 1969 and 2016 and the climate that prevailed after the 2020 and 2021 wildfires.
In the other study, researchers also noted that the seedlings that sprouted after the 2020 and 2021 wildfires are growing under new climate conditions, including higher average temperatures and the most severe drought conditions of the 121-year historical record. These conditions will probably reduce the survival of giant sequoia seedlings. In areas where the mature seed trees have died, chances of natural recovery will most likely be greatly diminished.
“Sequoia regeneration has varied widely across the range since the recent mega-fires,” said lead author David Soderberg, Ph.D., ecologist at the U.S. Geological Survey’s Western Ecological Research Center. “Some areas are doing well, but in other sites where the seeds and seed trees burned, or where young seedlings have died from high heat and drought, natural long-term recovery appears to be unlikely. Our data suggest that sequoia grove areas that were severely impacted by the fires may not have enough reproduction to replace the sequoias lost.”
The U.S. Geological Survey’s Western Ecological Research Center published “Assessing giant sequoia mortality and regeneration following high-severity wildfire” in the March 2024 issue of Ecosphere. This study assessed the likelihood of natural recovery of giant sequoias in areas affected by the 2020 and 2021 mega-fires.
In the study, Dr. Soderberg and co-authors reviewed evidence that recent wildfire severity in the Sierra Nevada has been far more extreme than the historic norm, and that if no interventions are taken, there might be a long-term or permanent loss of sequoia grove area. This is especially true in areas where mature seed trees were killed, where cones were burned out of the trees’ crowns and where the amounts of new seedlings are well below historic norms in the years since the wildfires. The results of this study are based on extensive surveys of four groves in Sequoia and Kings Canyon National Parks that were severely impacted by the 2020 SQF Complex Fire and 2021 KNP Complex Fire.
To provide conservative comparisons of the effects of the recent mega-fires to historically normal fire effects, the researchers contrasted second-year reference densities presented in the Stephenson et al. paper with giant sequoia seedling densities in the four groves that burned in 2020 and 2021. They found that in some areas, seedling amounts were well below historic norms and could not be expected to maintain or recover the groves to their pre-fire structure. Seedling density also depended on the proximity to nearby sequoia crowns that were either alive and green or scorched (brown from fire’s heat) but not torched (blackened and consumed by fire).
The study also provides a statistical tool for managers who are in need of answers and context to make restoration and management decisions following large, severe wildfires.
“These are important findings that identify the density of seedlings, the number per area, that is enough for the tiny, young trees to survive and become majestic sequoia adults,” said Joanna Nelson, Ph.D., director of science and conservation planning for Save the Redwoods League and a lead scientist with the Giant Sequoia Lands Coalition.
Nelson says this new research could help land managers determine how to best care for the giant sequoia groves, especially in emergency-response windows post fire.
“Working from the best science available, we can better understand when and where sequoia reproduction may be too low for future success and be equipped to take action,” Nelson said. “At the same time, we continue to prioritize fuels reduction treatments that favorably change fire behavior and reduce the severity of potential future wildfires.”
Ongoing research and monitoring are supported by the Giant Sequoia Lands Coalition to continue providing a scientific basis for restoration programs across the sequoia range. To date, GSLC has completed wildfire resilience work across half of California’s giant sequoia acres, planted more than 500,000 native seedlings in severely burned areas where reproduction has been insufficient and conducted scientific research to support evidence-based restoration techniques. The GSLC’s recently published “Progress Report for Saving the Sequoias” offers more information about their research.
