2021 fires burned 27 sequoia groves; 2,261–3,637 large trees estimated killed or dying

Two lightning-sparked wildfires in September 2021 burned into 27 giant sequoia groves, and the best preliminary estimate is that 2,261 to 3,637 sequoias over four feet in diameter have been killed or will die within three to five years. The report, by Kristen Shive (The Nature Conservancy) and Christy Brigham, Tony Caprio and Paul Hardwick (National Park Service, Sequoia and Kings Canyon National Parks), draws its data and analysis from the Burned Area Emergency Response plan for the KNP Complex, with added explanation.

Redwood Mountain Grove: a large high-severity patch at the southern end of the grove, October 12, 2021. Image from the report on NPS's page.

The setting

Giant sequoias cover only about 28,000 acres in about 70 groves on the western slope of the Sierra Nevada. Sequoia and Kings Canyon National Parks (SEKI) have 38 delineated groves on roughly 10,000 acres and manage approximately 37% of all sequoia grove area. The 2020 Castle Fire killed an estimated 10 to 14% of the range's sequoias over four feet in diameter, so managers and the public are alarmed.

Both fires, the KNP Complex (the Colony and Paradise fires) and the Windy Fire, were ignited by lightning in one late-evening thunderstorm on September 9, 2021. The KNP Complex had burned 89,398 acres by October 18, 2021, and the Windy Fire 97,528 acres by its containment date of November 15, 2021. Both were fought with a full suppression strategy from discovery.

What burned

Together the fires burned 6,109 acres of sequoia groves. The Windy figure leaves out two groves on the Tule River Reservation, for which no public spatial data exist. Severity comes from the Forest Service's RAVG satellite product, where high severity has been associated with 95% tree mortality.

Estimated losses

That is an estimated 3 to 5% of the Sierra Nevada's large sequoias. Added to the Castle Fire's 10 to 14%, the authors call it a significant threat to the persistence of large sequoias. The estimates combine satellite severity with death rates from surveys of groves burned in three earlier fires.

Fire is not new, but high severity is

Sequoias are highly fire-adapted but not to extensive high-severity fire. Historically fire returned about every 15 years, mostly at low to moderate severity, and small high-intensity patches thinned the canopy and helped open cones. After Euro-American settlement, lightning fires were put out and Indigenous burning was prohibited, building up surface fuels and tree density. Since 2015 the area burned and the share burned at high severity in groves have risen dramatically, consistent with warmer, drier fire seasons and hotter droughts. In groves that burned in three fires from 2015 to 2017, average mortality in high-severity areas was about 84%. The Castle Fire burned 9,531 acres of sequoia grove, 2,810 of them at high severity.

Map of groves showing fire severity for the KNP Complex Fire. Image from the report on NPS's page.

Regrowth and the benefits of fire

Sequoias generally regenerate well after fire, but there are reports of poor regrowth in high-severity areas, where cones or seeds may have burned, failed to survive smoldering, or washed away. In the KNP Complex, 436 acres of high severity lie more than 100 meters from live sequoias and may lose sequoias entirely if regeneration from surviving seed fails. Erosion modeling suggests those areas are also at high risk of losing any surviving seed.

Much of the burned grove area burned at low severity, which the authors expect to help sequoias through fuel reduction, small canopy openings and ash-covered ground that suit seedlings. Earlier prescribed fire reduced severity in parts of Redwood Mountain Grove and Giant Forest, and earlier prescribed fire, thinning and burn-unit preparation gave crews safer chances to fight the KNP Complex.

Sources

• National Park Service, Sequoia and Kings Canyon National Parks, https://www.nps.gov/articles/000/2021-fire-season-impacts-to-giant-sequoias.htm, report by Shive, Brigham, Caprio and Hardwick; rewritten in hubnx's own words.