Ancient trees and modern wildfires: Declining resilience to wildfire in the highly fire-adapted giant sequoia

Ancient giant sequoias Sequoiadendron giganteum (Lindl.) J. Buchholz are highly valued trees with limited distribution. They are the most massive trees on earth, and they have exceptional longevity (>3,000 years). Given their extraordinary resilience to wildfire, insects, and disease, most managers hoped giant sequoias would be resistant to anthropogenic change. However, the warming climate and fire exclusion-caused fuel build-up has dramatically increased the amount of high-intensity wildfire in giant sequoia groves since 2015. To better understand recent wildfire effects on large, legacy giant sequoia trees, we surveyed high- and moderate-severity fire areas in groves that burned in three wildfires between 2015 and 2017. Within areas classified as high severity via remote sensing, ∼84% of the 162 large sequoias (>1.2 m diameter at breast height) were killed; in moderate severity, ∼28% of the 239 large sequoias were killed. These rates varied between fires but overall were at the lower end of the mortality range that has been documented in Sierra Nevada mixed-conifer forests. Across all three fire areas, we found large sequoia mortality was linked with presence of significant prior fire damage to the lower tree bole (i.e., fire scars or “catfaces”), lower crown ratios, and lower elevations. At Black Mountain Grove, we tracked trees through time and found total crown damage and the presence of fire scars were the best predictors of mortality by year three, where large sequoias sustained up to ∼ 85% crown damage before succumbing to injury. There, 52 of 204 trees were dead one-year postfire, with 71 dead by year three. Overall, these fires caused significant and concerning mortality levels of these ancient trees, and yet the impact on sequoia groves between 2015 and 2017 was dwarfed in comparison to the more recent 2020 Castle Fire, 2021 Windy Fire and 2021 KNP Complex Fire. With the push to increase forest and fuel treatments to reduce catastrophic fire risk across the Western U.S., more immediate triage to protect irreplaceable resources is needed as fire activity rapidly increases. Our study suggests that prioritizing management action that creates fire resilience in unburned groves, and maintains the fuel reduction benefits accrued by past treatments and lower severity wildfires, will be critical to avoid continued losses of these exceptional, irreplaceable old growth forests.