Giant sequoia forests losing resilience to wildfires
Ancient giant sequoias (Sequoiadendron giganteum) are highly valued trees with limited distribution. They are the most massive trees on earth, and they have exceptional longevity — more than 3,000 years. Given their extraordinary resilience to wildfire, insects, and disease, most managers hoped giant sequoias would be resistant to anthropogenic change. However, 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, USDA Forest Service scientists 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, about 84 percent of the 162 large sequoias were killed; in moderate severity, about 28 percent of the 239 large sequoias were killed. These rates varied between fires but were at the lower end of the mortality range documented in Sierra Nevada mixed-conifer forests.
Across all three fire areas, scientists found large sequoia mortality was linked with presence of significant prior fire damage to the lower tree bole, such as fire scars or “catfaces”, lower crown ratios, and lower elevations. At Black Mountain Grove, they 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 about 85 percent crown damage before succumbing to injury. 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.
The extent of high severity fire in giant sequoia forests is rapidly increasing, with 84 percent of large sequoias dying in high-severity burn areas. Forest treatments that reduce fire severity could help protect this unique species. 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.