The 2021 KNP Complex Fire was a wildfire, or wildfire complex, that burned primarily in Sequoia National Park and the Sequoia National Forest in Central California's Tulare County. After lightning ignited several fires in the southern Sierra Nevada on September 9, two of the fires merged, and the combined blaze burned over 42 days. The KNP Complex Fire was not declared contained until mid-December, with attendant firefighting costs of over $170 million, after rain and snow from several atmospheric rivers in October curbed the fire's growth.
The KNP Complex Fire was not as destructive to communities as other fires during the 2021 California wildfire season, such as the Dixie Fire and Caldor Fire in Northern California. It burned in rugged, remote terrain, though Sierra communities such as Three Rivers, Wilsonia, and Cedar Grove were forced to evacuate, and the fire forced the closure of much of Sequoia and Kings Canyon National Parks for long periods while damaging park resources such as roads, trails, and cabins.
The KNP Complex Fire also heavily impacted the endangered giant sequoia (Sequoiadendron giganteum) population, which grows in under a hundred natural groves scattered throughout the western Sierra Nevada. The National Park Service estimates that the KNP Complex Fire resulted in the death of roughly 1,300–2,400 large giant sequoias, part of the heavy toll that high-severity wildfires have taken on the species in the 21st century. The Windy Fire, ignited by the same spate of dry thunderstorms, burned and killed hundreds more large giant sequoias in the Sequoia National Forest south of the Complex. The two simultaneous fires are estimated to have killed as much as 3–5% of the total population of large giant sequoias.
Background
The KNP Complex Fire took place during an exceptionally severe fire season for the western United States, and particularly for California, in which nearly 2.6 million acres () burned: the second largest area on record after 2020. In the northern Sierra Nevada, the Dixie Fire became the largest single wildfire in state history. The national preparedness level, representing the mobilization of wildland firefighting crews, hit the maximum level of 5 on July 14 and remained there until September 20 in the longest such period on record. Officials took drastic measures to try and limit new ignitions: between August 31 and September 15, the U.S. Forest Service’s Pacific Southwest Region closed all of California's national forests to public use because of fire conditions.
Fuels and climate
In the decades preceding the fire, average temperatures in the Sierra increased measurably even as precipitation did not. This trend was driven partly by climate change. Climate change and consequent warming in California has helped produce hotter and more severe droughts, such as the one California endured between 2012 and 2016. Acute stress from that drought killed many trees in the Sierra Nevada, particularly at middling elevations.
The lack of water also crippled trees' abilities to resist the predations of bark beetles, which resulted in "greatly elevated mortality" for many major tree species in Sequoia National Park, including the ponderosa pine, the sugar pine, the incense-cedar, and the white fir. Annual tree mortality rates nearly doubled in the park just between 1983 and 2004. A park representative estimated in 2021 that Sequoia and Kings Canyon National Parks held over a million trees killed by beetles that were helping drive the KNP Complex Fire.
California saw its second-driest water year ever in 2020–2021, exceeded only by that of 1923–1924. It was the driest ever water year on record for the southern Sierra Nevada in particular, with only of rainfall compared to the region's average of . The summer of 2021 was also California's hottest ever recorded. Temperatures in the vicinity of the fire were cooler, but the hot and dry summer still kept vegetation moisture levels "critically low".