Amidst another devastating and record-setting wildfire season, the recent efforts to protect California’s iconic giant sequoia trees from the Windy and KNP Complex wildfires highlight a new reality of global warming: Wildfires are passing the threshold from existential necessity to existential threat for ecosystems around the world.
Thresholds such as this — where human activities can push historically stable plant communities past the point of no return — are common features of many ecosystems, but their exact locations are difficult to predict and, tragically, may become apparent only after they are crossed. In this case, the combination of fire suppression and climate change have increased the size and intensity of California wildfires to the point at which giant sequoias — quite literally, a textbook example of fire-adapted species — are burned to death en masse.
Giant sequoias are part of an ancient lineage of trees. Over thousands of years, they have been a prominent fixture — ecologically, aesthetically and culturally — of the landscapes in which they occur. The superlatives of these trees are well known. They are the largest trees by volume and among the very tallest and most long-lived. Their massive stature and charisma have galvanized a number of efforts to protect and celebrate them.
The giant sequoia and its close relatives have changed little since evolving around 150 million years ago, but the environment, and therefore their geographic distribution, has. Giant sequoias grow best in habitats that have a temperate climate, ample soil moisture — and perhaps most characteristically, frequent low to moderate-severity wildfires. Their current range — patchy groves in the Sierra Nevada — is a minute fraction of the expansive forests once widely distributed across the northern hemisphere.
Giant sequoias have evolved very specific adaptations that allow them to not only survive wildfires, but also regenerate and colonize afterward. Their thick, fibrous bark (up to 18 inches thick) prevents all but the most severe fires from damaging the living tissue beneath; their self-pruning lower branches keep most fires from climbing into the canopy and damaging leaves. Low-intensity fire stimulates the seed-bearing cones to drop thousands of seeds that quickly develop into fast-growing, highly competitive seedlings in the burned and exposed forest floor.
It is common in nature for species in stable environments to become increasingly adapted to specific features of their habitat. Over evolutionary time, the giant sequoia’s relationship with wildfire has strengthened to an existential dependence. Without wildfires, the seed-cones won’t open and release seeds, and shade-loving, fire-intolerant species like white fir choke out sequoia seedlings and replace mature trees as they die off. We learned this from solving a decades-long mystery: Young giant sequoias were inexplicably non-existent for much of the 20th century, despite efforts to protect and regenerate the forests. Young sequoias returned only to the areas where low to moderate-severity fires were reintroduced.
Wildfires have been in California for as long as giant sequoias, and they have played a prominent role in shaping landscapes across much of the state. Historically, more than 40 percent of California had average fire return intervals of less than 35 years. This is due in large part to the state’s Mediterranean-type climate, with cool, wet winters and hot dry summers.
California’s climate supports an abundance of winter and spring vegetation that dries as fire-prone weather becomes the norm in summer and fall. Before the last century, frequent fires consumed these dry fuels before they accumulated to dangerous levels. This all changed in the 20th century with efforts to actively suppress fires — efforts that reversed the practices of California’s indigenous peoples who used fire as part of their land stewardship for thousands of years.
Fire suppression was codified in 1935 with the U.S. Forest Service’s “10am policy,” which stipulated that all wildfires, once detected, must be suppressed by 10am the next morning. In landscapes that evolved with frequent fires, fire suppression leads to fuel build-up, increased vegetation density, smaller trees and other ecological impacts. Scientists still do not understand all of the repercussions of these policies.
More recently, California’s wildfire regime has gotten a brutal kick from climate change, with long-term warming combining with declining seasonal precipitation to more than double the frequency of extreme wildfire weather days. The combined effects of fire suppression and climate change have been dramatic: The 13 largest fires in California’s history have occurred since 2000 — the eight largest have all been in the last five years. These fires are not only larger. In many cases, they are so much more severe that they destroy forests that have survived many thousands of years of earlier fires.
The Castle Fire in 2020 killed up to 42 percent of all “large” sequoias. These trees, many 1,500 years old or older, each survived dozens of fires in the historical regime before being killed by the Castle Fire. The death of these archetypes of fire-adaptation is a tragic demonstration of the profound differences between the world to which California ecosystems are adapted and the world that we are transitioning to.
The giant sequoia’s vulnerability to the recent Castle and KNP Complex fires is one of many abrupt system-wide shifts that we can expect within the next century. These tipping points foreshadow fundamental changes in the structure, function and composition of the natural world and human-dominated ecosystems. From the melting of alpine glaciers to the death of warm-water coral reefs, we are seeing more and more iconic systems pushed beyond the point of no return. Scientists are struggling to define these tipping points before they are crossed.
This is a frustratingly difficult challenge, even when the risk is clear. In the face of this uncertainty, avoiding the cliff’s edge is the best insurance. Beyond the loss of beloved natural icons, the real significance of the death of giant sequoias in recent wildfires is the stark demonstration that these tipping points exist —and that, once passed, we can’t rewind for a second effort.
Avery P. Hill is a research assistant for the Stanford Woods Institute for the Environment’s “Zombie Forest Project,” a consortium that also includes the California Academy of Sciences.
Christopher B. Field is the Perry L. McCarty director of the Stanford Woods Institute for the Environment and the Melvin and Joan Lane professor for Interdisciplinary Environmental Studies at Stanford University.
Noah S. Diffenbaugh is the Kara J Foundation professor and Kimmelman Family senior fellow at Stanford University.