Wildfire

This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.

It is tempting to treat wildfire purely as a disaster, and in the places where it threatens human life and property it plainly is one. But across large stretches of the natural world, regular fire is not a disruption of the ecosystem's normal functioning; it is part of that normal functioning, and some species have evolved to depend on it directly. The clearest example is serotiny, a trait carried by the giant sequoia and many pine species, in which the tree's cones stay sealed, sometimes for years, resisting opening under ordinary conditions. Only the heat of a passing fire melts the resin holding the cone shut, releasing the seeds onto a seedbed the same fire has just cleared of competing vegetation and enriched with a fresh layer of mineral-rich ash. A giant sequoia grove that never burns is not a healthy grove left alone; it is a grove that has stopped reproducing.

Grassland and savanna ecosystems depend on fire in a different but equally direct way. Left unburned for long enough, a grassland is gradually invaded by woody shrubs and trees, which shade out the grasses and shift the ecosystem toward forest. Periodic fire, historically both lightning-ignited and deliberately set by grazing animals' own foraging pressure and by human land management, kills back the encroaching woody growth while the grasses themselves, whose growing point sits at or below ground level, survive and resprout quickly, maintaining the open, grass-dominated structure that defines the ecosystem and the grazing animals it supports.

Every fire-adapted ecosystem has its own characteristic fire-return interval, the typical span of years between fires under natural conditions, and that interval varies enormously: some savanna grasslands naturally burn every year or two, while some conifer forests may naturally go many decades or even a century or more between fires. An ecosystem denied fire for far longer than its natural interval does not simply stay safely unburned; it accumulates dead wood, litter and dense undergrowth well beyond what its natural fire regime ever allowed to build up, so that when a fire eventually does arrive, deliberately or by accident, it burns far hotter and more destructively than the smaller, more frequent fires the ecosystem evolved alongside.

This article records tradition as it has been passed down and reported. Its sources are not yet part of the atlas's verified catalogue.

For most of the twentieth century, the official policy of the United States Forest Service toward wildfire was simple: put it out, as fast as possible, every time, a policy popularly reinforced from 1944 onward by the Smokey Bear advertising campaign's message that only visitors could prevent forest fires. The policy made intuitive sense to a public and a government focused on protecting timber and property, and for decades it was pursued with real success, measured in acres burned per year.

The trouble was that success, on its own terms, was quietly creating a different and larger problem. Forester Harold Weaver argued in a 1943 paper that decades of total fire suppression in western pine forests had left them unnaturally dense with small trees, brush and dead fuel that the historical, more frequent fire regime would have cleared out gradually, and that this buildup was setting the stage for far more severe fires than the region had previously experienced. Weaver's argument, and the broader body of fire ecology research that followed it across the second half of the twentieth century, gradually shifted official policy toward prescribed burning, the deliberate, controlled reintroduction of fire under conditions land managers can control, rather than treating every fire as an emergency to be extinguished regardless of the ecosystem's own needs.

The giant sequoia groves of California's Sierra Nevada became one of the clearest illustrations of what a century of suppression had cost. Sequoia groves that had burned naturally, and lightly, every decade or so for thousands of years went unusually long without fire under twentieth century suppression policy, accumulating dense undergrowth of smaller trees beneath the ancient giants. When severe wildfires reached several sequoia groves in 2020 and 2021, fed by that accumulated fuel and by drought conditions, they killed mature sequoias, some of them thousands of years old, in numbers without precedent in the trees' known history, a loss that finally moved land managers to apply prescribed fire directly inside groves that had specifically been protected from it for generations, restoring, deliberately and under control, the fire regime the sequoia's own reproductive biology had depended on all along.