It is the cleanest example there is of an idea that is hard to accept: that a forest can need to burn. And also of how long it takes to correct a management error when the tree it affects takes millennia to grow.
01 A cone that waits
A mature giant sequoia can carry tens of thousands of cones at once, and many have hung there for years, green and closed. They do not readily fall or open by themselves: they hold the seed inside the tree, sometimes for more than a decade. Botanists call it an aerial seed bank.
What opens those cones is heat. When a fire runs across the forest floor, the hot air rises into the crown, dries the scales and they separate. The seed then falls onto ground that has just burnt. The tree is not reacting to fire: it was waiting for it.
The scale of it is what surprises. The cone is the size of a hen's egg and the seed is barely more than a flake; the tree that comes out of it can pass eighty metres and live two thousand years.
02 The seed needs ash
Here is the second half of the mechanism, the half usually forgotten. The sequoia seed is tiny and has no reserves to push through a built-up layer of litter. It needs to reach bare mineral soil, with enough light and enough moisture.
Fire produces exactly that: it burns the litter, leaves the soil clean and opens a gap in the canopy through which light comes down. Without fire, the seed lands on a carpet of dry needles and comes to nothing.
In other words: the fire is not an accident the sequoia survives. It is the act that reproduces it. A sequoia forest without fire is a forest of old sequoias with no successors, and that takes a century to become visible because the trees are still there, enormous and apparently fine.
03 The tree is built to burn
For that bargain to work, the adult has to survive what it itself requires. Hence its bark: fibrous, spongy, of considerable thickness and almost free of resin, which is what burns readily in other conifers. It works as thermal insulation.
Even so, fire leaves its mark. Many old sequoias have bases hollowed by fires of earlier centuries, with a black cavity that sometimes goes right through the trunk. The tree stays alive because what carries the sap is the outer layer, not the heartwood.
Those scars are, in fact, the forest's archive. Counting the rings between one burn and the next reconstructs how often each grove burnt before anyone arrived to put anything out. The answer, in nearly every case, is: often, and with small fires.
04 The century when everything was put out
For almost a hundred years forest policy consisted of putting out any fire as quickly as possible. The intention was to conserve the forest; the effect was the opposite.
With nothing burning, the understorey filled up. Branches and litter accumulated, and young trees of other, smaller and more flammable species grew up beneath the sequoias. That forms what the trade calls a ladder: continuous fuel that lets a ground fire climb into the crowns, which is where a sequoia is vulnerable. At the same time, and for the same reason, the sequoias stopped producing offspring.
The change of approach began in 1968, when Sequoia and Kings Canyon national parks started burning under controlled conditions. It is the oldest prescribed-burning programme in the country. The idea was easy to state and hard to carry out: give the forest back the small, frequent fire it always had, so that it would stop stockpiling the material for the big one.
05 What happened in 2020 and 2021
The bill for that century arrived all at once. In 2020 the Castle Fire killed, according to the published estimates, between 10 and 14 per cent of all the large sequoias in existence. In 2021 the KNP Complex and Windy fires took between 3 and 5 per cent more.
The percentages are hard to absorb because they describe a tree of which a few tens of thousands of large specimens exist in total, all in the same range. Another figure places it better: between 2015 and 2021 more than 85 per cent of the sequoia grove acreage in the Sierra Nevada burnt. In the century before, around a quarter had.
And there is a scene from September 2021 that sums it up better than any statistic: crews wrapping the base of the General Sherman — 83.8 metres tall, 7.7 across, 1,487 cubic metres of trunk — in fire shelter material as the smoke came into the grove.
The tree survived. But the image makes the scale of the change clear: a species that spent millennia needing fires has reached a point where it has to be protected from fire, because the fire that reaches it now is nothing like the fire that made it.
It is worth knowing before walking into a grove. The black trunks at the base are not damage: they are the tree's biography. The worrying thing is not seeing a burnt sequoia; it is seeing no young ones.
