The fire boundary (how flame draws the line)

Stand at the edge where rainforest meets gum forest and you would swear the line was drawn by the soil: rich red ground on one side, poorer ground on the other. Soil does draw most of it. Along the whole coast, what a place grows depends less on rainfall than on the ground beneath it. But right at the join, the soil hands the decision to fire. And the line fire draws is a frontier, not a border — advancing and retreating, as it has for a very long time.

The two forests are enemies. The eucalypt, as anyone who has watched one burn will tell you, is a fire hazard with leaves on: volatile oils in every leaf, a heavy litter of slow-rotting bark and twig piling up beneath it, and long streamers of loose fibrous bark that carry flame up into the crown like a wick. It does everything but strike the match. None of it is aimed at anything, and all of it pays. Fire clears away its slower, softer competitors and leaves it holding the ground. Then it walks calmly away from the blaze, on thick insulating bark, on dormant buds hidden deep in the wood, on a swollen woody rootstock that resprouts as though nothing had happened.

The rainforest does the reverse. It does not resist fire; fire does not start inside it. A closed canopy shuts out the sun and the drying wind, so the interior stays dark and humid and still, and the deep litter does not dry into kindling. A fire racing through the eucalypt forest next door reaches the rainforest edge, finds nothing dry to feed on, and gutters out against the green wall. One forest feeds fire. The other puts it out.

So whoever controls the fire controls the boundary. Burn the country often, and the eucalypts and the heath hold the open ground, penning the rainforest into the gullies and wet tops too damp to catch. Withdraw the fire for long enough, and the rainforest quietly goes on the offensive. Its pioneer trees creep out of the gullies, their shade thickens, their moist litter smothers the grassy understorey until the floor will no longer carry a flame, and the eucalypts, unable to raise a single seedling in the gloom of their own usurped forest, are slowly shaded into local extinction. The rainforest never has to win a fight. It only needs the fires to stop, and time.

What settles it is not whether fire comes but how often — and that can be measured. A long-term prescribed-burning experiment in the region’s heath and woodland, tracked across many years of records, found that it was fire frequency, rather than the mere presence or absence of fire, that shaped the understorey within each of heath, woodland and forest (Dooley and colleagues, 2023). Change the interval and you change which forest holds the ground.

Here is the catch, and it should keep you up at night. The two shifts are not equal. The eucalypt forest is built to be knocked flat and spring back, with bark, buds, rootstocks and fire-triggered seed, a whole armoury for catastrophe. The rainforest is built for the opposite, for stability and permanence. It has none of the eucalypts’ tricks, because for a hundred million years it never needed them. So the frontier moves easily one way and stubbornly the other. Withdraw fire and the rainforest advances, and if fire returns the eucalypts simply take the ground back; the change undoes itself. But let a single bad fire breach a dried, fragmented rainforest and the change tends to stick, because a burnt rainforest does not grow back as rainforest. It comes back as gum forest, or as a tangle of weeds, and the green wall, once it comes down, mostly stays down.

That asymmetry is why the fires of recent drought years have been so alarming, with flame licking into rainforest gullies and the wet floors of sand islands that are barely supposed to burn at all. A boundary that has been renegotiated, back and forth, for millennia can be redrawn in an afternoon, and may not come back. On a map, the line between the two forests looks like the most permanent thing on the coast. Few things on it are more provisional.