Concept · Ch 6
The fire boundary (how flame draws the line)
The sharp line between rainforest and eucalypt forest is drawn as much by fire as by soil: eucalypts make themselves flammable and profit from burning, rainforest survives by staying too wet to burn, and how often fire comes decides which one holds the ground.
First, meet: Sclerophylly (the tough-leaf economy)
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. But right at the join, the deciding hand is fire, and the line it draws is a frontier that moves.
The two forests are enemies with opposite tactics. The eucalypt makes itself flammable on purpose — oily leaves, deep litter, bark that wicks flame into the canopy — and profits from the burn, because fire kills its softer rivals and it walks away on thick bark and hidden buds. The rainforest refuses to burn at all: its closed canopy keeps the floor dark and damp, so a fire racing through the gum forest reaches the green wall and gutters out for want of dry fuel. One forest summons fire; the other shuts it out.
So whoever controls the fire controls the boundary. Burn often, and the eucalypts hold the open ground while the rainforest hides in the wet gullies. Stop the fire, and the rainforest quietly climbs out of the gullies, shades the grass away, and smothers the eucalypts in the gloom of their own woods. The catch is that the two shifts are not equal. Withdraw fire and the change can be undone; but let a fire finally breach a dried, broken rainforest and the change tends to stick, because a burnt rainforest grows back as gum forest or weeds, hardly ever as rainforest.
In depth
It is tempting to think soil alone sorts rainforest from eucalypt forest — rich red ground grows one, poorer ground the other — and soil does most of the work along the gradient rule. But where the two forests meet, the line between them is drawn and redrawn by fire, and the boundary is a frontier, not a fixed edge.
The two forests are opposites that want opposite things. The eucalypt is, frankly, a fire hazard with leaves on: volatile oils, heavy slow-rotting litter, and loose fibrous bark that carries flame into the crown like a wick. It is closer to an arsonist than a victim — it has made itself flammable on purpose, because fire clears away its slower, softer competitors and leaves it holding the ground, and it walks away from the blaze it invites on thick bark, epicormic buds and lignotubers (its leaves defended and long-lived — see sclerophylly). Rainforest does the reverse. It excludes fire rather than resisting it: a closed canopy keeps its interior dark, humid and still, its litter too damp to ignite, so a fire arriving from the dry forest outside falters at the green wall and goes out.
So the boundary moves with the fire regime. Where fire comes often enough, the eucalypts and the heath hold the country and the fire-sensitive rainforest is penned into the gullies and wet tops too damp to burn. Withdraw the fire for long enough and the rainforest advances — its pioneers creep out of the gullies, their shade and moist litter smothering the grassy understorey until the ground is too damp to carry a flame, and the eucalypts, unable to regenerate in the gloom of their own usurped forest, are slowly shaded out. The rainforest does not need to defeat the eucalypts; it only needs the fires to stop, and time. Fire frequency is therefore a master switch on which forest you get — a point borne out by long-term local burning experiments showing frequency, not fire-or-no-fire, doing the sorting (Dooley et al. 2023). The asymmetry that matters: the eucalypt forest is built to be knocked down and spring back, while the rainforest is built for stability and has no recovery mode — so a boundary that shifts one way (fire withdrawn) is easily reversed, but one that shifts the other way, when a fire finally breaches a dried, fragmented rainforest, tends to stay shifted, because a burnt rainforest comes back as eucalypt forest or weeds, rarely as rainforest.
Primary sources & further reading — the doorway
- Bowman, D.M.J.S. (1998). The impact of Aboriginal landscape burning on the Australian biota. New Phytologist 140: 385–410. — Fire as a central force shaping the Australian vegetation, including the rainforest–sclerophyll boundary.
- Dooley, M., Lewis, T. & Schmidt, S. (2023). Fire frequency has a contrasting effect on vegetation and topsoil in subcoastal heathland, woodland and forest ecosystems, south-east Queensland. Austral Ecology 48(8). (Long-term Peachester burning experiment; regular fire maintains understorey diversity, with a topsoil-carbon cost. First author Madeline Dooley — initial confirmed "M." against the paper, July 2026.) — Long-term SE Qld burning experiment: fire frequency as a master switch on vegetation.