Subtropical rainforest (the big scrub)

At the top of the long climb up from the sea, on the richest soil and under the heaviest rain, sits the forest everyone agrees is glorious: the rainforest, the tallest and oldest and by a distance the most crowded forest on the coast. Step off a mown edge into one of the survivors and the change is instant and physical — the light drops by half, the air cools and dampens and goes still, and the Queensland glare falls away behind you as if a door had shut. You have walked out of the sun and into a green room, and the room is stacked in layers: giant trees emerging above a closed canopy, smaller trees and palms and ferns filling every level below, buttressed trunks braced like the flying buttresses of a cathedral, strangler figs standing as hollow woven cylinders where another tree used to be.

Hold it against the wallum and the logic snaps into focus, because the two are the poles of the whole gradient. The wallum grows on the poorest ground, in the open, demanding fire, and answers its poverty with a riot of small, hard-leaved specialists. The rainforest grows on the richest ground, in deep shelter, excluding fire, and answers its wealth with sheer mass and structure. Where the wallum is a crowded democracy of the poor, the rainforest is a towering aristocracy of the well-fed. Both are diverse, but at opposite ends of the same slope — and between them they bracket everything else on the coast.

The wealth is real, and it starts in the rock. Basalt rots down to a deep red clay that holds water and nutrients better than any other soil in the region, and the ranges that carry it are where the coast pulls the most rain out of the sky. Richest rock, richest soil, most rain: this is just the wet, fertile top corner of the gradient rule. But the forest keeps its fortune overhead, not in the ground — the roots and their fungi snatch each fallen leaf back almost before it lands, running the whole system on a tight, fast, hand-to-hand cycle — which is exactly why clearing does such lasting harm. Fell the forest and the wealth does not stay behind for the paddock; most of it walks away in the logs, and the rest washes off the bared red slope in the first summer storm.

The defining fact about rainforest, though, is how it handles fire, and it is the opposite of everything the fire-country plants do. It survives flame by refusing to let it start. The closed canopy keeps the interior dark, humid and still, the litter too damp to burn, so a fire tearing through the eucalypt forest next door arrives at the green wall and simply runs out of fuel. Where the forest is whole this works almost perfectly, which is why rainforest survives as islands in a sea of fire, holding the wet gullies and the range tops. But the defence is all-or-nothing. Break the canopy — clear it, fragment it, run a road through it, or let a single bad fire breach its edge — and it dries out and becomes, for the first time, able to burn, and a rainforest that burns does not come back as rainforest. It has no reverse gear. That is why so little of the old big scrub is left, and why the fragments that remain matter as much as they do.