Subtropical rainforest (the big scrub)

At the top of the long haul up from the sea, on the richest ground and under the heaviest rain, stands the forest nobody argues about: the rainforest, the tallest and most crowded thing growing on this coast. It is the wet, fertile ceiling the whole reef-to-range sequence has been climbing toward. Why it grows in this corner and hardly anywhere else is the story of basalt fertility; how it packs so much life into stacked tiers belongs to rainforest structure. This entry is about the two things that make a rainforest itself: how old it is, and how easily it dies.

Set it against the wallum at the far end of the coast and the logic clicks shut. That heath lives on the poorest sand, out in the open, and answers its poverty with a few hundred small, wiry specialists. The rainforest lives on the best clay, in deep shade, and answers its wealth with sheer mass, a comparable crowd of species stacked into one volume of air. Between those two poles lies everything else the coast can grow.

Take the age first, because nothing else here is old in the same way. Lift your eye to where a pale, hoop-ringed trunk or a great dark dome breaks clear above the canopy, and you are looking at an araucaria, a conifer off a line that clothed the forests of Gondwana while the southern continents were still one landmass and dinosaurs browsed woods much like this. Flowering trees came later and elbowed the old conifers aside almost everywhere on Earth. But in the wet, sheltered, rarely burnt folds of the eastern ranges, whole lineages of that vanished world held on, and hold on yet. Stand in a rainforest gully and you are not among trees that will grow back in a lifetime or two. You are at the frayed edge of something that took the better part of a hundred million years to assemble, and that was cleared, across most of its range, in about fifty.

Now the fragility, which is the same fact turned over. What makes a rainforest a rainforest is the way it handles fire: it survives flame because one rarely starts inside it. The closed canopy keeps the inside dark and windless, the litter too damp to carry a spark, so a fire tearing through the eucalypt forest next door meets the green edge and starves for want of fuel. Where it is unbroken this works so well that rainforest survives as islands in a sea of fire, holding the wet gullies and high shoulders that flame cannot reach. But the defence is all or nothing. Clear the canopy, fragment it, cut a road through it, or let a single bad fire in at the margin, and the interior dries and, for the first time, will burn. A rainforest that burns has none of the eucalypt’s fire tricks, neither the thick insulating bark nor the reserve buds and smoke-triggered seed that let a gum turn a fire to its own advantage. It comes back, if it comes back at all, as eucalypt forest or a snarl of weeds, and hardly ever as rainforest. It has no reverse gear.

The big scrub was the broad reach of subtropical rainforest that once covered the red basalt of the Blackall Range. It was doomed by the very thing that made it magnificent: the best soil in the region, carrying saleable timber on top of it. Felled within a few decades of European arrival, it survives now only where steep ground, luck or somebody’s foresight held it back. A forest that could shut fire out had no defence whatever against the axe.