Eucalypt forest and woodland

Say “the Australian bush” and a picture arrives unbidden: tall straight trunks, hanging grey-green leaves, dappled light, the smell of eucalyptus on a hot afternoon, the laugh of a kookaburra. That picture is the stretch of the Sunshine Coast that looks Australian to the bone. It is also the country we notice least. The rainforest is dramatic, the wallum is curious, the beach is the beach; the gum forest is merely there, the green-grey backdrop we drive through on the way somewhere else. But the backdrop turns out to be the stage: the largest and most fiercely contested ecosystem on the whole gradient.

The first thing to learn is that it is not one forest but two, and the difference is written, as ever, in the ground. Give a eucalypt forest more water and a little more nourishment and it grows tall, dense and shrubby. This is the towering wet sclerophyll of the foothills and gully floors, blackbutt, tallowwood and flooded gum rising clear for fifty metres and more over a soft, almost rainforest-like understorey. Starve it on the harder, drier ridges and it opens into grassy woodland, spotted gum and ironbark over a sunlit floor. Same tribe of trees, the same tough leaf that scarce phosphorus levies on every plant that grows here, the richness of the soil turned up or down, and two quite different worlds.

The strange thing is where the wealth sits. In the tall wet forest your eye goes up, but the real variety lies on the floor of the dry one, down around your boots: kangaroo grass and wiregrass between the trunks, and threaded among them flax lilies, native peas, ground orchids, trigger plants, grass-trees and cycads. A single patch can hold a startling number of small plants, and most of them hold on only because fire keeps the canopy open and stops any one shrub from shading them out.

Which is the second thing to learn: running through both forests, holding them in place and drawing the boundaries, is fire. These trees are flammable. The leaves are oily, the litter will not rot, and ribbons of loose bark carry flame into the crown. They are also armoured against the blaze that follows, with thick insulating bark, dormant buds that resprout the crown within weeks, and lignotubers waiting underground. The whole forest is shaped by the fire its own litter feeds.

That dependence is most dramatic in the tall wet forest, which lives a curiously double life. Its giants cannot reproduce in their own shade. Their seedlings need open, ash-cleared, sunlit ground, while the palms, ferns and rainforest saplings crowding the gully floor beneath them need no such help. So without fire reaching up the gully every few decades to knock the rainforest back and open the soil, the shade-lovers slowly close over and shut out the next generation of gums, and the wet forest turns to rainforest for good. It is a strong tendency rather than a certainty; it needs a rainforest seed source close by, and no drought or harsher fire cutting across the process first. But where the conditions line up, the giants overhead are the last gum generation of a forest already on its way to becoming something else.

The dry woodland tells the same story lower down, in its floor. A cool burn every few years clears the litter and lets the grasses and small flowering plants come away again from their roots. Withhold it and the floor begins to close: wattles come up in dense flushes, the black she-oak thickens the mid-storey, lantana smothers the disturbed patches, and the grassy, flowering ground layer, starved of light, vanishes. Ecologists call it woody thickening, one of the commonest ways an Australian woodland loses its diversity without losing a single tree to the chainsaw; where rainforest plants do the thickening, dry open forest elsewhere in eastern Australia has been measured to lose more than half its understorey species.

Fire, then, is a dial the bush is built around, and a dial with no single correct setting. The long-term fire-frequency experiment run at Beerwah Scientific Area and in the Cooloola Recreation Reserve, going now for close on half a century, has watched this play out: plots burnt often keep their open, herb-rich floor but draw down the carbon stored in the topsoil, while in its heathland plots the withholding of fire brought on a smothering dominance of coral fern. On the dry ridges it is the black she-oak that thickens when fire is withheld. Too little fire and the woodland thickens; too much or too fierce and the slow growers never get the years they need, and fire kills the old veterans outright — and those you do not get back in a human lifetime. Nor is fuel simply banked while you wait: it peaks a decade or so after a burn and then falls, so skipping the small fires does not quietly store up safety. None of this was discovered by science. It was known and worked, as a fine-grained mosaic, through tens of thousands of years of deliberate Aboriginal burning.

Read the floor, then, and you have read the fire. Grassy and in flower means the flame came through recently and gently; dim, shrubby and litter-deep, thick with black she-oak, means a very old conversation has lapsed. And read the oldest trees for the thing only time can make: the hollow, a century and more in the excavating, an afternoon to fell. Do that, and a stretch of bush you might once have driven past as a green blur becomes a legible record of soil, fire and time.