Why the poorest ground grows the richest flora

Behind the beaches the sand is bleached, rinsed of its nutrients by hundreds of thousands of years of rain until almost nothing usable is left. Hundreds of plant species grow on it, packed in where you would expect near-bare ground. The second fact is true because of the first.

Common sense insists it should be the other way round. Surely the rich red basalt of the Blackall Range, the best soil in the region, ought to carry the greatest variety of plants, and the starved white sand the least. Common sense, here, has it almost exactly backwards. The rainforest on the fertile ground, for all its cathedral grandeur, runs on a short list of large trees. The wallum heath on the oldest, poorest sand, banksias and grevilleas, peas and boronias, ground orchids and wildflowers and glittering carnivorous sundews, is one of the most species-rich plant communities in the whole region. The poor ground is the crowded one.

The first half of the reason is competition, and it is almost a parable. Where soil is rich, a few fast, greedy plants grab the nutrients, shoot up, and shade and crowd everyone else into oblivion; abundance, perversely, narrows the field to a handful of bullies. Where soil is desperately poor, no plant can grow fast enough to lord it over the others, so the door stands open to hundreds of specialists, each scratching a living a slightly different way. Poverty is a great leveller, and a greater diversifier.

What makes the poverty permanent is a single asymmetry between two nutrients. Nitrogen can be hauled out of the air by the right microbes and banked in the soil; phosphorus can only ever come from rock. Once the original sand’s phosphorus has weathered out and washed away to sea, there is no resupply, not ever. On the ancient dunes of Cooloola the available phosphorus drains away dune by dune until the last reachable trace is gone. That is what holds the field open: a shortage no plant can outgrow.

The second half of the answer is stranger. As the soil grows poorer, the number of different tricks plants use to find nutrients goes up, not down. Work on long, ageing dune chronosequences has shown that nutrient-acquisition strategies multiply as phosphorus dwindles, and that this very variety largely explains why species diversity runs so high. You can collect nearly the full set on a single morning’s walk through the wallum. Most plants here run their roots in partnership with mycorrhizal fungi, each striking a slightly different underground bargain, trading sugars for a share of the soil’s meagre stores. The Proteaceae, banksias and grevilleas and hakeas, carpet a patch of soil with short, dense root-clusters that pour out acids and enzymes and unlock phosphorus that lies beyond the reach of almost everything else: the botanical equivalent of dissolving the safe to get at what is inside. Where the ground cannot supply enough nitrogen, some plants take it from meat, and the sundews on the damp sand and the bladderworts in the swamps trap and digest insects outright. The many native peas and wattles rent nitrogen-fixing bacteria, banking the one major nutrient the air will hand over for free.

One thread runs through nearly all of these plants: the hard, long-lived leaf that Australian botany named itself after. Why a starved plant should build so tough and thrifty a leaf is the business of sclerophylly; here it is enough that the leaf is the outward badge of a life run on almost nothing.

Two arcs, then, cross on these dunes. The mass of the vegetation peaks back on the middle-aged sand, where the tall forest stands, and falls away to almost nothing on the oldest ground; the variety of it climbs to its maximum on that same hungriest sand. The wallum is what a landscape looks like when no one is allowed to win, one of the few places on Earth where you can walk out and watch it happen.