Plant partnerships (the deals for poor ground)

Few things look more self-reliant than a plant. It stands in one spot, spins its own food out of sunlight and air, and seems to ask the world for nothing but a place to root. Look underground, though, and that independence is a fiction. A plant is the hub of a web of deals: trading sugar with fungi, mining the soil by chemistry, renting bacteria to pull nitrogen from the air, and in a few cases eating animals outright. None of it comes free, and the poorer the ground, the more partners it takes to get by. That is why the starved sands of this coast run such busy underground economies.

The most widespread of these partnerships is also the one almost nobody can see. Nearly every plant here — rainforest giant, wallum banksia, gum and paperbark alike — runs its roots in league with soil fungi, a relationship called mycorrhiza, “fungus-root.” The deal is simple. The fungus threads its impossibly fine filaments out through the soil, far beyond the reach of the plant’s own roots, and harvests scarce water and nutrients; in return the plant pays in sugar, the energy it caught from sunlight, which the fungus, living in the dark, cannot make. Sugar for minerals, photosynthesis for plumbing: one of the oldest trade agreements in the history of life, older than forests, and possibly the very thing that let plants colonise dry land at all.

On rich red basalt a plant can afford to be casual about its fungal partners; on the bleached white sand of the wallum, where the phosphorus has all but leached away, they are the difference between life and death. Local work comparing the wallum’s scribbly gum with a pampered, fertile-country relative found that what decides how hard a tree leans on its fungi is not which tree it is but what ground it stands in: on wallum sand both depended heavily on the partnership, on rich soil neither much bothered (Schmidt et al. 2006). The wallum, in that sense, is really a community of plant-and-fungus partnerships, each tuned to its own sliver of the meagre soil, the plants merely the parts that show above ground.

Not everything solves the poverty the same way. The banksias, grevilleas and hakeas, the Proteaceae, largely skip the fungal route and go mining instead, growing dense brushes of “cluster roots” that ooze carboxylates to prise sorbed phosphorus loose by chemistry. They do the extraction in-house, and it works best on the very poorest ground of all. Others turn to predation. The sundews of the wet wallum spread leaves studded with what look like dewdrops and are in fact glue, then digest whatever sticks for the nitrogen the sand will not give; in the swamps their cousins the bladderworts do the same underwater, with minuscule suction traps that snap open and swallow a mosquito larva in a few thousandths of a second.

And a great many plants simply rent the one trick no plant can do for itself: pulling inert nitrogen straight out of the air. The wattles and native peas house rhizobial bacteria in nodules on their roots; the she-oaks lodge actinorhizal Frankia; the cycads run cyanobacteria in specialised coralloid roots. The plant feeds and shelters the microbe, and the microbe pays rent in nitrogen: the same logic as the fungal deal, trade what you have for what you lack. It is why wattles are so often first onto bare or burnt ground — they arrive with their own fertiliser factories pre-installed.

All this variety of strategy drives the wallum’s variety of species. Where the ground is desperately poor there is no single best way to make a living, so many different ways coexist, each supporting its own set of specialists. The plant that mines, the plant that rents and the plant that hunts stand side by side, in different trades rather than in competition. The measurement comes from the Jurien Bay dunes in Western Australia, where the number of these strategies climbs as the phosphorus runs down (Zemunik et al. 2015). A principle borrowed rather than a Cooloola result, but it is the mechanism running under the sand country here too.

Self-sufficiency is a luxury of rich soil. On poor country no plant stands alone; it survives by partnership, by chemistry, or by predation, and most often by some mix of all three. Poverty runs on cooperation.