The underground loop

Nearly every plant standing in the Adelaide bush is married to a fungus, and the wedding is the oldest deal in the book. The fungus spins a mesh of threads through the soil, far past the reach of any root, and gathers scarce minerals — phosphorus above all — that the tree cannot easily find on its own; the tree repays the debt in sugar, manufactured from nothing but sunlight and air. Eucalypts, being eucalypts, run two versions of this at once: a looser partnership as seedlings, and by adulthood a second fungus that wraps their fine roots in a dense living sheath, until an old gum tree’s roots are, functionally, half fungus (Chen et al. 2000).

The deal is a loop, not a simple trade, because many of these root fungi never bother raising a mushroom into daylight at all: they fruit underground, as small truffles, and a buried truffle cannot fling a spore to the wind the way a toadstool can. It has to be found, dug up and eaten. In the bush that once covered the Mount Lofty Ranges, that job belonged to small digging marsupials, bettongs and potoroos, which nosed out the truffles, ate them, and carried the living spores off in their gut to be sown, viable, wherever they next dug (Claridge 1994; Danks et al. 2020). Fungus feeds mammal, mammal plants fungus, tree is fed — no party in the deal intending any of it — and the digging itself picked up sharply after fire, exactly when a burnt landscape most needed reseeding underground.

That is the loop as it used to run. What has changed: the brush-tailed bettong is extinct across the South Australian mainland outside a handful of fenced reintroductions, so here this is a lost dynamic, not a live one — a machine still standing with one gear missing.

You have probably heard a neighbouring claim: that forests are wired into a single sharing “wood-wide web,” complete with wise mother trees feeding their seedlings down the line. The fungal linkages are real. But a careful 2023 reassessment found the popular claims about them — networks everywhere, measurable benefits, deliberate maternal care — running well ahead of the evidence, the last with no field support at all (Karst et al. 2023). Overstated, not a myth: the marriage is genuine, the fairy tale bolted onto it is not yet earned.