The mallee eucalypts

Drive past a stand of mallee and your eye, trained on ordinary trees, keeps looking for the trunk.

There isn’t one.

Where a jarrah or a river red gum puts everything into a single rising column of wood, a mallee eucalypt has given up on that idea entirely and instead grows a dozen or more thin stems, all much the same size, springing together from a single woody swelling buried in the ground.

That swelling is the lignotuber, and it is doing something no ordinary trunk can do: banking a reserve of dormant buds and stored starch below the reach of heat, so that when fire burns every one of those stems to a blackened stump — as in this country it regularly does — the plant is not dead, merely pruned (Auld 2020).

Within weeks a fresh cluster of shoots breaks through the ash in a ring around the old base, and the mallee carries on as if the fire had been an inconvenience rather than a catastrophe.

Eucalypts carry two separate fire-recovery tricks.

The first is a strand of epicormic buds tucked just beneath the bark of the trunk and branches, insulated from a fire that scorches but does not kill the stem outright — the mechanism behind the “green fuzz” you see clothing a burnt trunk within a month of a fire passing through (Burrows 2013).

The second is the lignotuber itself, the deeper and more drastic account, drawn on only when the stems above ground are finished for good.

A mallee’s whole identity is built around leaning on that second account as a matter of course rather than a last resort — which is why the growth habit looks the way it does: low, shrubby, many-stemmed, not the tall single trunk of the forest eucalypts on the wet side of the ranges.

Behind Adelaide, in the rain shadow east of the Mount Lofty crest, the eucalypts doing this belong to a fairly short cast of species — red mallee, white mallee, yorrell and their relatives — standing a few metres high over a floor of saltbush or spinifex, on country too dry to grow anything taller (Foulkes & Gillen 2000).

None of that species list was guessed at through a windscreen; it was walked, one quadrat at a time, by field ecologists who counted what was there rather than assuming one grey shrub looked much like the next.

What all of them share, whatever the species name on the label, is that buried, multi-budded rootstock and the many thin stems it throws up in its place.

Stand in front of a mallee clump, and you are not looking at a young tree that never got around to growing a trunk.

You are looking at the visible record of a plant that has been burned to the ground and grown back from underground, over and over, for longer than anyone has been keeping count — a gum tree that solved the problem of living in fire country by refusing, ever again, to keep all of itself above the soil at once.