She-oak

Stand under a she-oak in any breeze and you will hear it before you understand it: a soft, continuous sighing, a sound like surf breaking somewhere just out of sight. It is the most atmospheric noise in the dry forest, and it comes of the tree’s strangest feature. Those fine, dark, weeping “needles” are not leaves at all. They are green branchlets, cladodes, jointed like bamboo, with the true leaves shrunk to a ring of tiny scale-teeth at each joint. It is a low-surface, drought-thrifty way to photosynthesise on hard, poor country, and it is why the wind combs a she-oak into an endless whisper where it merely rustles an ordinary tree.

The she-oak is the coast’s great generalist of hard ground. The black she-oak (Allocasuarina littoralis) holds the dry, poor ridges; the forest oak (A. torulosa) takes the slopes; the swamp oak (Casuarina glauca) suckers out across the brackish estuarine mud where the freshwater turns salt. Between them the family colonises the kind of ground that defeats fussier trees, bare rhyolite included. It manages that by carrying its own fertiliser. In nodules on its roots the she-oak houses Frankia, an actinorhizal bacterium that fixes nitrogen straight out of the air. So the tree quietly enriches the very soil it stands on, and gains a foothold where there is, as yet, nothing to feed on. It is, in a small way, its own gardener.

She-oaks keep their sexes apart, male and female on separate trees. In autumn the males flush a rusty orange with pollen, briefly colouring whole stands, while the females carry crimson-styled flowers that harden, over months, into small woody cones. And those cones are what ties this tough generalist to one very fussy bird.

Look down before you look up. If little cones lie scattered beneath a she-oak, chewed and splintered like cracked pistachio shells, you have found the dinner table of the glossy black-cockatoo, a bird that eats almost nothing but she-oak seed and is fanatical about it, returning to the same favoured trees year after year and snubbing perfectly good ones next door. The limit is in the tree, not the bird. A hot fire kills a she-oak outright, and it takes the best part of a decade to grow back to cone-bearing size. So a bad burn does not merely scorch the larder; it switches it off for ten years or more. A rare bird’s living hangs on how fast a slender tree grows and how often fire passes through it.

The same toughness has a second, less flattering side. On the dry ridges the black she-oak is the most persistent of the woody plants that thicken up and crowd the open forest when fire is withheld too long. The grassy, flowering floor of a healthy woodland, the flax lilies and native peas and ground orchids down around your boots, needs regular, gentle fire to keep the canopy open above it. Take the fire away, and that ground-holding vigour wins. The long-running fire-frequency experiment run at Beerwah Scientific Area and in the Cooloola Recreation Reserve has watched plots left unburnt for decades close over and lose the small plants beneath, though in its heathland plots the thing that shut the ground down was a smothering blanket of coral fern rather than she-oak. What a thickening mid-storey costs a dry open forest has been counted elsewhere in eastern Australia, where an advancing understorey displaced more than half the plant species on the floor. On these ridges it is the black she-oak that does the thickening, and nobody has yet measured what it takes with it.

So the she-oak reads two ways. It is a coloniser that mends poor ground and a linchpin a threatened bird can hardly live without; it also, in the absence of fire, crowds out the country it fertilises. Neither reading cancels the other. Burn it too hot and the cockatoo’s larder shuts for a decade. Leave it unburnt too long and the woodland floor goes dark. Stand under the tree, listen to it sighing like far-off surf, and it will tell you almost none of this. You have to read the ground beneath it.