The rain-shadow mallee

Come over the top of the Mount Lofty Ranges and drive down the eastern side, and within a few kilometres the world changes its mind about what it is. The tall wet forest thins, then quits altogether; the air dries out; the horizon opens; and the country flattens into a low, grey-green scrub running east toward the Murray and the dry interior beyond. You have crossed into the rain shadow, the dry lee where the ranges have already wrung the rain out of the sea wind before it can arrive, and into the last community on the gradient: the mallee.

“Mallee” is not really a place; it is a way of being a eucalypt. A mallee tree has given up on having a single trunk and instead grows a dozen or more thin stems from one buried, woody rootstock — a lignotuber, packed with dormant buds and stored starch, that lets the whole plant resprout from the ground up whenever fire burns everything above it away (Burrows 2013). The rain-shadow mallee is built from a handful of these multi-stemmed eucalypts — red mallee, white mallee, yorrell and their relatives — standing a few metres over a floor of saltbush or, on the sandier dune crests, of spinifex, and none of it was guessed at from a car window: it was walked, plot by plot, across hundreds of quadrats by field ecologists counting what was there (Foulkes & Gillen 2000).

Fire runs this country even more openly than it runs the hills behind it. Spinifex — the spiky, hummock-forming desert grass properly called Triodia — takes something like fifteen years after a burn to thicken up enough to carry the next fire, and a whole cast of small, secretive animals depends on that slow rebuilding: they need spinifex old and dense, and a single large fire can wipe them from an enormous area in one afternoon (Haslem et al. 2011). A mallee eucalypt’s low, shrubby shape is nothing but the visible record of having burned and grown back this way, over and over, for longer than anyone has been counting.

And yet the mallee is, of all the ecosystems on the gradient, the one that has kept the most of itself. The reason is the same one that ruined the settlers who pushed wheat north of Goyder’s Line: this ground was too dry to farm with any confidence, and much of it, cleared and cropped and then abandoned in the bust years, was in the end left alone (Goyder 1865). So while the fertile plains close to Adelaide were cleared almost completely, great blocks of mallee scrub survived behind the ranges — the dry, unwanted country turning out, as it does again and again on this gradient, to be the country still standing.