Spinifex (Triodia)

Most grasses you can mow, trample or ignore. Spinifex you route around. It grows as a dense, spiky ring of tough, resin-coated leaves radiating out from a dead centre, and a mature hummock is less a plant than a small fortress, packed tight enough to turn back a boot, let alone a predator. Despite the name, it has nothing to do with the runners of coastal dune grass also called spinifex; this is Triodia, the hummock grass of Australia’s dry interior, and in the rain-shadow mallee behind the Mount Lofty Ranges it is the understorey that matters most, standing alongside — and on the sandier rises, instead of — the low chenopod shrubs of the saltbush flats (Foulkes & Gillen 2000).

What makes spinifex worth a chapter of its own is not the plant but its clock. Burn a hummock down to ash and it does not vanish. The resprouting is slow, and uneven. Cover climbs only gradually through the first five years, then rises steeply through the next fifteen or so, and keeps building toward a peak of density and structure at somewhere near thirty years unburnt (Haslem et al. 2011). Around the fifteen-year mark the hummocks have thickened enough to carry a fire themselves, closing a loop that has been turning for longer than anyone has been measuring it: burn, rebuild, burn again. That timetable is not fixed to the year — mallee fire regimes bend with drought, rainfall and fuel type, and litter-driven fires in wetter years can move through much faster than the spinifex cycle proper (Bennett et al. 2021) — but the broad shape holds: old spinifex is a different, denser structure than young, one that takes over a decade to rebuild.

That distinction between young and old spinifex is the whole story for a particular guild of small, secretive, poor-flying mallee animals that will use nothing else. What they need is old spinifex — spinifex that has gone the better part of two decades without burning, dense enough to hide a bird that can barely cross open ground. The clearest case is the mallee emu-wren, a four-gram scrap of a bird so tied to long-unburnt Triodia that a single large fire through the wrong patch can remove it from an entire region in one afternoon, with no fifteen-year-old hummock left standing anywhere nearby to retreat to (DEWHA 2009). Read a stand of spinifex on the ground and you are reading a stopwatch — one that tells you, hummock by hummock, how long it has been since fire last passed through, and whether the earthbound lives that depend on its slowest rebuilding have anywhere left to go.