The gulf-to-range transect

Pick any point on the map and you can ask it one question: how far up the slope are you? Everything alive here, from the seagrass on the gulf floor to the mallee behind the last ridge, is arranged along a single short climb — some twenty-five to thirty kilometres from the coast to the top of the Mount Lofty Ranges — and once you see that climb as one thing, the region reads as a sentence, not a scatter of facts.

Start below the waterline. Gulf St Vincent is a sea that runs backwards: instead of freshening as it narrows, it grows saltier toward its head, because evaporation off the shallow northern gulf outpaces the rain and runoff that would otherwise dilute it (de Silva Samarasinghe & Lennon 1987). Step ashore through mangrove and blue-green samphire — the coast’s salt threshold — onto plains that once carried open grassy woodland of spear-grass and wallaby-grass under scattered blue gum and grey box on four or five hundred millimetres of rain a year. Almost none of it is left; barely four per cent of the plains’ native vegetation survives the plough (Kraehenbuehl 1996). Keep climbing and the country changes fast. Force the same sea winds up the range front and they cool and let go of their water, so the high hills catch something like twice the plains’ rain, the air turns cooler and damper, and alkaline plains soil gives way to ground finally rinsed of its lime and salt. Stringybark and messmate crowd the range front, and the wettest folded gullies hide fern, moss, and a thread-tailed scrap of a bird, the southern emu-wren, needing cover this dense and this wet just to exist.

Here is the twist that turns a climb into an arch. The ranges do not merely catch the rain; they steal it, so air spent on the summit forest arrives over the crest already wrung dry, and the country just behind is thrown back into a different, inland dry: the rain shadow. Tall wet forest thins within a few kilometres into mallee — low, multi-stemmed eucalypts resprouting from a woody underground swelling after fire, over spinifex or saltbush, on perhaps a third of the summit’s rain (Foulkes & Gillen 2000). None of this stepped terrain is an accident of erosion; the transect has been built, and is still being built, by faults shoving the ranges skyward faster than their summits wear down (Quigley et al. 2010).

That is the gradient, walked once: salt, plain, range front, summit, dry again over the back — one climb, four kinds of country, and every other unit in this guide is a closer look at one step of it.