The gradient rule (substrate writes the country)

What grows in any given spot is set less by how much rain falls on it than by what the ground is made of when the rain arrives. That is the rule, and once you have it the Sunshine Coast stops being a scatter of unrelated habitats.

Rainfall barely gets a vote, though not because it is everywhere the same. It does change across the coast — the range is genuinely wetter than the sand behind the beaches — but it changes gently, sliding up as you climb, with none of the hard edges the vegetation has. Ecosystems change in patches, sometimes across a single step, and a smooth gradient cannot draw a sharp line. Look instead at the two banks of one creek, one basalt and one sand, taking the same rain out of the same sky and growing two different worlds. The deciding factor is not how much water arrives but what the country does with it once it lands: how deep the soil runs, how much it can hold, and whether it keeps the water or lets it slip straight through.

The country is built of a handful of different grounds, and only some of them are volcanic. Between twenty-six and twenty-seven million years ago magma forced its way up toward the surface; in places it never quite arrived, cooling underground into hard plugs of rhyolite and trachyte that erosion has since stripped bare and stood up on the skyline — the Glass House Mountains, Mount Cooroora, the rest. Elsewhere a runnier basalt lava spread across the high, wet ranges and rotted over a few million years into deep red clay that holds both water and nutrients. The sand is no relation at all: along the coast, over hundreds of thousands of years, wind and sea built and rebuilt vast fields of it, and it drains fast and starves. Between the two lie sedimentary rocks laid down long before any of the volcanoes, and the alluvium the rivers have spread since, the softer ground they have chewed through most easily. Rich basalt on the cool ranges grows rainforest, hungry sand on the warm coast grows heath, and the middling ground grows the open eucalypt forest that nine people in ten picture the moment you say “the Australian bush.” Different ground, different outcomes, under the one sky.

What the rock is handing out is nutrients, and one nutrient above all. As a soil ages it runs short first of nitrogen and then, over far longer spans, of phosphorus, which comes only from rock and cannot be recovered once it has washed below the reach of roots. That slow bleed is why old ground is poor ground. Where the phosphorus goes, and what poverty does to diversity, are separate questions. Here the point is only that nutrients, not rainfall, are the currency the rule is written in.

Get soil depth, nutrient status and drainage clear in your head and the rule runs in either direction. Forwards, from ground to plant, it predicts: you can often call what will be growing before the car has rolled to a stop, which is a strange and slightly thrilling thing to do from behind the wheel. It works at almost any scale, from the whole coast down to a single hillside. Backwards, from plant to ground, it explains what you are looking at.

The exceptions say the most. A forest red gum left standing alone in a cane paddock, like the last guest at a party nobody told it had ended, is not reporting what the ground is; the soil beneath it would still grow forest if it were let. It is reporting what happened. Vegetation that does not match its substrate is the record of what people did to a place rather than what the place would do left alone. That is the fourth clause of the rule at work, people change them. Learn to tell what the ground would grow from what it has lived through, and every cane paddock and every lonely roadside gum becomes a sentence you can read.