Hard leaves (sclerophylly)

Walk into almost any patch of bush behind Adelaide and run a leaf between your fingers. It will not feel like a leaf from a wet-country forest — soft, broad, quick to wilt. It will feel like a leaf built by an accountant: small, stiff, faintly waxy, leathery enough to survive a squeeze. That toughness has a name, sclerophylly, from the Greek for “hard leaf,” and it is the single most obvious signature of a Mediterranean climate written onto a living plant.

The logic is simple arithmetic. A leaf is a hole in a plant’s skin, evolved to let carbon dioxide in, which means it also lets water vapour out, and in a country that gets months on end without rain, an ordinary broad, thin leaf is a leak the plant cannot afford. So the local flora shrank the leak: smaller leaves lose less water to start with, a thick waxy cuticle and stomata tucked into pits slow the loss further, and staying evergreen all year means never having to pay the expensive one-off cost of flushing a fresh crop of leaves the moment the rain returns. Much of the ground under this vegetation is old and starved of phosphorus, so a leaf that is expensive to build is worth holding onto for years rather than replacing every season (Keeley et al. 2012). Drought writes the deadline; poor soil writes the budget.

What makes the story remarkable is that nobody had to invent this trick more than once. Southern Australia’s kwongan and mallee heaths, the fynbos of South Africa’s Cape, Californian chaparral, Chilean matorral and Mediterranean maquis are five unrelated floras, on five separate continents, that never shared so much as a common ancestor in the relevant lineages — and yet, handed the same rainless-summer, poor-soil bargain, all five arrived at hard, grey-green, evergreen shrubland (Cowling et al. 1996). It is convergent evolution at the scale of whole landscapes, and it is why a botanist who knows the Cape can walk into the Adelaide Hills and feel, oddly, at home.

So the grey-green, spiky, unglamorous look of so much of this country is not a lack of imagination. It is five separate victories over the same hard problem, and Adelaide’s ranges are where you can read that solution in your own hand.