Scribbly gum

Some trees you know by their leaves, some by their shape. The scribbly gum you know by its handwriting. Climb up off the open dunes into the sandy country behind, and among the low heath stand pale, smooth-barked gums covered in looping, doubling lines that run over the trunk for all the world as though a child had been let loose on them with a biro. The lines are real, and they were alive. Each is the feeding tunnel of the caterpillar of a tiny native moth, Ogmograptis, which mines the soft layer between the tree’s old bark and its new, wandering and turning back on its own track. Every summer, when the gum sheds its old skin to stand fresh and pale, the tunnels emerge on the bare surface, a signature scrawled in the millimetre of space between two coats of bark by an animal that never meant to be read.

A small, elegant partnership: a tree and an insect that have between them made a living out of almost nothing at all. And almost nothing is what this whole tree is built on.

Because the pretty bark is the least of it. The real marvel of the scribbly gum is that it has no business being as big as it is. On this coast it is Eucalyptus racemosa, and its country is the leached sand behind the beaches, the wallum and the sandy lowlands, where the phosphorus has all but drained away and nitrogen is desperately scarce. By any ordinary accounting there is not enough in that bleached sand to raise a shrub, let alone a full-sized tree you can stand a ladder against. And yet here it is, throwing shade. No hidden seam of rich soil runs down through the sand for it to tap; there is none to find. What holds the tree up is thrift, a frugality so complete it reaches all the way down into its dealings with the ground.

Like almost every plant on this coast, the scribbly gum runs its roots in partnership with fungi, trading sugar it makes in the light for minerals it could never reach alone. Careful local work compared it with a cousin off fat, fertile country — E. grandis, the flooded gum of the rich forests — and the finding was not quite the one you would expect. It is not that the wallum tree is built to lean on fungi while its pampered relative is not. Grown on rich ground, neither leaned on them much. Grown on wallum sand, both leaned on them heavily. The partnership turns out to be less a fixed trait of the species than an answer to the dirt: what the sand demands, the tree arranges. The gum you are looking at is the visible half of a plant-and-fungus economy that the poverty of the ground called into being. Its nitrogen is rented, not owned.

The leaves keep the same accounts. They are tough and long-lived, because on ground this starved a plant cannot afford to build a soft leaf and shed it every season; every scrap of nutrient locked into a leaf is too dear to throw away, so it makes leathery ones and keeps them for years.

The doodled bark is the charm, the reason everyone stops and points. The real work is underground: a full-grown tree held up on that sand by spending almost nothing and sharing the little there is. It writes its story twice. Once in a moth’s wandering hand across the bark, and once, unseen, in the thrifty accounts of its roots — the version most people walk straight past.