Rainforest structure (the layered forest)

A rainforest only looks like a wall of green. Walk into one and it turns out to be a building, and a building has floors. Most of what else is worth knowing about the place follows from that one fact of architecture: the forest is organised into storeys, and each storey makes its living off whatever the storey above lets fall past.

Count them from the top down. A few giants, the emergents, push their crowns clear above all the rest. Below them the closed canopy knits into an unbroken green ceiling of interlocking crowns. Under that runs a sub-canopy of smaller trees and palms, and under that again a dark, shade-hardened understorey of ferns and seedlings, filling every level that remains down to the litter. More storeys here than in any other forest on the coast means more niches and surfaces, and more sheer plant packed into a single hectare.

The stacking is not ornament; it is how a forest this lucky turns its luck into living tissue. Where the ground is this well watered and this well fed (the basalt’s doing, set out under basalt fertility), the thing in short supply is no longer nutrient but light, and tiering is the answer to a light problem. Each layer lives off the fraction the layer above spills, so that between the emergent crown and the fern brushing your boot almost no daylight reaches the ground unused.

Once you can see the floors, the fittings begin to read. Broad, thin fins of wood flaring from the base of a big grey trunk are buttress roots, most often the flanks of a booyong (Argyrodendron), the towering signature tree of the scrub. They look like proof of deep rooting and they mean the reverse. A booyong is enormously tall and heavy yet roots shallow, so in place of a taproot it braces itself sideways, casting these wide flanges out around its foot like guy-ropes turned to timber, the same fix an engineer reaches for at the base of a tower.

Now look up into the forks and out along the heavy horizontal limbs, because the branches themselves have been planted. The stiff apple-green shuttlecocks wedged into the crotches are bird’s-nest ferns, cupped to funnel falling leaves and rainwater into their own hearts and grow a private compost heap in mid-air. Around them ride the staghorns and elkhorns, one set of fronds pressed flat to the bark and another thrown out like antlers, and, harder to pick out, the orchids. Not one of them is a parasite. They take nothing from the tree but a foothold up in the light, and by taking it they turn a single big rainforest tree into a hanging wetland, with pockets of soil and water, and frogs and insects of its own, held tens of metres above the ground.

At your feet is the last floor, and its darkness is what the whole structure produces. So little light survives the fall that almost nothing grows on a closed rainforest floor but the shade-proof seedlings of the canopy trees and a scatter of ferns and palms, biding time for a gap to open overhead.

Because a floor held this dark is also held cool and damp, and a forest kept that cool and damp is a forest too wet to burn. The architecture buys the shade, and the shade holds fire out and lets the whole edifice stand. Here is the catch. Every storey beneath depends on the ceiling staying shut. Open that ceiling and the entire stack is bared at a stroke, the interior dries, and the most elaborate structure on the coast becomes, for the first time, something that will carry a fire. The building only works while the roof holds.