Hollow dependence (the century-old apartment)

Open a nest box and you install a possum’s front door in an afternoon. Grow the real thing and you wait a century and more, because a eucalypt does not, in any ordinary sense, make hollows. It makes wood. The hollow comes later, excavated out of the heartwood of an old tree by fungi and termites and fire and slow rot, working inward over decades from the first wound the tree can no longer wall off. Grown versus excavated: that one distinction decides a great deal about who gets to live in this forest.

A young eucalypt is nearly all living sapwood, defended and sealed, with no dead heartwood worth rotting. So the hollowing waits on age, and in the sclerophyll forests of south-east Queensland it barely begins before a tree turns a hundred. A blackbutt carries nothing a possum could use until it passes a hundred and sixty. The big chambers, the ones a powerful owl or a glossy black-cockatoo needs, come only at two hundred and fifty years and more, which means a hole large enough for the forest’s biggest nesters may be older than European settlement on this coast. The hollowing begins quietly, too, and earlier than the fire scars you might blame: fungal decay reaching into wood the tree has stopped defending.

Count who is staked on it. The gliders den in hollows through the day; the powerful owl, the parrots and the pale rosellas nest in them; the microbats roost in them; and the glossy black-cockatoo, fussy about everything, will hardly breed anywhere but a large old hollow. None of these animals can build the thing they need. Every one of them inherits it, second-hand, from a tree that spent two centuries making it by accident.

Which sets the trap. The hollow is the slowest thing in the forest to build and the quickest to lose: a century or two to make, an afternoon to fell. That big, broken-crowned, half-dead veteran with the riddled trunk is the tree that looks most ready to come down, the one a tidy-minded manager most itches to clear. It is not really one tree at all. It is two hundred years of housing. Cut it and you have not removed a tree so much as evicted a whole population and left it nowhere to go.

Nest boxes help a little, and volunteers across the coast put them up, but they are a stopgap and anyone who nails one to a trunk knows it. No shortcut makes an old tree. Here the hollow-bearer parts company with the humble feed tree: a cleared patch of gums can grow back within a young animal’s lifetime, but a felled veteran carries off centuries no goodwill can conjure back.

And here a hotter, drier century bites hardest, because the parts of the forest slowest to build are the parts a fiercer fire or a deeper drought is likeliest to take. Severe or too-frequent fire degrades these old stands rather than renewing them; it thins and kills the very veterans the hollows depend on. So a stretch of forest can stand green and loud with birdsong by day and still quietly fail the animals that breed in the dark, and each hollow tree lost to a severe burn is, for any timescale that matters to a cockatoo, gone for good.