Concept · Ch 12
Hollow dependence (the century-old apartment)
A eucalypt cannot grow hollows — they are excavated slowly, over a century and more, by fungi, termites, fire and rot. A whole cast of gliders, owls, parrots and cockatoos can't breed without one, which makes the old, half-dead trees the most valuable and most irreplaceable real estate in the forest.
A great many of the animals of the gum forest — the parrots, the owls, the gliders, the microbats, the glossy black-cockatoo — cannot breed at all without a hole in a tree to do it in. And here is the catch that decides everything: a eucalypt does not grow holes. Hollows are hollowed out, slowly, by decades of fungi and termites and fire and rot chewing into the heartwood of a tree that has already lived a long time. A hollow big enough for a cockatoo or an owl can take a hundred years or more to form.
Which means the most sought-after real estate in the forest is also the slowest to build and the easiest to lose. The big, old, half-dead veteran with the broken crown and the riddled trunk — the very tree that looks most like it ought to come down — is a century or two of housing that nothing can replace on any human timescale. Cut it and you have not removed one tree. You have evicted a whole population and left it nowhere to go.
That is the trap of the perfect fit sprung on a single resource. A cleared feed tree can grow back inside a lifetime; a felled hollow-bearing giant cannot. The forest can look green and sound loud by day and still be failing the animals that den and nest in the dark, because the thing they most need takes a century to make and an afternoon to fell — and a hotter, drier century is coming for exactly the trees that took longest to build.
In depth
The mechanism is an asymmetry of time. A eucalypt does not build hollows the way it builds leaves; a hollow is excavated, by decades of fungi, termites, fire and rot working into the heartwood of a tree old enough to have heartwood worth rotting. That takes a very long time. In the sclerophyll forests of south-east Queensland, hollow-formation only begins at around a century; a blackbutt carries no hollow a possum could use until it is past a hundred and sixty years, and a tallowwood's large hollows come only at two hundred and fifty years or more (Wormington & Lamb 1999; Gibbons & Lindenmayer 2002) — so a hole big enough for a powerful owl or a glossy black-cockatoo may be older than European settlement on this coast. So the most valuable housing in the forest is also the slowest to build and the first to go — the big, old, half-dead, "useless"-looking veterans a tidy manager is most tempted to clear.
A surprising share of the forest's animals are staked on it. Gliders den in hollows by day; the powerful owl, the parrots and the microbats nest and roost in them; and the glossy black-cockatoo is a near-obligate large-hollow nester as well as an obligate she-oak feeder (EPBC/SPRAT listing). Nest boxes help a little, but they are a stopgap — there is no fast way to make an old tree. The asymmetry is the whole point: a cleared feed tree might, given decades, grow back within a young animal's lifetime, but a felled hollow-bearing veteran carries off two centuries of housing that no goodwill can conjure back in time. It is why a stretch of forest can look perfectly healthy by day — green, loud with birdsong — while quietly failing the animals that need it most. And it is where a warming, drying climate bites hardest: the parts of the forest that take longest to build are the ones a fiercer fire or a deeper drought is most likely to take — severe and too-frequent fire degrades these old forests rather than renewing them (Furlaud et al. 2021) — so each hollow tree lost to a severe burn is, in practice, gone for good.
Primary sources & further reading — the doorway
- Leiper, G. et al. (2022). Mangroves to Mountains (3rd ed.). Native Plants Queensland. — Regional natural-history doorway to the gum forest and its hollow-dependent tenants.
- Furlaud, J.M., Prior, L.D., Williamson, G.J. & Bowman, D.M.J.S. (2021). Fire risk and severity decline with stand development in Tasmanian giant Eucalyptus forest. Forest Ecology and Management 502: 119724. (Older stands grow moister with fewer ladder fuels; frequent/severe fire keeps forest flammable — supports "both fire suppression and too-frequent severe fire are degradation pathways," Ch 11. NB: concerns flammability/severity, not hollow-tree mortality.) — How severe or too-frequent fire degrades old-growth eucalypt forest — the fire-side pressure on slow-built veterans.