Fire, giant eucalypts and the evolution of Australian forests

It’s a truism that fire is a driver of ecosystem function, and indeed ecosystem evolution, in Australia. Yet it’s surprising how little we still understand about what this means for the ecosystem niches occupied by even some of our most conspicuous endemic species, including the tallest flowering plant on earth, Eucalyptus regnans.

The tallest known living individual of this species is the Centurion, which grows in the Arve Valley of Tasmania. It has been measured at 99.6 m in height, but there is a convincing historical record of 114.3 m for a tree in Victoria that was felled in 1880.*

TERN-associated researcher Professor David Bowman and colleagues from the University of Tasmania have this month published a review paper that puts what we thought we knew about the ecological role of these iconic trees in an entirely new light. The crux of the problem is the well-known fact that although giant Australian eucalypts such as E. regnans are dependent on fire for regeneration, they are often found growing among temperate and tropical rainforest tree species that are not.

‘Giant eucalypts have been a puzzle because they require fire to regenerate, yet they tower over the rainforest tree canopy that regenerates without fire,’ David says.

‘They are incredibly competitive when they are young, yet once established, co-exist happily with invading rainforest species.’**

This has contributed to ongoing controversy about whether, ecologically, giant eucalypts should be considered long-lived pioneering species of rainforest communities, rather than the currently accepted view that they comprise mature, fire-adapted forest communities in themselves.

It is instructive that in some places, such as the south-west of Western Australia, forests of giant eucalypts (in this case E. diversicolor) do not give rise to rainforest.

‘There’s evidence that the extinction of rainforest species in that region around 3 million years ago occurred because the climate there became drier and more fire-prone. So it’s conceivable that increasing frequency of fire may have prevented those forest communities from developing towards rainforest, and instead driven the evolution of an alternative ecosystem dominated by giant eucalypts,’ David says.

While this debate may appear academic, there are practical implications. Current management practices such as logging and burning may be actively maintaining some Australian forests in fire-prone giant-eucalypt-dominated states, preventing long-term ecological succession towards rainforest.

‘And it may be that encouraging the growth of forests of giant trees that are hugely valuable for the timber industry, by suppressing ecological succession towards rainforest, is considered appropriate ecosystem management,’ David says.

‘But in any case, reclassification of giant eucalypts as globally unique, fire-dependent rainforest pioneers would better reflect their greater ecological potential, and would better inform debate about their management and conservation.’

* Perhaps the most believable maximum height recorded for a mountain ash was 114 m for a tree felled in 1880 at Thorpdale, South Gippsland. This tree was measured both before and after felling by a qualified surveyor, George Cornthwaite. He writes that this tree, and two other very tall trees nearby ‘were to a certain extent, freaks, standing 50 to 100 feet above the skyline’ (Cornthwaite 1925).

** Some sentences in this article have been adapted from an article on ABC Science online.

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