Symbol Of The Kimberley

What are the origins of Western Australia’s iconic boab tree, whose only living relatives survive far away in Africa?

- Alasdair Mcgregor

AUSTRALIA ISN’T KNOWN for its deciduous trees. Across vast swathes of forest on other continents, the annual cycle of shedding and regrowth is ubiquitous. Each year, as days shorten and temperatures drop, leaves fall. Trees shut down for winter, biding their time before embarking on a huge spring growth spurt.

But in Australia’s highly variable boom-bust climatic conditions, the evolutionary path to deciduousness was never really an option. Putting on leaves in response to seasonal change, only to have that foliage wither prematurely during tough times, is biologically wasteful. Unable to rely on predictable seasonal cycles, our trees have become opportunists, waiting out dry spells by tapping groundwater, their permanent leaf canopy always at the ready in case rain does come. The omnipresent eucalypts are a perfect example of Australia’s ascendant evergreens.

Although evergreens are ecologically triumphant here, Australia is home to a small number of native deciduous tree species. All but one are restricted to tropical or subtropical environments; the outlier is the deciduous beech of Tasmania’s cool-temperate mountains, a glorious autumnal relic of the supercontinent of Gondwana.

Australia’s other deciduous trees lose some or all of their leaves not in readiness for the winter cold, but to best survive the northern dry season. Species include the red cedar and that spectacular red-flowered endemic the Illawarra flame tree.

The red cedar is also found in Asia, and Australia serves as the regional genetic hotspot of the Illawarra flame genus, Brachychiton. But there is one remarkable Australian deciduous tree that doesn’t have a single native sibling, nor any presence elsewhere in our region. That total loner is the unmistakable boab of Western Australia’s Kimberley region.