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The curious case of Acacia peuce


 
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Dr Jon Luly from James Cook University looks at one of the rarest trees from Australia's arid zone - Acacia peuce or Waddy-wood.    
Almost certainly a relic of wetter climate periods, Acacia peuce is a plant that made significant structural adaptations to avoid being eaten by large browsers, like Diprotodons that roamed Australia, millions of years ago (click here for story).
Today Acacia peuce can be found virtually only around Birdsville and Boulia in south-western Queensland and the Mac Clarke Conservation Reserve in the Northern Territory.

 
The trees are quite magnificent, rising some 18 metres above the dry, flat landscape of the surrounding gibber plains. In the Mac Clarke reserve they are virtually the only tree and in their other sites at Boulia and Birdsville, they tower over other species that typically reach a height of four or so metres.

 
Below right: This tree, growing in the Mac Clarke Acacia peuce Reserve in the NT (about 320 km southeast of Alice Springs), shows both the size of the tree and the unforgiving landscape the trees will live in.
    
While the trees are very localised, they are quite common in these three remaining areas.
It seems that the formation of the Simpson Desert some 400 thousand years ago, meant that sand dunes replaced the trees' usual habitat, leaving the remaining stands as reminders of an earlier golden age. Acacia peuce is also known as the waddy tree or waddy-wood and as the Birdsville wattle.

 
While little is known about the physiology of the trees and how they survive today in such extreme environments where few (or no) other trees do, at JCU there is a series of research projects underway, focussing on the species' population health and physiology.

 
The studies are supported by the Australian Flora Foundation and the Australian Pacific Science Foundation, with a main interest in relevance to climate change issues.

Right: A band dendrometer being used to measure girth increment in waddi trees at Boulia. Data can be compared with a reasonably long time series collected in the drier Mac Clarke Reserve by the Conservation Commission of the Northern Territory.


 
How does Acacia peuce manage to make a living in such a harsh environment? At JCU a range of studies of the tree's physiology are being conducted to address this question.

Left: Here researcher 
Joe Holtum is measuring the rate of photosynthesis of waddi tree phyllodes.
Other studies measure the flow of sap up the trunk of the tree so that estimates can be made about the volume of water used, and water use efficiency, of the tree

 
It is also likely that the Australian Institute for Nuclear Science and Engineering will support a project that will use radio carbon dating on the trees.    
Jon Luly says that Acacia Peuce may well have a role in carbon capture and carbon trading because, while the trees grows slowly throughout their lifespan of about 300 years, they also releases carbon slowly when they die. Carbon dating has shown that felled trees can remain on the ground over 250 years without decomposing, suggesting benefits of a slow grow, slow capture, slow release carbon cycle and a plus for retaining such remnant vegetation in arid zones.

 

Left: There are several advantages to using Acacia peuce stumps for radiocarbon dating to determine the age of the tree.
Since the tree had obviously been felled by saw cutting, this must have occurred in the past thirty yeaars rather than the tree lying dead in the landscape for an unknown period of time.
In addition, using stumps avoided killing trees for the research.

 


Right: Waddi fence.

This fence near the Mac Clarke Reserve is made of waddi tree stems and illustrates the use that was made of the tree for fencing and construction of cattle yards.

Fencing is now essentially star pickets and barbed wire.

 

As the main (or only) tree in a harsh environment, Acacia peuce is host to various butterflies and their larvae and provides protective places for birds from grey falcons to finches.

While vertebrates are obvious predators of waddi trees (eg browsing, seed predation), the foliage is often chewed by insects.

Left: Ants tending scale insects which are sucking sap from the base of waddi tree phyllodes.

 


Right: Butterfly (Theclinesthes misk) laying eggs on the plant. This butterfly species uses quite a few desert Acacia species as larval food plants.


The benefits of retaining the current stands of Acacia peuce and its close relatives from the arid zone may have far reaching effects.

Text: V.B. January 2009 All images from Jon Luly

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