The black wattle is a boon for Australians (and a pest everywhere else)



File 20181005 72117 cgtzcu.png?ixlib=rb 1.1
Black wattle is part of the huge Acacia family.
Wikipedia/The Conversation

Gregory Moore, University of Melbourne

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The genus Acacia is Australia’s largest, containing nearly 1,000 different species. It includes our national floral emblem, the golden wattle, and is the source of the green and gold colours of many of our sporting teams. The variety of acacias is mind-boggling.

There are many well-known small, short-lived species that thrive both in their natural habitats and in suburban gardens, where they are known for attracting insects and birds. There are species that survive in the arid inland as inconspicuous, stunted shrubs that are more than 200 years old, and there are also tall forest trees such as the blackwood, Acacia melanoxylon that can live for centuries.




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The black wattle, Acacia mearnsii, falls somewhere between these extremes. It ranges from 6 metres up to (occasionally) 15 metres in height. It is generally called “short-lived”, but often makes it past 20 years old, and may persist for 30 years or more under the right conditions.


The Conversation, CC BY

It has attractive bi-pinnate (feathery) leaves, dark green foliage, and smooth, dark bark – hence its common name. It has the typical pale yellow to golden wattle flower. Its pea-like fruits are typically 10mm wide and up to 150mm long, which as they dry out can rattle on the tree. These fruits contain many tough, black seeds that can readily germinate if damaged, which breaks the hard seed coat. This explains the high weed potential, but makes them easy to propagate.

Different indigenous groups used wattles for various purposes. Seeds were often consumed as food. The bark of many species, including black wattle, was used for coarse rope and string, and the tannins and gums in the bark of black wattle were used as adhesives. Indeed, they are still used in the manufacture of some modern veneered and laminated timbers. An infusion of the bark in water has also been used for medicinal purposes.

Despite the name, black wattle have golden flowers – they’re named after their dark bark.
Vinayaraj/Wikipedia, CC BY

Unusually, black wattle is probably better known and used outside Australia. Its fame and infamy stem from the fact that it has been grown for more than a century as far afield as South Africa, Portugal and Germany.

It was grown in plantations here and overseas, as its bark and wood contain high levels of chemicals called tannins, used in tanning leather. Indeed, many of the famous horse-riding paths, such as the Melbourne tan track around the botanic gardens, were once surfaced with black wattle waste from tanneries. The infamy arises because in many places, including parts of Australia, it is regarded as a highly invasive weed.




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The tree was also grown locally and in places like India as a source of firewood and timber for light construction. It is easily killed by fire, but can also sucker prolifically, which can give rise to dense thickets that virtually eliminate other species. The bark and crevices are home to many insects, fungi and bacteria. Some arborists who have worked with the species dislike its brittle wood, as broken twigs and branches can easily cut workers with a risk of subsequent infection. So be wary when working with it!

Black wattle have feathery, bifurcated leaves.
KENPEI/Wikipedia, CC BY

Given its occurrence over a large part of southeastern Australia, black wattle occurs naturally in a diverse range of habitats, from open eucalypt forests to drier woodlands and grasslands in New South Wales, Victoria, South Australia and Tasmania. It grows well in low rainfall, and in the heavy clays of the great basalt plain that extends from the outskirts of Melbourne to beyond the South Australian border.




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This is a great fillip to the garden, as it requires little irrigation and has the added advantage of telling you when it needs water. When the plant is dry its leaves, which are usually open and in full display, noticeably close up. This is the time to give it a good drink.

In tougher environments where it is windy, frosty or very sunny, black wattle can be planted as a quick-growing tree among which other slower-growing and more sensitive species can be planted. The black wattle provides protection for these other tree species, and as it ages and starts to collapse (often at around 15 years) the other trees emerge and take over.

As for all acacias, black wattle is a nitrogen-fixing plant. This means its roots have bacteria that allow it to take nitrogen from the atmosphere and incorporate it into the plant’s structure, which also benefits the surrounding soil. This can be a real advantage to those establishing a garden in poorer soils or heavy clays.




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The black wattle can also provide excellent natural mulch both in large-scale revegetation projects and domestic gardens. The mulch forms from the leaves and bark as they are shed, but also from the twigs and branches as the plant dies.

In the right parts of Australia, where it grows naturally, the black wattle is a valuable plant for revegetation work and an asset in the garden.

Sign up to Beating Around the Bush, a series that profiles native plants: part gardening column, part dispatches from country, entirely Australian.The Conversation

Gregory Moore, Doctor of Botany, University of Melbourne

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Explosive lies: how volcanoes can lie about their age, and what it means for us



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Lake Taupo, in the North Island of New Zealand, is a globally significant caldera of a supervolcano that formed following a massive eruption more than 20,000 years ago.
from http://www.shutterstock.com, CC BY-ND

Richard N Holdaway; Ben Kennedy, and Brendan Duffy, University of Melbourne

Just like a teenager wanting to be older, volcanoes can lie about their age, or at least about their activities. For kids, it might be little white lies, but volcanoes can tell big lies with big consequences.

Our research, published today in Nature Communications, uncovers one such volcanic lie.

Accurate dating of prehistoric eruptions is important as it allows scientists to correlate them with other records, such as large earthquakes, Antarctic ice cores, historical events like Mediterranean civilisation milestones, and climatic events like the Little Ice Age. This gives us a better understanding of the links between volcanism and the natural and cultural environment.

Taupo’s last violent eruption

Lake Taupo, in the North Island of New Zealand, is a globally significant caldera supervolcano. The caldera formed after the collapse of a magma chamber roof following a massive eruption more than 20,000 years ago.

Now it seems that the Taupo eruption that occurred in the early part of the first millennium has been lying about its age. But like many lies, it was eventually found out, and it reveals exciting processes we hadn’t understood before.

The eruption of Taupo in the first millennium has been dated many times with radiocarbon, yielding a surprisingly large spread of ages between 36CE and 538CE.




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Radiocarbon dating of eruptions

Radiocarbon dating of organic material is based on the concentrations of radioactive carbon-14 in a sample remaining after the organisms’ death. Over the past two decades, the method has been refined greatly by combining it with dendrochronology, the study of the environmental effects on the width of tree rings through time.

Radiocarbon dating of tree ring records has allowed scientists to construct a reliable record of the concentration of carbon-14 in the atmosphere through time.

In principle, this composite record allows eruptions to be dated by matching the wiggly trace of carbon-14 in a tree killed by an eruption to the wiggly trace of atmospheric carbon-14 from the reference curve (“wiggle-match” dating).

Scientists presently use wiggle-match dating as the method of choice for eruption dating, but the technique is not valid if carbon dioxide gas from the volcano is affecting a tree’s version of the wiggle.




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The effect of volcanic carbon on eruption ages

Our study re-analysed the large series of radiocarbon dates for the Taupo eruption and found that the oldest dates were closest to the volcano vent. The dates were progressively younger the farther away they were.

This graph shows all of the ages obtained for the Taupo first millenium eruption, sorted by age, plotted on a digital model of the North Island of New Zealand. Lake Taupo is the caldera from which the eruption occurred. The oldest ages for the eruption are clustered around Lake Taupo, and older ages are located further from the volcano. We interpret this pattern as being caused by contamination of red areas with volcanic carbon dioxide.
Provided by authors, CC BY-ND

This unusual geographic pattern has been documented very close (i.e. less than a kilometre) to volcanic vents before, but never on the scale of tens of kilometres. Two wiggle match ages, taken from the same forest, located about 30km from the caldera lake, were among the oldest dates from the series of dates.

This enlarged influence of the volcano can be explained by the influence of groundwater beneath the lake and its surroundings. The Taupo wiggle-match tree grew in a dense forest in a swampy valley where volcanic carbon dioxide was seeping out of the ground and was incorporated in the trees.

This conceptual image shows how gas from the triggering event, decades before the eruption, works its way into the groundwater system and is eventually incorporated in the wood of the trees that we date.
Provided by authors, CC BY-ND

The ratio of carbon-13 to carbon-12 (the two stable isotopes of carbon) in the modern water of Lake Taupo and the Waikato River tells us that volcanic carbon dioxide is getting into the groundwater from an underlying magma body.

Can large eruptions be forecast over decades?

Our study shows that a large and increasing volume of carbon dioxide gas containing these stable isotopes was emitted from deep below the prehistoric Taupo volcano. It was then redistributed by the region’s huge groundwater system, ultimately becoming incorporated into the wood of the dated trees.

The increase was sufficiently large over several decades to dramatically alter the ratios of different carbon isotopes in the tree wood. The forest was subsequently killed by the last part of the Taupo eruption series. But the dilution of atmospheric carbon-14 by volcanic carbon made the radiocarbon dates for tree material from the Taupo eruption appear somewhere between 40 and 300 years too old.

The precursory change in carbon ratios gives us a way to gain insight into the forecasting of future eruptions, a central goal in volcanology. We found that the radiocarbon dates and isotope data that underpin the presently accepted “wiggle match” age reached a plateau (that is, stopped evolving normally). This meant that for several decades before the eruption, the outer growth rings of trees had ‘weird’ carbon ratios, forecasting the impending eruption.

We re-analysed data from other major eruptions, including at Rabaul in Papua New Guinea and Baitoushan on the North Korean border with China and found similar patterns. The anomalous chemistry mimics but exceeds the Suess effect, which reversed the carbon isotopic evolution of post-industrial wood. This implies that measurements of carbon isotopes in 200-300 annual rings can track changes in the carbon source used by trees growing near a volcano, providing a potential method of forecasting future large eruptions.

We anticipate that this will provide a significant focus for future research at supervolcanoes around the globe.The Conversation

Richard N Holdaway, Adjunct Professor; Ben Kennedy, , and Brendan Duffy, Lecturer in Applied Geoscience, University of Melbourne

This article is republished from The Conversation under a Creative Commons license. Read the original article.

Moving Break


I will be taking a break from Blogging for the next 2 to 3 weeks. It has become necessary for me to move home and this will be taking place over this period – so it’s packing, cleaning, transporting, etc, for the next few weeks. I may be able to get back to Blogging before 3 weeks, we’ll see how the move all goes. There is a lot to do though.