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Purplewood Acacia monitoring

Graeme Finlayson (Ecologist)
Published 16 Apr 2020 by Graeme Finlayson (Ecologist)

Purplewood Wattle (Acacia carneorum) is listed as ‘Endangered’ under South Australia’s National Parks and Wildlife Act 1972 and nationally under the Environment Protection and Biodiversity Conservation Act 1999 due to its restricted natural distribution, very slow growth and the rarity of production of viable seed.

On Boolcoomatta Station Reserve, Purplewood Wattles, are a feature of the ‘Sandy Sheets’, one of the five Targets that are the focus of the management plan.

They're slow growing and get their name from the striking purple-coloured heartwood.

They're likely to provide key refugia for species such as Fairy Wrens and Chirruping Wedgebills and are a host for other important plants such as mistletoe.

The sandy sheets are mainly on the eastern half of the reserve throughout the sparse chenopod plains, which have been subject to two years of below average rainfall and grazing pressure of overabundant kangaroos. Over the past six months, Boolcoomatta resident volunteer and a familiar face to many in the Bush Heritage family, Andrea Tschirner, has been collecting information about the status of this species across the reserve.

The ‘Purplewood Project’ was established to map and document the populations of Purplewood throughout the property and assess the health and status of the species in relation to threats and help with planning for the management of these species. In addition, it's hoped that genetic analysis that we aim to conduct in 2020, will provide further insight into the population viability of the species and determine whether further intervention may be required to protect the species into the future.

Previous research has identified that this species is almost entirely dependent on asexual reproduction through suckering, and the genetic diversity of most populations is low. From previous studies, there are potentially as few as 219 known stands in the arid zone, of which around only 30% are protected on conservation reserves, such as Boolcoomatta.

There’s a very real chance that with low genetic diversity and continued threats from pastoralism and rabbit grazing the species may be lost.

The key findings of this work so far include the mapping and assessment of up to 30 distinct stands of Purplewood containing over 5,000 standing individual stems. Patches were variable in size and health, with patches ranging from one to more than 1,000 individuals. Die-off is prevalent at a large proportion of sites, generally on the outer edges of groves, most likely attributed to drought but also strong evidence of impact from rabbits, that are likely impacting survival and persistence of juvenile stems.

Flowering was evident at 35% of sites, however no evidence of seed set, which seems to align with previous surveys conducted for this species. Despite a high abundance of kangaroos for most of the past 5 years, it would appear they're not impacting recruitment of Purplewood suckers to the same extent as rabbits.

The surveys also found that Purplewood stands in the sandy sheets, may well have provided sites of importance for Traditional Owners, with numerous sites containing a range of culturally significant artefacts including grinding stones, knapping flakes and stone ovens. These areas may have provided refuge sites, or temporary shade to escape the extreme conditions.

On Boolcoomatta, Purplewood patches have the ability to survive prolonged drought conditions as long as rabbit grazing is reduced to allow regeneration of suckers (and possibly seedlings) in higher rainfall years.

Rabbits are an identified threat to conservation targets on Boolcoomatta and both extensive warren ripping and the spread of Rabbit Haemorrhagic Disease Virus (RHDV) had a significant impact on rabbits in the early years of ownership. Rabbit control work through warren fumigation has commenced and will focus on key sites identified through this project to protect an endangered species of regional significance, and also protect areas of cultural importance on Boolcoomatta Station Reserve.

Flowering mistletoe in a Purplewood Wattle on Boolcoomatta. Flowering mistletoe in a Purplewood Wattle on Boolcoomatta.
A healthy specimen of Purplewood Wattle  (Acacia carneorum). A healthy specimen of Purplewood Wattle (Acacia carneorum).
Dead stand and surviving juvenile suckers. Dead stand and surviving juvenile suckers.
Flowering Purplewood on Boolcoomatta. Flowering Purplewood on Boolcoomatta.
A new plant. A new plant.
Juvenile sucker Purplewood. Juvenile sucker Purplewood.
Geri and Maia Tschirner, ready for rabbit control work. Geri and Maia Tschirner, ready for rabbit control work.
Filling in rabbit warrens. Filling in rabbit warrens.
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