Acacia pickardii - Mount Gason Wattle

Monday, Jan 02, 2012 at 13:14

Stephen L (Clare) SA

The Birdsville Track reveals many treasures to the observant traveller. One such treasure is a little known wattle, only discovered 40 years ago.

Known by 3 common names, Mount Gason Wattle, Pickard’s Wattle and Birds Nest Wattle, Acacia pickardii is a member of the Acacia victoriae group. It is at best a bushy tree that has been recorded reaching 5 metres tall, while on average the tree is usually seen as a spiny and woody shrub 2 – 3 metres in height. Only recently discovered as a new Botanical species in the Mt Gason area in northern South Australia on the 24th December 1971 by John Pickard, it was not officially named until the 16th November 1978. Trees are found in clumps due to the tendency of this long-lived species to reproduce from root suckers. A. pickardii has rarely been observed flowering, but when it has been observed, the flowers are golden globular flower heads. It is not known to produce viable seeds, with seed set extremely poor, with no seedlings ever observed and only one or two seed pods exist in collections. Studies on the plants have shown that this wattle will not produce any seeds until at least 11 to 20 years of age. The only time that it has been observed flowering has been after rare heavy summer rainfalls.

Acacia pickardii is only found in the far north eastern areas of South Australia and the south western area on the Northern Territory. The two main growing areas are in the Mt Gason/Pandie Pandie area, 150 kilometres south of Birdsville in South Australia, and at Andado Station – O’Neill Point area in the Northern Territory, 400 kilometres northwest of Mt Gason. There are fewer than 10,000 mature individual trees in total.

The easiest location to observe this species is in the vicinity of the northern end of the Birdsville Track near Mt Gason Bore, where it can be observed growing on both sides of the Birdsville Track. Other occurrences in South Australia are further north near Lake Etamunbanie, which is part of the Pandie Pandie Pastoral Lease, and a recently discovered occurrence on Cordillo Downs Station. At present, the wattle is found over an area of at least 104 Kilometres Square, but because of the remoteness, there may be higher numbers due to the lack of collections from that area.

In 1993, 4 monitoring plots were established in the Goyders Lagoon Pastoral lease populations by the South Australian Department for Environment and Heritage. The program includes sites inside and outside established rabbit and cattle proof fences. Since being discovered in 1971, an extensive population was discovered in an area north of Lake Etamunbanie in 1993 and during further studies in 1995, 2 populations were discovered to the south of Mt Gason on Cowarie Station, with one population of 30 trees on the eastern side of the Birdsville Track. Two other populations are known to occur in the Mt Gason area, with one population containing around 250 mature plants.

In the Northern Territory there are 3 known locations; 2 locations are on pastoral leases, Hubbard Hill on Andado Station, and 10 kilometres east of Numery Station, while the other population is on Allitra Tableland which is on Pmere Nyente Aboriginal Land Trust occurring in the Simpson – Strzelecki Dunefield bioregion on the Western edge of the Simpson Desert. The Andado population is by far the largest, containing an estimated population of a few thousands plants, consisting of a densely populated main stand and several smaller outliners within an area of approximately 220 kilometres square, with the largest concentration of trees on the summit of Hubbard Hill. Mapping of the Andado population is a high priority so that impacts associated with disturbance, including mineral exploration can be minimised with 33 individual stands so far mapped for the Andado population. In the Northern territory Acacia pickardii is extremely rare with no populations covering an area no greater than 2 kilometres square. The Biodiversity Unit of the Department of Natural Resources, Environment and Arts, Northern Territory undertakes routine visits to the Hubbard Hill population on Andado station to check on the trees progress, with 4 monitoring plots established in 2008 on Hubbard Hill. During the 2008 surveys, most of the populations were dominated by small plants, between 0.31 – 1.5 metres tall. Importantly though, most of the plants in this class were very old and seemed as likely to be reproductively mature as trees in the next height class. Further studies also showed that the plants had not been browsed and the small height was associated with environmental constraints, with as little as 2% of the Northern Territory’s population of Acacia pickardii taller than 3 metres in height, compared to the taller plant populations in the South Australian studies.

Acacia pickardii typically grows on gibber-covered sandplains, stony rises and low hills, including mesas and tablelands and adjacent flats in crusty alkaline and neutral red duplex type soils with an annual average rainfall of 150mm. The species usually forms a low woodland or low-open woodland with an understory dominated by either chenopod-shrubland or open-grassland. Galls are common on the species and are produced by thrips and are often confused as fruit by inexperienced observers. The galls are formed when thrips feed on the foliage and cause the leaves to curl or roll. The thrips that cause this are Onychothrips zygus and it is believed that they only live on Acacia pickardii.

Acacia pickardii is most closely related to Acacia cuspidifolia (Common name Wait-a-While Wattle), which is endemic to the Paraburdoo area in Western Australia, but is distinguished from this species by its pungent, sharp cylindrical phyllodes (leaves). The Acacia species which it is superficially similar to is Acacia atrox, (Common name Myall Creek Wattle) another rare and recently identified Acacia that is endemic to the Inverell region of New South Wales. Both of these species of Acacias have pungent phyllodes, spread by suckering, produce hakea-fruit-like galls on its foliage and both appear to rarely produce fruit, but Acacia pickardii differs from Acacia atrox most obviously in its erect, cylindrical phyllodes, and in having spiny stipules and free sepals. The phyllodes on Acacia pickardii are sharpest when the leaves are younger and palatable.

As a result of the restricted distribution of the species, all areas where Acacia pickardii occurs are considered to represent habitat critical for its survival. These sites provide suitable climatic and biological conditions for the continued persistence of the species. Of particular importance appears to be the presence of gibber-covered sandplains and stony rises and low hills and adjacent flats.

The main potential threats to this species have been identified as browsing by rabbits, which has greatly been reduced since 1995 with the introduction of the Rabbit Calicivirus Disease (RCD), bulldozing during road construction/maintenance and disturbance by cattle, and to a lesser extent feral animals from the Simpson Desert, mainly camels and donkeys.
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