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Cyclonic conditions give tropical weed a free ride
News release | 06 May, 2008
Cyclones that whip up strong seas and terrifying winds can spread weed seeds more than 40 times further afield than in regular weather conditions, a groundbreaking study commissioned by the Department of Primary Industries and Fisheries has found.
The study is the first time the potential sea dispersal of a significant weed threat has been modelled in such detail, and it gives a rare insight into the ability of pest plants to gain a foothold along stretches of coastline.
The results of the study will be presented by DPI&F Biosecurity Queensland researcher Melissa Setter at the upcoming Australian Weeds Conference to be held in Cairns from May 18 to 22.
"The weed studied in the research is pond apple; an invasive, semi-deciduous tree first imported into the country as root stock for custard apple," Ms Setter said.
"The plant can form forests with trees up to 15 metres tall that have indistinct pale yellow-cream flowers and fruit - containing up to 200 seeds - similar to the smooth-skinned custard apple.
"With its water-dispersed floating fruits and seeds, it has the potential to overrun whole areas of sensitive mangrove and tidal forest areas, and is already established in areas of the tropical east coast of Queensland," Ms Setter said.
The findings of the study will assist in managing and controlling the spread of the weed, which is a Weed of National Significance. The study was funded by the Australian Government.
The Marine Modelling Unit of the Australian Maritime College was enlisted to model ocean dispersal of pond apple.
Using sophisticated computer modelling techniques based on historical wind, tide and ocean current data from 1997 to 2002, the "virtual" seeds were tracked over six seasons after release near the mouths of 16 major Queensland rivers.
The rivers are located both north and south of current known infestations, and stretched from the Gulf of Carpentaria and western Cape York to the Pioneer River near Mackay.
"Results have shown that virtual seeds could be dispersed in either direction, but generally moved southwards during calm and monsoonal conditions and northward during south easterly trade winds.
"Distances travelled varied greatly depending upon prevailing conditions. For example, virtual seeds released from the Johnstone River were mostly beached within 31 kilometres of the river mouth.
"In extreme events such as Tropical Cyclone Justin however, north east coast rivers could potentially transport seed over 1,300 kilometres to the Torres Strait, Papua New Guinea and beyond," Ms Setter said.
Most of the virtual seed, which beached did so within three months. Separate studies by DPI&F show that the seed can survive more than one year in salt water, so many of the beached seeds will be viable.
"Of course, not all seed which reaches land will be in a suitable site for germination and growth, but those which are able to enter the coastal vegetation strip via tidal movement are more likely to survive and spark new infestations."
Media contact: Andrea Corby, +61 7 4722 2648
andrea.corby@dpi.qld.gov.au
