Key points
- Athel pine, planted for shade, shelter and erosion control throughout arid and semi-arid especially around riverine habitats. Australia, can escape cultivation and naturalise.
- It causes significant environmental and economic damage by using up valuable water resources, hindering mustering, and altering vegetation and river structure.
- New infestations should be prevented because control is difficult and costly.
- Mechanical and chemical methods are the main control options. Care must be taken using either method around waterways.
The problem
Athel pine has infested hundreds of kilometres of the Finke River in central Australia. Photo: Colin G. Wilson
Athel pine is a Weed of National Significance. It is regarded as one of the worst weeds in Australia because of its invasiveness, potential for spread, and economic and environmental impacts.
Athel pine affects the pastoral industry by forming dense stands along inland rivers. It consumes water more quickly than native plants, thereby reducing the number and quality of watering holes.
It concentrates salt, which is excreted by its leaves. This makes the ground beneath athel pines more salty and excludes native pasture grasses and other salt-sensitive plants. It can change river flow patterns and cause overland flooding and bank erosion.
It is harder and more expensive to muster cattle in athel pine infestations. Because they are drought tolerant and fire resistant, athel pines decrease the frequency of fires and alter vegetation structure. Infestations reduce the cultural and aesthetic value of affected land and may impact on tourism in the region.
There are several other Tamarix species, all commonly known as tamarisks, that are weeds in Australia.
The weed Athel pine is a spreading tree to 15 m with pendulous, jointed branches. Immature trees have light grey trunks and stems. Mature trees have a thick, rough, dark grey to black bark, and grey-brown stems, and can be up to 1 m in diameter. The minute, dull green leaves superficially resemble pine tree ‘needles’. However, athel pine is misleadingly named as it is a flowering plant, not closely related to true pine trees (conifers).
Its small flowers are pinkish-white without stalks, growing on 30–40 mm long spikes from the ends of the previous year’s branches. The fruit is bell shaped with a hairy tuft, and contains numerous small cylindrical seeds. The seeds have a tuft of fine hairs which assists wind dispersal. The trees have strong woody roots which penetrate and spread deeply throughout the soil.
How it spreads
Athel pine can reproduce by dropping seeds or, more commonly, by revegetation of plant parts. Although athel pine seeds die quickly if not kept moist, they are easily dispersed by both wind and water and may also be spread by animals. A single tree can produce thousands of seeds every year.
Athel pine is classified as a ‘sleeper’ weed because it was present in Australia for some time before it became weedy. A native of northern Africa and Asia, it was first introduced into Whyalla, South Australia, in 1930 via California. Since then it has been extensively planted as shade and wind breaks and for erosion control around rural South Australia, New South Wales, Queensland, Western Australia, and the Barkly Tablelands and Alice Springs regions of the Northern Territory.
The worst infestations of athel pine occur along 600 km of the Finke River in Central Australia near Alice Springs. The explosion in its abundance and range is thought to have been caused by large floods in the 1970s and 1980s, which washed seeds and vegetation downstream and provided the moist conditions required for germination. Its habit of making nearby soil saltier may be assisting its expansion because it thrives in saline conditions.
Other athel pine outbreaks have occurred throughout inland Australia since the 1990s at Starvation Lake and Tilcha Flow (SA), Burnett and Darling Downs regions (Qld) and Menindie Lakes (NSW). Infestations on the Gascoyne and Avon Rivers (WA) have recently been shown to include both athel pine (Tamarix aphylla) and another weedy tamarisk species Tamarix parviflora.
Where it grows
Athel pine is drought resistant and is well suited to arid and semi-arid rangelands. It is tolerant of saline and alkaline soils and, although it flourishes best in and around rivers, is not restricted to the riverine environment. It has escaped cultivation and become naturalised in all mainland states and territories except Victoria.
Potential distribution
Based on climate, athel pine could potentially infest inland watercourses throughout Australia, including parts of northwestern Victoria. A few infestations exist outside of the projected distribution, perhaps surviving on below-ground water resources.
What to do about it:
Prevention of spread–plant other species instead
Preventing the further spread of athel pine in Australia is critical to the successful management of this problem. As part of the prevention of spread measures, the planting of athel pine for windbreaks, shade or erosion control is now actively discouraged. Weedy Tamarix species should not be imported or further planted, and alternative species should be used. Generally, a native Casuarina or Allocasuarina species will make a good alternative, especially for windbreaks. However, local councils or state/territory government agencies will be able to provide appropriate advice.
Control athel pine near rivers
Athel pine needs to be carefully controlled to ensure that it does not escape cultivation. Its potential to threaten environmental integrity and human interests, especially in the extensive arid and semi-arid parts of Australia, warrants an aggressive management approach.
Athel pine in the upper catchments of rivers are the highest priority for control.
Experience clearly indicates that athel pine spreads fastest along waterways, especially when summer flooding aids the downstream dispersal of vegetative material and germination of seeds. Therefore, mature athel pines in the uppermost parts of catchments are the highest priority for eradication. Control can then focus on downstream infestations. The lowest priority for control are mature trees away from water.
Early control efforts
Athel pine was not formally recognised as a weed in Australia until the late 1980s when control attempts first examined its susceptibility to different herbicides and different application techniques. In the mid 1990s mechanical control was attempted on the Finke River, and since then integrated control methods using both mechanical and chemical means have been used to combat the spread of athel pine.
Remove seedlings by hand and mature trees mechanically
Seedlings can be easily removed by hand in sandy ground, and large trees can be removed by ripping and bulldozing, taking care to remove as much of the root system as possible. A large bulldozer is required if the trees are fully grown. If possible the area should be deep ripped to bring any root material to the surface and, where appropriate, a suitable pasture should be sown to outcompete any regrowth of athel pine. Otherwise, care must be taken to reduce the amount of soil covering felled stems and exposed roots as they may re-shoot. Follow-up treatments will be required as some re-shooting is likely. Permits may be required to conduct mechanical control if native species will be affected. Weed control contacts (see table p. 4) will be able to provide relevant advice.
Herbicides may be better suited where erosion is a problem
Herbicides may be used as part of the follow-up to initial mechanical control, and are preferred in sensitive environments (eg riverbanks) where mechanical control may damage non-target species and cause erosion and habitat loss. Herbicide control generally entails treating each stem separately.
An appropriate registered herbicide can be applied in several different ways. Frilling, where small notches are cut into the bark until the white sapwood is reached and herbicide is injected immediately into the notches, has been used successfully in the Carnarvon area.There should be about 50 mm between notches, and drenching guns or veterinary syringes can be used to deliver herbicide into each notch. An alternative approach with larger stems is the cut-stump technique, where the main stem is cut off by chainsaw and the stump is immediately painted with herbicide. Care must be taken to reach as close to the roots as possible.
Smaller trees that have not developed rough bark can be treated by the basal bark technique, which involves soaking the circumference of the stem, to a height of 250 mm above soil level, with herbicide to the point of run-off. Very small stems can be snapped or cut, and herbicide applied to the stem. Foliar spray over the entire plant is effective on small trees (less than 2 m). However, the impacts on non-target species (both natives and crops) prevent this method being used in the Carnarvon area.
Other weedy tamarisk species
The family that athel pine belongs to – the tamarisks – includes other closely related species that are major pests in the United States, such as Tamarix parviflora and Tamarix ramosissima. These species have shown weedy tendencies in both New South Wales and Western Australia, and also deserve attention because their potential impacts are similar to Tamarix aphylla (athel pine).
Biological control
Experience in the United States tamarisks may also help to provide solutions for controlling athel Australia. For example, the US Department of Agriculture has introduced biological control agents against tamarisk species. Although there currently no biological control being investigated for use in Australia, this option remains a desirable of any integrated weed management control program.
Acknowledgments
Information and guide revision: John Gavin (NT DIPE), Richard Carter (NSW Dept of Agriculture/Weeds CRC), Philip Maher (Qld DNRM), Damian Collopy, John Peirce and John Stretch (WA Dept of Agriculture), Les Tanner (North West Weeds County Council) and John Thorp (National Weeds Management Facilitator). Maps: Australian Weeds Committee.
