Fighting the willow menace
In the aftermath of the 2003 fires in northeast Victoria, a new threat emerged on the Bogong High Plains.
The fires burnt most of the plains, laying bare wet sediments previously thickly covered by bog and wet heath plant communities. In these newly-exposed substrates, there was an opportunistic mass germination of willow seedlings.
The seeds are thought to have blown up from surrounding areas which house dense mature populations of the willow, and from isolated mature individuals on the High Plains themselves.
By the following year, hundreds of thousands of willow seedlings were seen throughout the burnt bogs on the High Plains, and alarm bells started ringing.
Willows are considered a grave threat to alpine bogs and associated fen communities on the Bogong High Plains. Indeed, willow invasions Australia-wide cause increased water use, displace native vegetation and disrupt aquatic nutrient regimes.
The particular willow species here is Grey Sallow Willow (Salix cinerea), a shrub willow introduced from Europe for soil stabilisation. One of the few willows that produce seed in Australia, it's the only one known to have invaded Australia's relatively weed-free alpine and subalpine regions.
Although restricted to moist environments, it is not limited to strictly riparian areas.
S. cinerea is a pioneer species with seeds that have a light pappus, facilitating long-distance dispersal and aiding rapid colonisation of disparate areas (although thankfully it is not long lived and so does not form a seed bank).
A ready coloniser of disturbed environments, the species is capable of establishing in semi-natural and natural environments, and is also able to regenerate after fire. These were just the conditions created by the alpine fires in 2003.
Control programs to eradicate the willows have been carried out since 2004 by Parks Victoria and other government bodies, including the North East Catchment Management Authority, the Victorian Government and Falls Creek Alpine Resort. More than $900,000 has been spent to date, representing 1633 people hours of control effort.
Contract teams, and volunteers like the Friends of Bogong, the Victorian National Parks Association, bushwalking groups, 4WD clubs, Landcare groups, and school and university students have been involved, often hiking into the more inaccessible areas.
The usual control method is to cut and paint with herbicide each willow sapling if small, or every branch of larger willows - the aim being to kill the plant. Larger willows can be more than two metres high.
The accompanying map, prepared by Parks Victoria, shows the results of their work. The seemingly impressive reduction in density of willows on the Bogong High Plains, however, does not leave room for complacency, as substantial numbers remain.
And while the germination of new willow seedlings has declined as native vegetation in the bogs has regrown, with fewer gaps for willows to germinate, many established willows remain, constituting an ecological time-bomb - they would be a seed source if another fire event occurred.
Dr Joslin Moore has been leading research aimed at maximising the effectiveness of willow control, working closely with Parks Victoria staff to identify how to manage the current willow infestation efficiently, as well as developing long-term management strategies to minimise the threat of further willow invasions in the future.
For the shorter term, research collaborator Kate Giljohann has developed a model of S. cinerea distribution, historical control effort and estimates of control effectiveness to map out priority locations for control. The mapping can be updated each year.
Top areas for control in 2010-11 included Falls Creek Alpine Resort, Red Robin Battery, the north bank of Rocky Valley storage dam and the areas surrounding Langford's Gap, Mt Cope and Cope East Aqueduct.
The research team has also helped Parks Victoria to develop a longer term management strategy. In particular, it was not clear how much effort to put into controlling willows on the Bogong High Plain, and how much to allocate to treating potential seed source populations in nearby creeks and valleys, given the uncertainty about fire frequency, post-fire bog recovery rates and seed dispersal distances.
A model has been developed to predict the long-term performance of management on willows on the Bogong High Plains and to consider a range of possible fire frequencies, bog recovery rates and willow seed dispersal distances. For current budget levels, the model shows that the best strategy for protecting the endangered bogs is to apply all resources in the bog areas on the Bogong High Plains.
As part of this strategy, surveys have been established to enable Parks Victoria to monitor the state of the willow invasion across the Plains, evaluate the effectiveness of management and identify areas for future treatment. Joslin is working with Parks Victoria to analyse the monitoring data and estimate dispersal potential of willows across the high plains and surrounding areas.
Victorian National Parks Association members (aka the WillowBusters) have been helping Parks Victoria fight willows since 2006. Much of this work has been done during Alps Rehabilitation weekends based at Falls Creek. Volunteers have removed small seedlings and, as the willows have grown bigger, cut and painted them with herbicide.
In 2010 and 2011, VNPA members have also been helping with willow monitoring surveys, which are crucial for ongoing willow control as they enable Parks Victoria staff to monitor the status of the bogs and identify key areas for ongoing works. A total of 63 bogs have now been surveyed; 75% contained willows, and 50% had at least one large willow that could be a source of seed in the future.
There is still plenty to do, but progress is being made - and VNPA members can continue to contribute much-appreciated help!
- This story was compiled with the help of Joslin Moore (Senior Ecologist, Royal Botanic Gardens, Melbourne) and Elaine Thomas (Ranger, Alpine National Park, Mt. Beauty) and VNPA member Kaye Oddie.
If you would like to help fight willow infestations phone the Victorian National Parks Association on 03 9347 5188 and find out how you can become a WillowBuster!