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Management Priorities |
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Willow Management Priorities The prioritisation of willow control is based primarily on the threat to the wider environment, and the cost effectiveness of control. The decision and method of controlling any particular willow will depend on site specifics and should adhere to the guiding principles outlined in the Plan. The following six priorities apply to the entire survey area, and are consistent with the principles outlined in the Upper Murrumbidgee Willow Management Strategy. The following recommendations should be used in conjunction with the associated maps. An interactive series of GIS layers is available as part of the Plan. Priority 1: High priority species for control The highest priority for control is the Black Willow, Purple Osier and Tortured Willow.
Other willow seedlings including the Basket Willow and Golden Upright Willow need to be controlled as a very high priority also, but occur in larger numbers and thus are of lower priority than the three species mentioned above. All these species should be controlled using a range of control methods appropriate to the specific site. These species are currently low in numbers and tend to be smaller and more manageable than other larger infestations. This reduces the risk of unwanted impacts on banks, aesthetics and recreation.
The seeding willow infestations tend to be increasing in density and size. In this regard, it is important that they are eliminated as soon as possible in order to minimise future control costs. Seeding willows are somewhat limited in that there appears to be only a small number of sites that are suitable to support major seeding events. This will assist in their control and follow up management.
An innovative approach to minimizing the impact of seeding willows on the environment would be to time water releases from Scrivener Dam so they don’t coincide with seeding events (which usually occur around spring). Willow seed is only viable for a short length of time and trapping seed within the Lake would go a long way to reducing downstream impacts in the Lower Molonglo and Murrumbidgee Rivers.
Map 1 shows the location of isolated seeding willow species in yellow, purple and green. Priority 2: Control unidentified hybrids Unidentified hybrids should also be controlled as a high priority since they are few in number and pose a high risk of producing viable seed.
Map 1 shows the location of unidentified hybrids in red. Priority 3: Strategic control of the Golden Upright Willow Strategic control of the Golden Upright Willow is important as these are seeding willows and they are in large numbers around the Lake. They also tend to be in high profile areas such as around the Carillon. While these willows should be removed wherever feasible this could create problems for recreation, visual amenity and other Lake uses. Therefore control should focus on removal of females and retention of males only as the first stage of a control program. In the longer term, for example ten years or more, it might be possible to remove all of these willows.
Map 1 shows location of Golden Upright Willows in orange. Map 9 shows the most common sex of the dominant species. Used in conjunction with Maps 2 and 5, the location of female Golden Upright Willows can be pinpointed as targets for control. Priority 4: Strategic control of Basket Willow Basket willow wasn’t present in very large numbers except for the lower reaches of the Molonglo River; however, it was found in higher numbers than the other seeding willows. It was found in isolated patches around the Lake and the Upper Molonglo but not as the dominant species (which tended to be Crack Willow). Basket willow presents a particular problem as it spreads both by seed and vegetatively. In addition to this, pollination events are timed more closely with the Weeping Willow than with other willow species, and the majority of Basket Willows were found to be male. This poses a particular threat as the Weeping Willow are all female. Basket willow should be removed where possible although this may be difficult along the Molonglo River where it is growing among dense infestations of Crack Willow. This could pose operational difficulties, and may be an expensive proposition. When removing Basket Willow in sites heavily colonised with Crack Willow there is a high likelihood of Crack Willow reinvading the site. Specimens of Basket Willow would need to be identified and marked during their flowering periods as the timing of their flowering differs from the flowering period of the Crack Willow. This would need to occur prior to a control program.
Maps 5, 6 and 7 show the location of Basket Willow as the second dominant species (i.e. less than half of the total distribution of other willows on site). It was not found to be the dominant species in any of the survey area. On Map 7, whilst identified as being the second dominant species, Basket Willow made up less than 5% of the total willow population. In other areas they made up just under 50% of the willows present. Priority 5: Strategic control of Crack Willow While Crack Willow were identified as one of the major problematic willow species, the infestation tends to be extremely dense and will be very costly to control. Any attempt made to control Crack Willow would need to have defined goals. This is due to the high source of willows coming from upstream. A strategic program to control Crack Willows in the shorter term should relate to specific goals as follows:
Another more innovative approach to minimize the spread of Crack Willows is to reduce their contact with water by trimming in areas where they are creating a problem or potential problem. This is undertaken routinely along the water-ski area for public safety reasons and reduces the incidence of willow limbs resprouting in the water. Maps 2, 3 and 4 show the distribution of Crack Willows as the dominant species (i.e. they make up more than half the willows on site). Weeping Willows and Golden Weeping Willows pose very little threat to the natural environment and should be left for visual amenity, shade and heritage value. If certain trees are posing a problem in any given location, they should be assessed on an individual basis, as would be the case with any vegetation type around the Lake. Only female Weeping Willows were introduced into Australia and have different flowering times to other willows preventing their spread by seed. However, it has been noted in some catchments that flowering times of Weeping Willows and problem willow species have aligned to produce Weeping Willow hybrids. The potential for this around the Lake should be recognized and monitored. Surveys revealed no evidence of this as yet. Weeping willows also provide habitat for Darter and Cormorant breeding events in certain areas (discussed further in the section addressing bird breeding events) and roosting sites for the Nankeen Night Heron along with other bird species.
Maps 1 and 8 show the distribution of Weeping Willows.
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