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Survey & Mapping |
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Willow Survey and Mapping All willows around Lake Burley Griffin and the Molonglo River to the NSW border were surveyed by boat and on foot. Due to its invasive nature, Black Alder was also surveyed by special request of the NCA. Separate funding was provided by the NCA for this task. Lake tributaries were not surveyed as part of the LBG Willow Management Plan (except for the Molonglo River) although these will be highlighted where information is available since they are an integral part of a willow management program for the Lake.
Each individual willow, or in the case of dense infestations group of willows, was recorded using a GPS and hand held computer. Information recorded was as follows:
This information was transferred to a Geographic Information System (GIS) and recorded as a series of layers. Photos were also taken at either end of each infestation. The results of the surveys are presented visually in the attached maps. Information collected is not only designed to assist with prioritisation but also to help determine the best course of action for on-ground works. This information will also provide a guide to the likely cost of control works based on densities, heights, stems per tree, etc. Management issues are also listed to assist with decision making. GIS layers are available to stakeholders by request through the Molonglo Catchment Group as part of this Management Plan. There were 9 distinct willow species found to occur around and within the Lake Burley Griffin and Molonglo River areas. In addition to these, a few specimens could not be clearly identified due to hybridisation and were recorded as unknown hybrids. Black Alder has been included in findings as they were found to be a dominant species in many areas. Species were as follows:
Except for the Weeping Willow, all of these species are on the list of ‘Weeds of National Significance’, although some are more significant in terms of their environmental impacts than others. Seeding willows - particularly the Black Willow, Purple Osier and the Basket Willow - are of concern as their potential to spread to areas outside the Lake is very high. Seeds can be distributed by wind and water making the threat of them spreading greater than those that depend on vegetative propagation.
Basket Willows use both seed and vegetative propagation to spread. They are present in moderate numbers around the Lake, are relatively dense in the lower Molonglo River but constitute less than 5% of the willow species in the upper part of the Molonglo. Surveys found that they tended to be mostly male but they are of concern because their pollination time is closest to the flowering time of the female Weeping Willow. While there was no evidence of the two crossing in the survey area, this has been observed in other areas around NSW.
Crack Willows appear to be one of the most problematic species around Lake Burley Griffin and the Molonglo River. It is not a seeding willow but spreads very aggressively through vegetative propagation. Crack Willow spreads at a very high rate and doesn’t require the suitable seed beds required by the seeding willows to become established. It was found to be the dominant species in many parts of the Lake and the Molonglo River. It also has the ability to cross with species that can produce fertile propagules such as the Golden Upright Willow. These appear to be more problematic than the seeding willows around the Lake area, even though seeding willows are a higher priority for control under best practice management guidelines. Crack willow is also a very expensive willow to control due to the need to treat each one of its multiple stems.
Black Alder, although not a willow, was found to be very invasive, particularly around the north-western end of the Lake (refer to Map 2) where it was found to be the dominant or second dominant species. In this regard, any willow control program would need to consider the likelihood of infestation of control areas by Black Alder and/or the possibility that the Black Alder may be posing more of a threat to the immediate surrounds than the willow species. As mentioned, Black Alder was surveyed at the request of the NCA. Where relevant to the Plan, it is mentioned but technically is outside the scope of this Plan (given that other exotic plants are not considered). Other willow species tended to be present in lower numbers and/or were found not to pose the same level of threat as some of the more problematic willows. These included the Weeping Willow and Golden Weeping Willow, which are relatively benign in terms of an environmental hazard. They are also aesthetically pleasing and play a significant role in maintaining heritage values of the Lake Burley Griffin precinct. |
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