NSW Bitou Bush Threat Abatement Plan - Department of ...

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Threat Abatement Plan - Invasion of native plant communities by Chrysanthemoides monilifera
8.1.1 Measuring the effect of control programs on bitou bush infestations
The effectiveness of bitou bush control programs can be measured directly from the response of
bitou bush. For example, through plant survival and recruitment following control. While the
exact techniques for collecting such information have not been finalised with regards to the
monitoring of this plan, each measure is discussed with examples below.
Measuring survival
Most control methods do not result in 100% mortality, so it is important to monitor the number of
plants that survive control treatments, both as an indicator of the success of a control program,
and as an indicator for determining the interval for follow-up control.
Plants that survive control treatments can produce seed in a shorter timeframe than plants that
germinate following the same control treatment. Hence, plants that survive initial treatments will
require earlier follow-up treatment if the input of fresh seed is to be limited. In order to control
weed seed banks, a measure of bitou bush plants that survive control treatments (e.g. missed
totally, treated but re-sprouting, or unaffected) needs to be determined for each method used. This
can be measured using quadrat counts or photopoints.
Measuring recruitment
Recruitment from the seed bank will have important implications for the long-term success of any
bitou bush control program. Firstly, the time taken to exhaust bitou bush seed banks is unknown,
but is thought to be 5–10 years (Stanley et al. 1989; NPWS 2001a). An understanding of seed
population dynamics will allow more effective control programs to be developed, in part from
better predictions of the follow-up control period. Secondly, some control measures will reduce
bitou bush seed bank densities more than others. For example, fire stimulates seeds to germinate
as well as killing others (as examined for a range of weed species, see Downey 1999). Thus, a
combination of fire and herbicide is likely to reduce soil seed banks to a lower level than either
method would if used alone.
The probability of a seed becoming a reproductive adult is unknown. Therefore, the optimal
period in terms of maximum impact on bitou bush between the initial control and any follow-up
treatment is also unknown. Data from the environmental weed Scotch broom (Cytisus scoparius)
showed that less than 2% of seedlings became reproductive adults (Downey and Smith 2000).
This information provides insights into when follow-up control programs should be undertaken. If
seedling mortality in bitou bush is similarly high, follow-up treatments should target older
seedlings because they have a greater probability of becoming reproductive. In addition,
premature follow-up control treatments may kill seedlings that would die anyway, as well as miss
many seeds that have not yet emerged. Lastly, the location of bitou bush seedlings may suggest
invasion patterns (i.e. under roosting trees). This information can be used to target specific areas
in follow-up control programs or subsequent stages of control, to prevent re-invasion.
The recruitment levels of the other species present also have important implications for the longterm
success of any bitou bush control program. The number of seedlings of priority taxa can
indicate their ability to recover/regenerate following bitou bush control programs. A lack of
seedlings may require additional recovery techniques like revegetation, but such techniques are
outside the scope of this TAP. The number of other weed seedlings will indicate the need for