Individual study: A demographic study of an invasive shrub, broom Cytisus scoparius, to assist in management control techniques, Barrington Tops, New South Wales, Australia
Downey P.O & Smith J.M.B. (2000) Demography of the invasive shrub Scotch broom (Cytisus scoparius). Austral Ecology, 25, 477-485
Broom Cytisus scoparius (a shrub native to Europe) has invaded large areas of scrub and woodland in parts of Australia. At Barrington Tops, New South Wales, it forms dense stands within eucalyptus woodland and has significant impacts on vegetation structure and native flora and fauna. The demography of the plant has been studied at this site over two decades in order to gain a better understanding of its invasion ecology and thus assist in best practice for control.
Study site: A demographic study of the highly invasive shrub broom Cytisus scoparius was undertaken in an area of eucalyptus woodland, Barrington Tops, New South Wales, that had been invaded by broom.
Methodology: Four permanent, 25 m² plots where established in 1985. Two plots were located in already uniform broom thickets of different ages. The second two were located across the boundaries of broom stands, which have subsequently expanded and now completely cover these plots. All broom plants in the plots (other than young seedlings, which were counted) were mapped, tagged and monitored annually (up to 1999).
Each year, new seedlings appeared but there was no relationship between their numbers (which varied between years due to factors such as rainfall) and subsequent recruitment of older plants. The probability of seedlings reaching first flowering was low (< 2%), with seedlings mainly dying through shade suppression. Some individuals (< 50 cm high) were also browsed which either killed plants or retarded growth. The proportion of seedlings surviving to mature size (i.e. > 10 cm² basal area) was negligible. Recruitment occurred only where light levels were high, either before closure of the broom canopy or after senescence and die-back resulted in the canopy opening.
From approximately 12-30 years after initial invasion, the broom stands at Barrington Tops underwent self-thinning as mature plants died back. This thinning was promoted by the collapse of plants on to each other. Recruitment of new maturing plants, after this period produced a stand that was less dense than that found after initial invasion.
Broom is creating a more disturbance-prone environment due to its impacts on other biota e.g. suppression of other vegetation and by harbouring feral pigs Sus scrofa (in its dense undergrowth) which root around in the soil. Such disturbance favours broom seed germination, and elsewhere it has resulted in massive seedling regeneration.
While fire or increased physical soil disturbance/thinning etc. can be used to stimulate germination, and thereby reduce a large part of the soil seed bank, denser broom infestations are likely to result unless follow-up treatments can be applied over long time periods.
From this study, evidence suggests that management to control invasive broom should avoid disturbance, especially of stands of mature broom, unless resources are available to eradicate it entirely.
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