Conference Paper

Demography of the invasive shrub Scotch broom (Cytisus scoparius) at Barrington Tops, New South Wales: insights for management.

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Abstract

The exotic shrub Scotch Broom (Cytisus scoparius) has invaded large areas of eucalypt woodland at Barrington Tops, New South Wales, where it forms dense stands that have significant impacts on vegetation structure, flora and fauna. Data are presented from four 25 m(2) plots, which have been studied since 1985. Two plots were located in uniform broom thickets of different ages, and two were located across the margins of broom stands, which have since expanded to cover the entire plots. All broom plants in the plots (other than young seedlings, which were counted) were mapped, tagged and monitored annually. New seedlings appeared annually, but there was no relationship between their numbers (varying between years) and subsequent recruitment of older plants. The probability of seedlings reaching first flowering was less than 2%, and of surviving to mature size (> 10 cm(2) basal area) was negligible. Seedlings mainly died through suppression (shade). Individuals less than 50 cm high were also browsed. Recruitment occurred only where light levels were high, either before closure of the broom canopy or after senescence had led to canopy opening. From approximately 12-30 years after initial invasion, broom stands underwent self-thinning of mature plants, accelerated by 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 more disturbance-prone environments due to its impacts on other biota, likely alterations to the fire regime, and by harbouring feral pigs. Further disturbance favours broom, and elsewhere it has resulted in massive seedling regeneration. While fire or other disturbance 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. A wiser management option, at least in the short term, may be avoidance of all disturbance, especially for stands of mature broom.

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... Mechanical disturbance of the standing cover (such as mulching or crushing) may create initial opportunities for the indigenous seed-stock or planted seedlings. However, such control may do little to assist recovery of indigenous vegetation if the conditions that are created are too harsh for establishment of indigenous species, if competitive woody weeds recover quickly through resprouting or re-establishment from long-lived seed banks, or if the disturbance created by the treatment, or the mere effect of weakening or removing one weed, results in unexpected invasion by another weed (Allen et al. 1995; Rees & Paynter 1997; Williams 1998; Downey & Smith 2000; Buckley et al. 2007; Harris et al. 2013). Spraying the broom cover with herbicide eliminates the resource competition, retains (at least initially) the deadstanding cover, and results in little mechanical disturbance, but it also eliminates any indigenous woody species that are present. ...
... Spraying the broom cover with herbicide eliminates the resource competition, retains (at least initially) the deadstanding cover, and results in little mechanical disturbance, but it also eliminates any indigenous woody species that are present. Therefore, an Australian study suggested that a 'wiser management option, at least in the short term, may be avoidance of all disturbance, especially for stands of mature broom' (Downey & Smith 2000). Unlike gorse, broom is not generally regarded as a useful nurse crop for recovery of indigenous woody vegetation in New Zealand. ...
... However, it is important to note that for species with higher seed germination and survival rates (particularly Sophora microphylla), sowing seeds may be a viable option in restoration projects in these dryland zones. For any mechanical-disturbance treatments to be effective in removing broom cover, follow-up control of regenerating broom would be required over many years (Downey & Smith 2000). While this might be effective in slowing broom regeneration, it inevitably causes additional disturbance, which in turn can result in other opportunistic introduced species colonising the disturbed areas, providing increased competition for the indigenous species (Buckley et al. 2007). ...
Article
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It is common practice in New Zealand dryland areas to chemically or mechanically control invasive woody weeds, including Scotch broom (Cytisus scoparius). Such weed control is not always effective in achieving the often implicit aim of advancing the restoration of indigenous woody vegetation. We used a field experiment on a braided river terrace on the Canterbury Plains to test how five different management treatments of broom cover affected the germination, survival and growth of six indigenous tree and shrub species in a dryland setting. Mulched, root-raked and crushed treatments resulted in low seed germination and high mortality of planted seedlings, which was apparently due to the associated soil disturbance and microsite conditions. Significantly higher germination and survival rates of indigenous woody species under the living broom canopy indicated that the facilitative effects of the living canopy outweighed any negative effects. With no evidence of unassisted regeneration of indigenous plants from local sources during our experiment, our results suggest that retaining a live broom canopy was most beneficial for the germination and establishment of planted indigenous woody seedlings at this site. Compared with sowing and planting after mechanical or chemical broom control, sowing seeds and planting seedlings under the living broom canopy was also the cheapest management strategy to advance the succession of indigenous woody species in these dryland weed communities.
... Timeframes: Based on the definition of extinction (see above), the time required to collect data to determine that any native plant species has been driven to extinction from an alien plant exceeds that of most of the very few long-term studies in invasion ecology (e.g. Downey and Smith 2000), virtually none of which span more than 50 years. The problem is compounded because very few species (either native or alien) are monitored systematically over sufficiently long periods. ...
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Biological invasions are widely acknowledged as a major threat to global biodiversity. Species from all major taxonomic groups have become invasive. The range of impacts of invasive taxa and the overall magnitude of the threat is increasing. Plants comprise the biggest and best-studied group of invasive species. There is a growing debate, however, regarding the nature of the alien plant threat - in particular whether the outcome is likely to be the widespread extinction of native plant species. The debate has raised questions on whether the threat posed by invasive plants to native plants has been overstated. We provide a conceptual framework to guide discussion on this topic, in which the threat posed by invasive plants is considered in the context of a progression from no impact through to extinction. We define six thresholds along the “extinction trajectory”, global extinction being the final threshold. Although there are no documented examples of either “in the wild” (Threshold 5) or global extinctions (Threshold 6) of native plants that are attributable solely to plant invasions, there is evidence that native plants have crossed or breached other thresholds along the extinction trajectory due to the impacts associated with plant invasions. Several factors may be masking where native species are on the trajectory; these include a lack of appropriate data to accurately map the position of species on the trajectory, the timeframe required to definitively state that extinctions have occurred, and management interventions. Such interventions, focussing mainly on Thresholds 1 to 3 (a declining population through to the local extinction of a population), are likely to alter the extinction trajectory of some species. The critical issue for conservation managers is the trend, because interventions must be implemented before extinctions occur. Thus the lack of evidence for extinctions attributable to plant invasions does not mean we should disregard the broader threat.
... To minimize the risk of fire spreading into the open grasslands, the localized prescribed burns must be conducted early in the dry season when the grassland biomass is not too dry. Prescribed burning might be more pragmatic than other methods such as aerial broadcast of herbicides (Swezy and Odion 1997) or introduction of biocontrol agents (discussed in Downey and Smith 2000); the former is dangerous and can be particularly damaging to the native community in general, while the latter, apart from being a threat to non-target species, has been shown to be ineffective. Burning alone may not be effective to initiate the recovery of the gamut of mainstream grassland plants; perhaps supplemental seeding of mixed species or planting plugs of competitively dominant grasses will greatly enhance grassland recovery. ...
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... The establishment and spread of non-native species are long-term processes that do not depend on the success of a single reproductive season (e.g. Downey and Brown, 2000). In short-lived perennials such as our study species, oneyear seed production might be more related to population demography (Parker, 1997) than in long-lived non-native species. ...
... To minimize the risk of fire spreading into the open grasslands, the localized prescribed burns must be conducted early in the dry season when the grassland biomass is not too dry. Prescribed burning might be more pragmatic than other methods such as aerial broadcast of herbicides (Swezy and Odion 1997) or introduction of biocontrol agents (discussed in Downey and Smith 2000); the former is dangerous and can be particularly damaging to the native community in general, while the latter, apart from being a threat to non-target species, has been shown to be ineffective. Burning alone may not be effective to initiate the recovery of the gamut of mainstream grassland plants; perhaps supplemental seeding of mixed species or planting plugs of competitively dominant grasses will greatly enhance grassland recovery. ...
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... The introduction and subsequent spread of introduced species can lead to broad-scale land-cover changes. For example, Scotch broom (Cytisus scoparius) was planted along selected highways in western Washington state for beautification but has now become a regional pest that competes with native species, widely disseminates pollen to which many people are allergic, disrupts fire regimes, and provides habitat for feral animals [9]. Kudzu (Pueraria lobata Ohwi) is another example of a deliberately introduced plant that has become a regional pest along roads. ...
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