Invasion and establishment of Scotch broom (Cytisus scoparius) - key threatening process listing

The Scientific Committee, established by the Threatened Species Conservation Act, has made a Final Determination to list the Invasion and establishment of Scotch Broom, Cytisus scoparius, as KEY THREATENING PROCESS in Schedule 3 of the Act. Listing of key threatening processes is provided for by Part 2 of the Act.

NSW Scientific Committee - final determination

The Scientific Committee has found that:

1. Scotch Broom, Cytisus scoparius (L.) Link (Fabaceae), is a leguminous shrub native to Europe, first introduced to Australia in the early 1800s. Subsequent introductions were made for ornamental purposes and by 1901 it had spread significantly and was declared a noxious weed in NSW (NPWS 2005). It is now listed as a Weed of National Significance (Cooperative Research Centre for Weed Management Systems 1998, Thorp and Lynch 2000). Outside Australia it is a weed of temperate areas of the western USA, Hawaii, Canada (British Columbia), New Zealand, India, Iran and South Africa (Hosking  et al. 1998).

2. C. scoparius is estimated to infest more than 200 000 ha in south-eastern Australia and has become an environmental weed in higher rainfall areas (Hosking  et al. 2000). It grows most successfully in cool temperate areas on moist, fertile soils (Victorian Department of Resources and Environment 2002). It is continuing to spread through both expansion of existing infestations and colonization into new areas (Smith 2000). In some locations it has formed near monocultures (Downey 2002).

3. C. scoparius can grow to over 6 m but more commonly to 1-2 m. Flowering occurs in spring and seed set, which is the only method of reproduction, mainly occurs from late December to early February (Hosking  et al. 1998). Germination can occur at any time of the year if conditions are suitable (P. Downey unpubl. data). Young plants usually do not flower until their third year (Smith and Harlen 1991). Overall seedling mortality is high in the first three years (Downey and Smith 2000) and the majority of small plants experience browsing by herbivores until they achieve a height of about 50 cm. Most seeds fall within 1 m of the parent plant (Smith and Harlen 1991; Paynter  et al. 1996) but some may occasionally be flung several metres. The seeds are not buoyant in water but can be carried long distances in the bed load of rivers and streams. Scouring in the stream bed may scarify the seed coat, preparing it for germination when it washes up on the bank. Seeds may also be locally dispersed by ants (Bossard 1990a), and a variety of other seed vectors may occasionally disperse an unknown, but small portion, of seeds over much greater distances (Smith and Harlen 1991). Dry pods containing seeds can be blown short distances by wind. Seeds can also be carried within the digestive tracts of horses and other animals (Downey and Smith 2000). Seeds remain dormant after submersion in water, although the level of dormancy may be reduced compared with seeds buried in the soil. After experimental submersion in water for 1,000 days, 43% of seeds were viable (Smith and Harlen 1991).

4. C. scoparius readily invades sub-alpine grasslands and grassy woodlands as well as associated riparian habitats (Hosking  et al. 1998; Sheppard and Hosking 2000). Seedlings establish readily on sites where the soil has been disturbed or after fire.  C. scoparius can also invade areas subject to minimal disturbance (Downey and Smith 2000; Downey 2002), including eucalypt forest and woodland, alpine grassland and pasture. Major infestations occur in eastern New South Wales, particularly at Barrington Tops, the upper Shoalhaven River catchment (including parts of Deua National Park), Kosciuszko National Park and the Blue Mountains area. Disturbance may have contributed to invasion of native forest and pasture at Barrington Tops which was grazed by cattle and occasionally burnt to manage fodder until the 1960s (Downey and Smith 2000).

5. After initial invasion of a site, C. scoparius plants grow erect for about 6 years, sometimes achieving heights of more than 4 m and foliage projective cover of about 40%. Seed production increases after 3 years of age. At about 10 years of age plants begin to lean and eventually become decumbent; they may then grow in a semi-prostrate state for several years (Downey and Smith 2000). Development and decline of the canopy is accompanied by changes in the understorey. Eucalypt seedlings are virtually eliminated by  C. scoparius cover and the re-establishment cycle of trees in the woodland nearly ceases (Waterhouse 1988). The herbaceous ground cover may also be reduced to nearly zero where the  C. scoparius canopy completely shades the ground (J. Hosking, pers. com.).

6. C. scoparius possesses many attributes that are thought to contribute to its ability to aggressively invade native communities and compete with native species. These include (i) ability to fix nitrogen (Wheeler  et al. 1979) (ii) physical structure and longevity - in a few cases Scotch Broom can grow over 6 m in height and live for more than 30 years (Downey and Smith 2000) (iii) photosynthetic stems which enable it to remain photosynthetically active during its deciduous stage (Bossard and Rejmànek 1992) (iv) rapid growth, > 1 m per year (Downey and Smith 2000; Downey 2002; Sheppard  et al. 2002) (v) long-lived seeds (eg. over 80 years, Turner 1933) (vi) high seed production, >8 000 seeds per plant per year (Sheppard  et al. 2002) (vii) larger seeds (Buckley  et al. 2003) and larger seed banks, >60 000 seeds/m2 (Downey 2002), within the exotic range compared to the native range (viii) formation of monocultures or dense thickets, especially in early stages of invasion (Downey 2002) (ix) ability to grow in a range of habitats in cool moist areas (Hosking  et al. 2000) (x) higher densities in its exotic range (Paynter  et al. 2003) (xi) continual seedling emergence (Downey 2002) (xii) massive seedling flushes (Downey 2002), and (xiii) ability to alter disturbance regimes, particularly fire and soil disturbance (Downey 2002).

7. C. scoparius competes strongly with native vegetation, reducing recruitment of seedlings and growth of understorey species in open forest areas (Fogarty and Facelli 1999, Downey 2000, Sheppard  et al. 2002, Odom  et al. 2003). While seedling establishment can be suppressed in shade (Downey and Smith 2000), massive seedling recruitment can occur after manual removal of  C. scoparius cover by slashing, burning or herbicidal treatment of mature stands, at least in the absence of heavy mulching of the exposed soil surface (Odom  et al. 2003). Episodes of mass recruitment have followed disturbances (Bossard 1993, Robertson  et al. 1999, Downey 2002). While such recruitment events experience self-thinning, sufficient densities of seedlings may still exist to replace  C. scoparius stands (Downey 2002). Experimental removal of competing vegetation has little effect on seedling recruitment, age at flowering or the percentage cover in years following disturbance (Sheppard  et al. 2002). Grazing has been shown to increase the probability of recruitment from the soil seed bank by a factor of four (Sheppard  et al. 2002). Fire can also stimulate seed germination resulting in dense infestations of seedlings (Bossard 1990b, Tarrega  et al. 1992, Downey 2000).

8. The main pollinators of C. scoparius are honeybees  Apis mellifera (Parker 1997) and bumblebees  Bombus terrestris (Stout 2000, Simpson  et al. 2005). The Honeybee is an introduced species in Australia and is widespread throughout the range of  C. scoparius (Simpson  et al. 2005). The Bumblebee is also an introduced species, and is a more effective pollinator of  C. scoparius than the Honeybee (Simpson  et al. 2005) but is currently only known in NSW from isolated records. 'Introduction of the Large Earth Bumblebee,  Bombus terrestris' is listed as a Key Threatening Process in NSW.

9. Following invasion, C. scoparius can modify natural ecosystems in a number of ways. Establishment of  C. scoparius can change the vegetation structure (for example by converting grasslands to shrublands (Smith 1994a; Downey 2002)), modify the microclimate, and alter the floristic composition (for example by reducing the native grassland vegetation as well as the overstorey trees (Waterhouse 1988)). Broom thickets provide habitat for rabbits, foxes and feral pigs that can increase soil disturbance (Downey 2002).  C. scoparius is highly flammable and can burn intensely under suitable fuel and weather conditions. Thickets increase the fuel loads in native vegetation, thus altering natural fire regimes and fire behaviour (Downey 1999, 2000, 2002; Robertson  et al. 1999). Infestations can also impede access along watercourses (Odom  et al. 2003) and affect both native (Bell 1990) and exotic (Smith 1994b) bird diversity, reptile density (Smith 1994a), and native mammal density (Green 2000; Keating 2003).

10. The impacts of C. scoparius have been most thoroughly studied on the Barrington Tops Plateau, which has the largest infestation of the species in Australia (>14,000 ha, Schroder 2001). In this area  C. scoparius forms a dense shrub layer, in contrast to the native shrub distribution that tends to be more scattered. The dense shrub layer reduces the recruitment of white sallee,  Eucalyptus pauciflora, and modifies the native grasslands (Downey 2002).  C. scoparius has been identified as a contributing factor to the decline of the Broad-toothed Rat (Green 2000; Keating 2003) and disadvantages ground-feeding and ground-nesting bird species (Bell 1990) as a result of such habitat modifications.

11. Most of the information on the number and range of species that are threatened by C. scoparius invasion comes from the Barrington Tops area (eg. Coutts-Smith and Downey 2006). Threatened species, populations and ecological communities affected and potentially affected by  C. scoparius include the following:

Endangered Population

Threatened Ecological Communities

Species that are not currently listed as threatened but could become threatened as a result of invasion and establishment by C. scoparius include the following:

Birds

Plants

12. Current control options include herbicide application, manual removal and biological control. Herbicide application is also likely to affect native species in the vicinity of broom infestations. C. scoparius can be effectively controlled by several herbicides although their use in natural areas can be limited and expensive, and unless residual herbicides are used or treatments are ongoing, the effect is only temporary (Wapshere and Corey 1999; Odom  et al. 2003). Mechanical control is ineffectual without follow-up treatments as the site is then left open to  C. scoparius regeneration (Odom  et al. 2003). Fire can be used as a management tool to reduce soil seedbanks, depending on the duration and intensity of the burn (Bossard 1993, Downey 2000). Fire triggers a mass recruitment event which helps deplete the soil seedbank. Follow-up treatment is then required to kill the germinants before they reproduce. However, even after such treatment the soil seedbank may still be sufficient for re-establishment of the stand several years later (Downey and Smith 2000). Several cycles of burning and seedling elimination may therefore be required to control the species. This option is often not feasible as conditions are usually only suitable for fire in these recovering habitats during extreme fire weather (J.Hosking, pers. comm..). Three biological control agents have been released:  Leucoptera spartifoliella (a twig-mining moth),  Arytainilla spartiophila (a broom psyllid) and  Bruchidius villosus (a seed-eating beetle). A further two are currently being investigated:  Aceria genistae (a gall-forming mite) and  Apion fuscirostre (a seed-eating beetle) (P. Downey pers. comm..). The release program is in an early stage and its impacts have thus far been limited. A rust,  Uromyces pisi-sativi, is now also present throughout the range of  C. scoparius in Australia. Its presence was first noted in 2003 but its impact is uncertain (Morin  et al. 2006).

13. The Invasion and establishment of Scotch Broom, Cytisus scoparius is eligible to be listed as a key threatening process as, in the opinion of the Scientific Committee:

(a)it adversely affects threatened species, populations or ecological communities, or
(b) could cause species, populations or ecological communities that are not threatened to become threatened.

Professor Lesley Hughes
Chairperson
Scientific Committee

Proposed Gazettal date: 09/11/07
Exhibition period: 09/11/07 -18/01/08

References 

Bell SAJ (1990) Effects of the Weed Scotch Broom on Bird Communities in Open Forests on Barrington Tops. Bachelor of Science, Honours Thesis, University of Newcastle, Newcastle.

Bossard C (1990a) Tracing ant-dispersed seeds: a new technique. Ecology, 71, 2370-2371.

Bossard C (1990b) Secrets of an ecological interloper: ecological studies on Cytisus scoparius (Scotch broom) in California. PhD thesis, University of California, Davis.

Bossard C (1993) Seed germination in the exotic shrub Cytisus scoparius (Scotch broom) in California.  Madroño 40, 47-61.

Bossard CC, Rejmànek M (1992) Why have green stems? Functional Ecology 6, 197-205.

Buckley YM, Downey, PO, Fowler SV, Hill R, Memmott J, Norambuena, Pitcairn M, Shaw R, Sheppard AW, Winks C, Wittenberg R, Rees M (2003) Are invasives bigger? A global study of seed size variation in two invasive shrubs. Ecology 84, 1434-1440.

Cooperative Research Centre for Weed Management Systems (1998) Best practice management guide: Broom, Cytisus scoparius.

Coutts-Smith AJ, Downey PO (2006) 'The impact of weeds on threatened biodiversity in New South Wales'. Technical Series no. 11. CRC for Australian Weed Management, Adelaide.

Downey PO (1999) Fire and weeds: A management tool or Pandora's box? In 'Proceedings of the Bushfire '99 Conference', Albury, 7-9 July 1999, pp. 111-117, (Charles Sturt University: Albury)

Downey PO (2000) Broom and fire: Management implications. Plant Protection Quarterly 15, 178-183.

Downey PO (2002) Disturbance and invasion ecology of Scotch broom (Cytisus scoparius (L.) Link) in Australia. PhD thesis, University of New England, Armidale.

Downey PO, Smith JMB (2000) Demography of the invasive shrub Scotch broom (Cytisus scoparius) at Barrington Tops, NSW: Insights for management.  Austral Ecology 25, 477-485.

Fogarty G, Facelli JM (1999) Growth and competition of Cytisus scoparius, an invasive shrub, and Australian native plants.  Plant Ecology, 144, 27-35.

Green K (2000). 'A survey of the Broad-toothed Rat at Barrington Tops'. Unpublished report, NSW National Parks and Wildlife Service.

Hosking JR, Smith JMB, Sheppard AW (1998) Cytisus scoparius (L.) Link ssp.  scoparius. In The Biology of Australian Weeds. (Eds Panetta FD, Groves RH, Shepherd RCH) Volume. 2, pp. 77-88. (RG and FJ Richardson: Melbourne).

Hosking JR, Sheppard AW, Smith JMB (2000). 'Best Practice Management Guide, Broom Cytisus scoparius'. Cooperative Research Centre for Weed Management Systems, Adelaide.

Keating J (2003) 'Surveys of the Broad-toothed Rat in Barrington Tops National Park: Conducted as an assessment of the effectiveness of fox baiting programs'. Unpublished report, NSW National Parks and Wildlife Service.

Morin L, Sagliocco J-L, Hartley D, Hosking JR, Cramond P, Washington B (2006) Broom rust in Australia. In 15th Australian Weeds Conference: papers and proceedings (Eds C. Preston, JH Watts, Crossman ND) pp. 569-572. Adelaide.

NPWS (2005) Weed management in NSW National Parks, 'Scotch broom fact sheet'. Dept. of Environment and Conservation NSW.

Odom DIS, Cacho OJ, Sinden JA, Griffith GR (2003) Policies for the management of weeds in natural ecosystems: the case of scotch broom (Cytisus scoparius, L.) in an Australian national park.  Ecological Economics, 44, 119-135.

Parker IM (1997) Pollinator limitation of Cytisus scoparius (Scotch broom), an invasive exotic shrub.  Ecology 78, 1457-1470.

Paynter Q, Fowler S, Hinz H, Memmott J, Shaw R, Sheppard A, Syrett P (1996) Seed predation, seed banks, and seed-limitation: are seed-feeding insects of use for the biological control of broom? In Proceedings of the IX International Symposium on Biological Control of Weeds (Eds Moran VC, Hoffman JH) pp. 495-510. University of Cape Town, Cape Town, South Africa.

Paynter Q, Downey PO, Sheppard AW (2003) Age structure and growth of the woody legume weed Cytisus scoparius in native and exotic habitats: implications for control.  Journal of Applied Ecology 40, 470-480.

Rees M, Paynter Q (1997) Biological control of Scotch broom: Modelling the determinants of abundance and the potential impacts of introduced insect herbivores. Journal of Applied Ecology 34, 1203-1221.

Robertson DC, Morgan JW, White M (1999) Use of prescribed fire to enhance control of English broom (Cytisus scoparius) invading a subalpine snowgum woodland in Victoria.  Plant Protection Quarterly 14, 51-56.

Schroder M (2001) 'Scotch Broom Management Strategy /Works Program; Barrington Tops National Park/Polblue Crown Reserve'. Hunter Region: NSW National Parks and Wildlife Service.

Sheppard AW, Hodge P, Paynter Q, Rees M. (2002) Factors affecting invasion and persistence of broom Cytisus scoparius in Australia.  Journal of Applied Ecology 39, 721-734.

Sheppard AW, Hosking JR (2000) Broom management. Plant Protection Quarterly, 15, 134-186.

Simpson SR, Gross CL, Silberbauer LX (2005) Broom and honeybees in Australia: an alien liaison. Plant Ecology 7, 541-548.

Smith JMB (1994a) The changing ecological impacts of broom (Cytisus scoparius) at Barrington Tops, New South Wales.  Plant Protection Quarterly 9, 6-11.

Smith JMB (1994b) Blackbirds reach Barrington Tops, New South Wales, Australian Bird Watcher 15, 273-275.

Smith JMB (2000) An introduction to the biogeography and ecology of broom (Cytisus scoparius) in Australia.  Plant Protection Quarterly 15, 140-144.

Smith, JMB, Harlen, RL (1991) Preliminary observations on the seed dynamics of broom (Cytisus scoparius) at Barrington Tops, New South Wales.  Plant Protection Quarterly, 6, 73-78.

Stout JC (2000). Does size matter? Bumblebee behaviour and the pollination of Cytisus scoparius L.(Fabaceae),  Apidologie 31, 129-139.

Tarrega R, Calvo L, Trabaud L (1992) Effect of high temperatures on seed germination of two woody Leguminosae. Vegetatio 102, 139-147.

Thorp JR, Lynch R (2000) 'The determination of Weeds of National Significance'. National Weeds Strategy Executive Committee, Launceston.

Turner JH (1933) The viability of seeds. Bulletin of Miscellaneous Information. Royal Botanic Gardens. Kew 6, 257-269.

Victorian Department of Natural Resources and Environment (2002) 'Weed management in NSW National Parks: Scotch broom fact sheet'.

Wapshere T, Corey S (1999) 'Biological control of Scotch Broom (Cytisus scoparius)'. Division of Entomology, CSIRO, Canberra.

Waterhouse BM (1988) Broom (Cystisus scoparius) at Barrington Tops, New South Wales.  Australian Geographical Studies 26, 239-248.

Wheeler CT, Perry DA, Helgerson O, Gordon JC (1979) Winter fixation of Nitrogen in Scotch broom (Cytisus scoparius L.).  The New Phytologist 82, 697-701.