Impact Assessment - St Barnaby’s thistle (Centaurea solstitialis) in Victoria (Nox)

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Assessment of plant invasiveness is done by evaluating biological and ecological characteristics such as germination requirements, growth rate, competitive ability, reproduction methods and dispersal mechanisms. Assessment of plant impacts, however, is determined by the extent to which a plant affects a land manager’s environmental, economic and social resources.

The relative importance of these resources varies depending upon the value people place on them and, as such, the assessment process is subjective. For example, a farmer is likely to place a higher emphasis on the impact of a plant on production (economic resource) than its impact on areas of natural vegetation occurring on the farm. Conversely, a Landcare or Friends group would value environmental or social resources more than economic resources.

Recognising that the value of resources vary between different land tenures, plant impact assessments allow a prioritisation of resources by land managers. Assessments can apply at a local, regional or state level, and the relative values of each resource identified may differ at each level.

The impact assessment method used in the Victorian Pest Plant Prioritisation Process uses three broad resource categories: social, environmental and agricultural, each with a number of related attributes. For example, social resources include such attributes as how the plant affects human access for recreation, or if it creates a health risk due to toxicity or by producing allergens.

Question	Comments	Reference	Rating
Recreation
1. Restrict human access?	“An erect annual or short-lived perennial herb, to 75 cm high.” Stems much-branched which terminate in a single floret armed with spines approximately 1 to 2.5 cm long. “This thistle often occurs in dense patches which restrict stock movement." “It can also…reduce access to recreational areas.” People access recreational facilities with difficulty.	P & C (2001) DiTomaso (2001) 1	MH
2. Reduce tourism?	As above. Some recreational activities may be affected.		MH
3. Injurious to people?	“…injure dogs and grazing animals particularly in the eyes, mouth and feet.” Potential to harm humans as well. Spines present throughout the year.	P & C (2001)	H
4. Damage to cultural sites?	Dense infestations likely to cause moderate negative visual effect.		ML
Abiotic
5. Impact flow?	Terrestrial species.	P & C (2001)	L
6. Impact water quality?	Terrestrial species.	P & C (2001)	L
7. Increase soil erosion?	It has become a weed in Australia on neglected areas and roadsides, occasionally invading unimproved grazing land.” The plant has a taproot about 20 cm long with numerous laterals. Not likely to increase soil erosion.	P & C (2001)	L
8. Reduce biomass?	It has become a weed in Australia on neglected areas and roadsides, occasionally invading unimproved grazing land.” In dense patches, biomass may increase.	P & C (2001)	L
9. Change fire regime?	“Additionally, dense infestations of yellow starthistle may change the fire regime by changing the fuel characteristics at a given site. …dried skeletons of yellow starthistle can provide fuel for late summer wildfires.” Likely to increase the frequency of fire risk in late summer.	Zouhar (2002)2	ML
Community Habitat
10. Impact on composition (a) high value EVC	EVC=Plains grassland (E); CMA=Goulburn Broken; Bioreg=Victorian Riverina; VH CLIMATE potential. Not known as an environmental weed in Victoria. However in the U.S. “C. solstitialis is a winter annual that can form dense impenetrable stands. Over the past 40 years, C. solstitialis has spread exponentially to infest rangelands [and] native grasses. C. solstitialis is best adapted to open grasslands.” Major displacement of grasses/forbs.	DiTomaso (2001)	MH
(b) medium value EVC	EVC=Heathy woodland (D); CMA=Port Phillip; Bioreg=Central Victorian Uplands; VH CLIMATE potential. “C. solstitialis is best adapted to open grasslands.” Population density reduced due to overstorey cover.	DiTomaso (2001)	ML
(c) low value EVC	EVC=Montane dry woodland (LC); CMA=North East; Bioreg=Northern Inland Slopes; VH CLIMATE potential. Impact as in 10(b) above.	DiTomaso (2001)	ML
11. Impact on structure?	Not known as an environmental weed in Victoria (it is not documented in Carr et al 1992). However, in the U.S., “C. solstitialis is a winter annual that can form dense impenetrable stands that displace desirable vegetation in natural areas, rangelands and other places. C. solstitialis is best adapted to open grasslands.” Potential to have a major effect on grasses/forbs.	DiTomaso (2001)	ML
12. Effect on threatened flora?
Fauna
13. Effect on threatened fauna?
14. Effect on non-threatened fauna?	“In addition, C. solstitialis infestations can reduce wildlife habitat and forage…and decrease native plant and animal diversity.” Reduction in habitat for fauna spp. leading to a reduction in numbers.	DiTomaso (2001)	MH
15. Benefits fauna?	No documented benefits.		H
16. Injurious to fauna?	“This thistle often occurs in dense patches which…injure dogs and grazing animals particularly in the eyes, mouth and feet.” Potentially injurious to fauna spp.	P & C (2001)	H
Pest Animal
17. Food source to pests?	Not known as a food source to pests in Australia.		L
18. Provides harbor?	Not known to provide harbor.		L
Agriculture
19. Impact yield?	“St Barnaby’s thistle competes with pastures and crops for moisture and nutrients and the large rosettes shade useful pasture species. Stories are quoted from north-eastern Victoria…of wheat crops with a potential yield of 2 tonnes per hectare being reduced to practically nothing.” Serious impact on yield.	P & C (2001)	H
20. Impact quality?	“Contributes to vegetable fault in wool. Seed can also be spread…as a contaminant of agricultural produce including commercial seed.” Level of impact not quantified; consider to be a minor component	P & C (2001)	ML
21. Affect land value?	Although the plant can have a serious detrimental effect on agricultural yield, control is relatively straightforward by repeated shallow cultivations and by, “encouraging legume-based pastures which provide maximum competition to the weed.” Left untreated, the land value may be affected.	P & C (2001)	M
22. Change land use?	Although the plant can have a serious detrimental effect on agricultural yield, control is relatively straightforward by repeated shallow cultivations and by, “encouraging legume-based pastures which provide maximum competition to the weed.” Land use may be restricted during treatment period.	P & C (2001)	M
23. Increase harvest costs?	Not known to increase harvest costs.		L
24. Disease host/vector?	None evident.		L