Impact Assessment - Black willow (Salix nigra) 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.

Each resource attribute, or criterion, is assessed relative to a list of intensity ratings. Depending upon information found in relation to each criterion, a rating of Low, Medium Low, Medium High or High is assigned. Descriptions of the impact criteria and intensity ratings used in this process can be viewed here.

The following table provides information on the impact of Black willow

A more detailed description of the methodology of the Victorian Weed Risk Assessment (WRA) method can be viewed below:

Victorian Weed Risk Assessment (WRA) method (PDF - 630 KB)
Victorian Weed Risk Assessment (WRA) method (DOC - 1026 KB)
To view the information PDF requires the use of a PDF reader. This can be installed for free from the Adobe website (external link).

Common Name: Black willow
Scientific name: Salix nigra

Question	Comments	Rating	Confidence
Recreation
1. Restrict human access?	S. nigra is able to grow in the middle of streams (Ladson et al, 1997) and has roots that encroach into streams, trapping silt and reducing channel capacity and can create a shallow stream (Cremer, 1995) that boats.	H	MH
2. Reduce tourism?	S. nigra is able to grow in the middle of streams (Ladson et al, 1997) and has roots that encroach into streams, trapping silt and reducing channel capacity that can create a shallow stream (Cremer, 1995) that can block the passage of boats and swimmers can no longer use.	H	MH
3. Injurious to people?	No reference to human injury found in any reference. Toxicity rated as ‘none’ for the species summarised in the USDA Plants Database (2006) including S. purpurea, S. exigua, S. nigra, S. x sepulcralis, S. alba. Presume no toxicity for any Salix.	L	MH
4. Damage to cultural sites?	As semi-aquatic species, confined to stream banks or beds or moist locations (Carr, 1996; Carr et al, 1992; Cody, 1996; Davis, 1982; Howard, 1988; Ladson et al, 1997; Maloney et al, 1999; Munz, 1963; Voss, 1972; Webb, Sykes & Garnock-Jones, 1988) willows are unlikely to grow near enough to cause structural or visual damage to cultural sites. However, during floods, senescent trees can drop large branches or trunks into waterways and a build up of material behind these snags can destroy bridges (ARMCANZ, 2001). Major damage to buildings can occur when streams change route because they have become clogged with mats of willow roots. A Tasmanian Landcare group was formed in response to flooded homes, the cause of which was attributed to stream blockages by willow roots encroaching into streams (Sarah Holland-Clift pers. comm.).	H	MH
Abiotic
5. Impact flow?	S. nigra have extensive roots, which can grow out into the stream (Cremer, 1995) “trapping silt and layering new roots over the top of the old ones…creating a broad shallow stream” (Purtle et al, 2001b).	H	MH
6. Impact water quality?	As deciduous plants (Carr, 1996), all shrub and tree willows have mass autumn leaf fall, which leads to decreased oxygen levels (Ladson et al, 1997). Intense shading by willows, which tend to have more dense canopies than native species decreases water temperature (Ladson et al, 1997).	H	MH
7. Increase soil erosion?	“Willows encroaching into the centre of streams interrupt the flow of water which results in stream flows being directed into watercourse banks, causing erosion. In severe cases, willows can create complete blockages, causing the stream to change course (Purtle, 2001b). S. nigra are capable of encroaching into streams (Cremer, 1999).	H	MH
8. Reduce biomass?	As woody plants that can form dense thickets, or large shrubs or trees (Carr, 1996), willows are capable of increasing biomass where they replace lower or less dense vegetation as they commonly do in disturbed sites (Cremer, 1999).	L	MH
9. Change fire regime?	Willows are low flammable/combustible trees (Carcaillet et al, 2001), likely to greatly change the frequency and intensity of fire risk.	H	H
Community Habitat
10. Impact on composition (a) high value EVC	Basin= East Gippsland- Cann River East Branch (ISC= Excellent); CMA= East Gippsland; CLIMATE=VH. For willows in general, their “dense shade and mat-forming willow roots suppress and kill indigenous understorey” (Purtle et al, 2001b). Potential for major displacement of dominant species in a specific layer.	MH	MH
(b) medium value EVC	All Victorian waterbodies are assumed to be high value EVCs.	L	H
(c) low value EVC	All Victorian waterbodies are assumed to be high value EVCs.	L	H
11. Impact on structure?	For willow species in general, their “dense shade and mat-forming willow roots suppress and kill indigenous understorey” (Purtle et al, 2001b).	MH	MH
12. Effect on threatened flora?	No information found.	MH	L
Fauna
13. Effect on threatened fauna?	No information found.	MH	L
14. Effect on non-threatened fauna?	Intense shading decreases primary production in waterways, impacting on invertebrates and fish (Ladson et al, 1997). Reduce indigenous vegetation which would otherwise provide habitat (especially tree hollows) and pollen and nectar food sources (Ladson, 1997). “Dense shade and mat-forming willow roots suppress and kill indigenous understorey [which is] important habitat for insects, birds and mammals. Bare banks beneath willows provide little protection for frogs, water rats, snakes, lizards and other fauna. Willows do not provide nectar for native birds…Willows also have few hollows, important habitat for over half of our woodland birds and mammals” (Purtle et al, 2001b). Reduction in habitat for fauna, leading to reduction in numbers of individuals but not to local extinction.	MH	MH
15. Benefits fauna?	Possums graze and defoliate isolated trees (ARMCANZ, 2001). S. nigra is palatable to some mammals (Uchytil, 1989). Willows are capable at most of providing some assistance as either food or shelter to desirable species.	MH	MH
16. Injurious to fauna?	No reference to animal injury found in any reference.	L	H
Pest Animal
17. Food source to pests?	Rodents eat the buds and bark of S. nigra (Uchytil, 1989).	ML	MH
18. Provides harbor?	S. nigra tend to have single trunks (Cremer, 1995), making them unlikely to harbor vermin as they don’t tend to form thickets.	L	MH
Agriculture
19. Impact yield?	Species present as agricultural weeds: S. alba in NZ and USA, S. babylonica and S. cinerea in NZ, S. exigua in USA, S. nigra in USA and S. fragilis is a principal agricultural weed in NZ (Holm et al, 1979). As willows are associated with waterways and not recorded as invaders of pasture or crops, their ability to form dense thickets that impede access to waterways for irrigation is likely to be the only impact they have on agriculture. Willow root mats reduce access to flowing water for irrigation purposes (Sarah Holland- Clift pers. comm.). S. nigra is able to grow instream (Cremer, 1999), which may reduce access to waterways for irrigation include. They are likely to have a minor impact on the quantity of produce.	ML	MH
20. Impact quality?	Generally willows are valued for shade and browsing for livestock (Besaans, 1995).	L	MH
21. Affect land value?	“Fibrous willow roots and dense willow foliage trap large amounts of silt which can decrease channel capacity, exacerbate flooding and change flood patterns…Willows encroaching into the centre of streams interrupt the flow of water which results in stream flows being directed into watercourse banks, causing erosion.” (Purtle, 2001b). Whilst risk of floods and erosion may cause decrease in land value in some incidences, with a state wide view, this is likely to be negligible.	L	MH
22. Change land use?	Willows are associated with waterways and not recorded as invaders of pasture or crops in the extensive literature. Whilst control may be required (see Q. 23) there is little risk that land use would need to change as a consequence of their invasion of agricultural land.	L	H
23. Increase harvest costs?	Some willow control to maintain waterways for irrigation may increase harvest costs by a minor amount. This is likely to be true for S. nigra (Cremer, 1999). In the Jerilderie area 80% of farmers are donating a rebate to fund a project manager for willows in Yanco Creek, because their root mats reduce access to creek water for irrigation (Sarah Holland-Clift).	MH	MH
24. Disease host/vector?	Not found in the extensive literature.	L	H

This table can also be viewed as a PDF document (printer friendly).

Impact Assessment Record - Black willow (PDF - 34KB)
Impact Assessment Record - Black willow (DOC - 86KB)
To view the information PDF requires the use of a PDF reader. This can be installed for free from the Adobe website (external link).

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