Invasiveness Assessment - Hybrid crack willow (Salix x rubens) in Victoria (Nox)

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Plant invasiveness is determined by evaluating a plant’s biological and ecological characteristics against criteria that encompass establishment requirements, growth rate and competitive ability, methods of reproduction, and dispersal mechanisms.

Each characteristic, or criterion, is assessed against a list of intensity ratings. Depending upon information found, a rating of Low, Medium Low, Medium High or High is assigned to that criterion. Where no data is available to answer a criterion, a rating of medium (M) is applied. A description of the invasiveness criteria and intensity ratings used in this process can be viewed here.

The following table provides information on the invasiveness of Hybrid crack 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 - 1 MB)
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Common Name: Hybrid crack willow
Scientific name: Salix x rubens

Question	Comments	Rating	Confidence
Establishment
Germination requirements?	S. viminalis generally invade as seedlings (ARMCANZ, 2001), and as such are “virtually restricted to bare sediment that is wet for weeks or months following seed shed [and] seedlings need much light” which is predicted to occur “in most streams at intervals of about 5 to 20 years” (Cremer, 1999).	ML	MH
Establishment requirements?	Willows are described as heliophyllic (intolerant of shade) (Haines, 2004) and most species …must not be over-humg by larger plants or trees (Newsholme, 1992).	ML	MH
How much disturbance is required?	S. viminalis generally invade as seedlings (ARMCANZ, 2001), and as such are “virtually restricted to bare sediment that is wet for weeks or months following seed shed [and] seedlings need much light” (Cremer, 1999). This is a major disturbance with little or no competition from other plant species.	L	MH
Growth/Competitive
Life form?	Semi-aquatic- All willows are described as growing on the banks of water courses or in moist locations (see references below), with some able to grow in riverbeds: S. exigua (Cody, 1996), S. fragilis (Maloney et al, 1999), S. alba, S .x rubens, and S. viminalis (Webb, Sykes & Garnock-Jones, 1988), and few species S. cinerea (Cremer, 1999) and S. exigua (Voss, 1972) can also establish on drier soil.	H	MH
Allelopathic properties?	Of the naturalised willows in Australia, allelopathic compounds have been demonstrated in S. rubra (Bokarev, Kefeli & Kapeljusnikova, 1966), S. babylonica (Koul et al, 1991), S. fragilis (Davison, 1965), S. viminalis (Bowen & Hoad, 1968), S. alba (Banzhaf et al, 1983) and S. purpurea (Gilliver, 1947).	MH	H
Tolerates herb pressure?	Some Salix species provide good fodder for both ruminants and horses…particularly in Australia (Newsholme, 1992). However, many species tolerate this grazing and/or are able to resprout from stump, e.g. S. cinerea (Cremer, 1999), S. x rubens (Shafroth et al, 1994), S. viminalis (Newsholme, 1992), S. exigua (Uchytil, 1989).	MH	MH
Normal growth rate?	“Salix are fast growing trees or shrubs, rarely prostrate” (Spencer, 1997). Where growth rates are described, tree and shrub forms of Salix are recorded as vigorous or fast growing, e.g. S. alba (Spencer, 1997), S. glaucophylloides (Newsholme, 1992), S. nigra (Cremer, 1995), S. purpurea (Webb, Sykes & Garnock-Jones, 1988) and S. x rubens (Meikle).	H	MH
Stress tolerance to frost, drought, w/logg, sal. etc?	FIRE “If a tree is completely girdled by fire at ground level it may die without oppicing… however young trees tend to sprout from the buried portion of their stem” (Cremer, 1999), indicating some fire tolerance in tree and shrub willows in general. FROST All tree and shrub species (except S. humboldtiana) in this study are recorded as tolerating USDA hardiness zones of between 1 and 6 (average minimum temperatures between –46^oC and –17^oC) (Bailey & Bailey, 1976, Griffiths, 1992). S. humboldtiana is susceptible to frost (Newsholme, 1992). DROUGHT All willows are described as growing on the banks or in the riverbeds of water courses or in 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) suggesting a low drought tolerance; and only few species (S. cinerea and S. exigua) can also establish on drier soil (but are not necessarily drought tolerant), with S. purpurea alone tolerating extremely dry, sandy soils (Newsholme, 1992). WATERLOGGING Those species able to grow in riverbeds must have high tolerance of waterlogging (S. exigua, Cody, 1996; S. fragilis, Maloney et al, 1999; S. alba, S. x rubens, and S. viminalis; Webb, Sykes & Garnock-Jones, 1988) and also S. cinerea (Cremer, 1999). SALINITY S. alba is “the most tolerant of all willows to brackish water” (Zallar). S. cinerea “tolerates salt-laden air” (Newsholme, 1992), S. fragilis, S. matsudana & S. seringeana tolerate high levels of salinity (Crouch & Honeyman, 1986). S. nigra has moderately high tolerance of salt (Swift, 1997) and S. purpurea shows “considerable tolerance for salt water” (Newsholme, 1992). S. alba are highly tolerant of waterlogging, salinity and frost, have some tolerance of fire, but not of drought.	MH	MH
Reproduction
Reproductive system	S. viminalis is capable of regenerating both vegetatively and by seed (Ladson et al, 1997).	H	MH
Number of propagules produced?	“A large crown would produce over 500,000 seeds (Cremer, 1999). Seeds numerous, very small, each with a tuft of long hairs (Tutin, 1993).	H	MH
Propagule longevity?	“Willow seed is very short-lived” with longer-lived species recorded at surviving 8-10 weeks (Cremer, 1999).	L	MH
Reproductive period?	Following reproductive maturity “flowering tends to be ample and to occur every year” (Cremer, 1999). S. cinerea “stands are mostly monoculture excluding 97% of sunlight and most other species” (Cremer, 1999). The oldest stem S. exigua in this study was 31 years (Ottenbreit & Staniforth, 1992). An old S. fragilis tree was observed to 130 years (Meikle, 1984).	H	MH
Time to reproductive maturity?	“Flowering and the production of viable seed may begin two to three years after germination, provided the plant is at least 1 m tall if it is a shrub willow, or 2 m tall if it is a tree willow…In some taxa (e.g. S. alba) flowering tends to begin at larger sizes” (Cremer, 1999), which is probably still within 5 years. For plants that have reproduced vegetatively, this may be reduced, as in S. nigra, observed flowering on cuttings produced from < 1 year old growth (Crouch & Honeyman, 1986).	ML	MH
Dispersal
Number of mechanisms?	Vegetative propagules and seed spread by water. “Seed is easily carried by wind” (Cremer, 1999).	H	MH
How far do they disperse?	“Seed is easily carried by wind for more than 1 km and some travels for up to 50 km or even 100 km (Cremer, 1999) for species that commonly reproduce by seed such as S. viminalis (ARMCANZ, 2001).	H	MH

S. cinerea, S. fragilis, S. x rubens & S. nigra are identified by Ladson et al as outstandingly invasive because of their biology and ecology, specifically their reproductive ability.

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