Invasiveness Ranking System for Non-Native Plants of Alaska

Carduus nutans L.common names: musk thistle,C. acanthoides L. plumeless thistle,C. pycnocephalus L. Italian thistle,C. tenuiflorus W. Curtis slender-flowered thistleRanking SummaryEcoregion known or expected to occur inSouth CoastalInterior BorealArctic AlpineYesYesYesPotential Max. ScoreEcological Impact 40 22Biological Characteristics and Dispersal 25 17Amplitude and Distribution 25 14Feasibility of Control 10 8Relative Maximum 61Climatic ComparisonCollected inAlaska regions?CLIMEXsimilarity?South Coastal No YesInterior Boreal No YesArctic Alpine No YesNo Carduus species have been recorded in Alaska (Hultén 1968,AKEPIC 2004, UAM 2004). The CLIMEX matching programshows that climatic similarity between Juneau and areas wherethe species are documented is high. Musk thistle is naturalizedalong the coastal region of Norway, including the area aroundBergen and Kristiansand (Lid and Lid 1994), which have a 73%and 60% similarity with Juneau. The native range of the speciesincludes Bogolovsk and Sverdlovsk, Russia (Gubanov et al.1995), which has a 71% and 66% climatic match with Fairbanks,and 67% and 66% climatic match with Nome, respectively. Thissuggests that if introduced, establishment of species from thegenus Carduus in south coastal, interior boreal and arctic alpineecogeographic regions may be possible.Ecological ImpactScoreImpact on Ecosystem Processes (0–10) 5Overwintering rosettes can severely inhibit the establishmentof other plants. This may retard natural processes of secondarysuccession (Pitcher and Russo 1988, Rutledge and McLendon1996). Dead stands can trap snow in winter, increasing soilmoisture in the spring (Desrochers et al. 1988).Impact on Natural Community Structure (0–10) 5Carduus species are capable of forming a dense, tall herbaceouslayer of vegetation (Royer and Dickinson 1999, Whitson et al.2000).Impact on Natural Community Composition (0–10) 5Thistle stands can outcompete and reduce the number ofindividuals and may displace native herbaceous plants (Pitcherand Russo 1988, Royer and Dickinson 1999, Whitson et al.2000).Impact on Higher Trophic Levels (0–10) 7Infestations in meadows and rangelands reduce foraging sites andhinder the movement of grazing animals (Hull and Evans 1973,Royer and Dickinson 1999, Whitson et al. 2000). Thistle flowersare usually very attractive to insect pollinators and can alter thebehavior of native pollinators (Desrochers et al. 1988, Gubanovet al. 2004). Hybridization between musk thistle and plumelessthistle has been reported (Warwick et al. 1989).Total for Ecological Impact 22/40Biological Characteristics and Dispersal ScoreMode of Reproduction (0–3) 3Carduus species reproduce by seed only. Seed production can beas great as 11,000 seeds per plant (Desrochers et al. 1988).Long-distance dispersal (0–3) 3The majority of the seeds fall near the parent plant. Experimentalstudies in Virginia suggest that seeds do not travel far from thematernal plant, with over 80% of seeds deposited within 40 m ofthe parent plant (Smith and Kok 1984). However, seeds can alsobe dispersed by wind, small mammals, birds, and water (Beck2004, Butterfield et al. 1996, Rutledge and McLendon 1996).Spread by humans (0–3) 3Seeds may attach to animals, farm machinery, and vehicles. Theymay contaminate crops and hay (Rutledge and McLendon 1996,Zouhar 2002).Allelopathic (0–2) 2Aqueous extracts and dead plant material from musk thistle havean inhibitory effect on germination and growth rate of severalgrass species (Wardle et al 1993).Competitive Ability (0–3) 1Thistles are highly competitive plants; however, establishmentmay be negatively affected by grasses (Butterfield et al. 1996,Rutledge and McLendon 1996, Wardle et al. 1996). Carduusspecies are usually more productive in communities where levelsof competition are low (Austin et al. 1985).Thicket-forming/Smothering growth form (0–2) 2Members of the genus Carduus are capable of forming densestands, especially at highly disturbed sites where competition islow. Plants can be as tall as 6 feet (Desrochers et al. 1988).Germination requirements (0–3) 0Sufficient light is required for germination (Rutledge andMcLendon 1996), therefore, more seeds germinate andestablish on bare soils in open pastures and poorly vegetatedsites (Beck 2004, Hamrick and Lee 1987). In greenhouseexperiments, optimum levels of germination and establishmentoccurred in habitats with a light covering of litter that reducedevapotranspiration. Thick litter layers reduced germination andestablishment by preventing seeds from reaching the soil surface(Hamrick and Lee 1987).Other invasive species in the genus (0–3) 3The Carduus genus is comprised of a number of noxious pastureand range weeds (Royer and Dickinson 1999, USDA 2002,Whitson et al. 2000).Aquatic, wetland or riparian species (0–3) 0Carduus species can be found in waste ground, old fields,pastures, and along roads and railroads. They can invade opennatural areas such as meadows, prairies, and grasslands (Beck2004, Butterfield et al. 1996).Total for Biological Characteristics and Dispersal 17/25Ecological Amplitude and Distribution ScoreHighly domesticated or a weed of agriculture (0–4) 2Carduus species are not major agricultural pests; instead theyare mostly weeds of pastures and ranges (Beck 2004, Royer andDickinson 1999, Whitson et al. 2000).B-15