Plant Assessment Form

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Centaurea diffusa

Synonyms: Acosta diffusa (Lam.) Sojak

Common Names: diffuse knapweed

Evaluated on: 11/28/05

List committee review date: 10/01/2006

Re-evaluation date:

Evaluator(s)

Elizabeth Brusati, project manager
California Invasive Plant Council
1442A Walnut St. #462, Berkeley, CA 94709
510-843-3902
edbrusati@cal-ipc.org

Joseph M. DiTomaso/Coop. Ext. Specialist
University of California, Davis
Weed Science Program, Robbins Hall
530-754-8715
ditomaso@vegmail.ucdavis.edu

List committee members

Joe DiTomaso
John Randall
Peter Warner
Jake Sigg

General Comments

No general comments for this species

Table 2. Criteria, Section, and Overall Scores

Overall Score? Moderate

Alert Status? No Alert

Documentation? 3.5 out of 5

Score

Documentation

1.1

?Impact on abiotic ecosystem processes

B. Moderate

Reviewed Scientific Publication

Impact?
Four-part score BABD Total Score
B

1.2

?Impact on plant community

A. Severe

Reviewed Scientific Publication

1.3

?Impact on higher trophic levels

B. Moderate

Reviewed Scientific Publication

1.4

?Impact on genetic integrity

D. None

Other Published Material

2.1

?Role of anthropogenic and natural disturbance in establishment

B. Moderate

Reviewed Scientific Publication

Invasiveness?
Total Points
13 Total Score B

2.2

?Local rate of spread with no management

B. Increases less rapidly

Observational

2.3

?Recent trend in total area infested within state

D. Declining

Other Published Material

2.4

?Innate reproductive potential
(see Worksheet A)

A. High

Reviewed Scientific Publication

2.5

?Potential for human-caused dispersal

A. High

Reviewed Scientific Publication

2.6

? Potential for natural long-distance dispersal

B. Occasional

Other Published Material

2.7

?Other regions invaded

C. Already invaded

Other Published Material

3.1

?Ecological amplitude/Range
(see Worksheet C)

A. Widespread

Other Published Material

Distribution?
Total Score B

3.2

?Distribution/Peak frequency
(see Worksheet C)

D. Very low

Observational

Table 3. Documentation

Scores are explained in the "Criteria for Categorizing Invasive Non-Native Plants that Threaten Wildlands".

Section 1: Impact
Question 1.1 Impact on abiotic ecosystem processes? Consider the impact on the natural range and variation of abiotic ecosystem processes and system-wide parameters in ways that significantly diminish the ability of native species to survive and reproduce. Alterations that determine the types of communities that can exist in a given area are of greatest concern. Examples of abiotic processes include: - fire occurrence, frequency, and intensity; - geomorphological changes such as erosion and sedimentation rates; - hydrological regimes, including soil water table; - nutrient and mineral dynamics, including salinity, alkalinity, and pH; - light availability (e.g. when an aquatic invader covers an entire water body that would otherwise be open). Select the one letter below that best describes this species' most severe impact on an abiotic ecosystem process: A. Severe, possibly irreversible, alteration or disruption of an ecosystem process. B. Moderate alteration of an ecosystem process. C. Minor alteration of an ecosystem process. D. Negligible perceived impact on an ecosystem process. U. Unknown.	B Reviewed Scientific Publication
Identify ecosystem processes impacted: Increases erosion along streambanks. Sources of information: 1. Sheley, R.L, J.S. Jacobs, and M. F. Carpinelli. 1998. Distribution, biology, and management of diffuse knapweed (Centaurea diffusa) and spotted knapweed (Centaurea maculosa). Weed Technology. 12:353-362
Question 1.2 Impact on plant community composition, structure, and interactions? Consider the cumulative ecological impact of this species to the plant communities it invades. Give more weight to changes in plant composition, structure, and interactions that involve rare or keystone species or rare community types. Examples of severe impacts include: - formation of stands dominated (>75% cover) by the species; - occlusion (>75% cover) of a native canopy, including a water surface, that eliminates or degrades layers below; - significant reduction or extirpation of populations of one or more native species. Examples of impacts usually less than severe include: - reduction in propagule dispersal, seedling recruitment, or survivorship of native species; - creation of a new structural layer, including substantial thatch or litter, without elimination or replacement of a pre-existing layer; - change in density or depth of a structural layer; - change in horizontal distribution patterns or fragmentation of a native community; - creation of a vector or intermediate host of pests or pathogens that infect native plant species. Select the one letter below that best describes this species' impact on community composition, structure and interactions: A. Severe alteration of plant community composition, structure, or interactions. B. Moderate alteration of plant community composition. C. Minor alteration of community composition. D. Negligible impact known; causes no perceivable change in community composition, structure, or interactions. U. Unknown.	A Reviewed Scientific Publication
Identify type of impact or alteration: Reduces biodiversity (1). Compounds in shoots extracts reduce germination of other species through allelopathy (2,3) and it has a stronger negative effect on North American plants than on plants that evolved with knapweed in Eurasia (2). Can form monocultures, although populations in California are mostly small patches due to eradication efforts (1). Formation of monotypic stands is aided by the fact that seedling emergence is distributed over several weeks, allowing diffuse knapweed to occupy all available safe sites (4). Sources of information: 1. Sheley et al. 1998 2. Callaway, R., and E. T. Aschehoug. 2000. Invasive plants versus their new and old neighbors: a mechanism for exotic invasion. Science. 290:521-523 3. Muir, A. D., and W. Majak. 1983. Allelopathic potential of diffuse knapweed (Centaurea diffusa) extracts. Canadian Journal of Plant Sciences. 63:989-996 4. Sheley, R. L., and L. L. Larson. 1996. Emergence date effects on resource partitioning between diffuse knapweed seedlings. Journal of Range Management. 49:241-244.
Question 1.3 Impact on higher trophic levels? Consider the cumulative impact of this species on the animals, fungi, microbes, and other organisms in the communities that it invades. Although a non-native species may provide resources for one or a few native species (e.g. by providing food, nesting sites, etc.), the ranking should be based on the species' net impact on all native species. Give more weight to changes in composition and interactions involving rare or keystone species or rare community types. Examples of severe impacts include: - extirpation or endangerment of an existing native species or population; - elimination or significant reduction in native species' nesting or foraging sites, cover, or other critical resources (i.e., native species habitat), including migratory corridors. Examples of impacts that are usually less than severe include: - minor reduction in nesting or foraging sites, cover, etc. for native animals; - minor reduction in habitat connectivity or migratory corridors; - interference with native pollinators; - injurious components, such as awns or spines that damage the mouth and gut of native wildlife species, or production of anti-digestive or acutely toxic chemical that can poison native wildlife species. Select the one letter below that best describes this species' impact on community composition and interactions: A. Severe alteration of higher trophic populations, communities, or interactions. B. Moderate alteration of higher trophic level populations, communities, or interactions. C. Minor alteration of higher trophic level populations, communities or interactions. D. Negligible impact; causes no perceivable change in higher trophic level populations, communities, or interactions. E. Unknown.	B Reviewed Scientific Publication
Identify type of impact or alteration: Reduces forage for livestock and wildlife, but is grazed by deer. Elk foraging was reduced by 98% on plots of C. maculosa (1). Presumably, diffuse knapweed would act the same. There is anecdotal evidence that diffuse knapweed contains a compound that, when absorbed through the skin through cuts or abrasions, can cause benign tumors in humans, although there is no medical literature on this. Anyone working with diffuse knapweed should wear gloves as a precaution (2). Sources of information: 1. Sheley et al. 1998 2. Carpenter, A. T., and T. A. Murray. 1998. Element Stewardship Abstract for Centaurea diffusa. The Nature Conservancy, Arlington, VA. http://tncweeds.ucdavis.edu.
Question 1.4 Impact on genetic integrity? Consider whether the species can hybridize with and influence the proportion of individuals with non-native genes within populations of native species. Mechanisms and possible outcomes include: - production of fertile or sterile hybrids that can outcompete the native species; - production of sterile hybrids that lower the reproductive output of the native species. Select the one letter below that best describes this species' impact on genetic integrity: A. Severe (high proportion of individuals). B. Moderate (medium proportion of individuals). C. Minor (low proportion of individuals). D. No known hybridization. U. Unknown.	D Other Published Material
None. There are no native Centaurea spp. in California. Sources of information: Hickman, J. C. (ed.) 1993. The Jepson Manual, Higher Plants of California. University of California Press. Berkeley, CA
Section 2: Invasiveness
Question 2.1 Role of anthropogenic and natural disturbance in establishment? Assess this species' dependence on disturbance: both human and natural: for establishment in wildlands. Examples of anthropogenic disturbances include: - grazing, browsing, and rooting by domestic livestock and feral animals; - altered fire regimes, including fire suppression; - cultivation; - silvicultural practices; - altered hydrology due to dams, diversions, irrigation, etc.; - roads and trails; - construction; - nutrient loading from fertilizers, runoff, etc. Examples of natural disturbance include: - wildfire; - floods; - landslides; - windthrow; - native animal activities such as burrowing, grazing, or browsing. Select the first letter in the sequence below that describes the ability of this species to invade wildlands: A. Severe invasive potential: this species can establish independent of any known natural or anthropogenic disturbance. B. Moderate invasive potential: this species may occasionally establish in undisturbed areas but can readily establish in areas with natural disturbances. C. Low invasive potential: this species requires anthropogenic disturbance to establish. D. No perceptible invasive potential: this species does not establish in wildlands (though it may persist from former cultivation). U. Unknown.	B Observational
Describe role of disturbance: Invades highly disturbed areas such as roadsides and overgrazed grasslands, but can also invade after minor disturbance such as a hailstorm, rodent activity, or light grazing (1, 2,3). Disturbance greatly increases the the rate and final density of diffuse knapweed and allows it to invade a wider range of habitats (1). Sources of information: 1. Sheley et al. 1998 2. Myers, J. H., and D. E. Berube. 1983. Diffuse knapweed invasion into rangeland in the dry interior of British Columbia. Canadian Journal of Plant Sciences. 63:981-987. 3. Lacey, J., P. Husby, and G. Handl. 1990. Observations on spotted and diffuse knapweed invasion into ungrazed bunchgrass communities in western Montana. Rangelands. 12: 30-32.
Question 2.2 Local rate of spread with no management? Assess this species' rate of spread in existing localized infestations where the proportion of available habitat invaded is still small when no management measures are implemented. Select the one letter below that best describes the rate of spread: A. Increases rapidly (doubling in <10 years) B. Increases, but less rapidly C. Stable D. Declining U. Unknown	B Observational
Describe rate of spread: Has spread at very rapid rates in other western states. In California, the rate of spread appears to be a bit slower, probably due to the efforts of CDFA to manage the species. Sources of information: DiTomaso, observational.
Question 2.3 Recent trend in total area infested within state? Assess the overall trend in the total area infested by this species statewide. Include current management efforts in this assessment and note them. Select the one letter below that best describes the current trend: A. Increasing rapidly (doubling in total range statewide in <10 years) B. Increasing, but less rapidly C. Stable D. Declining U. Unknown	D Other Published Material
Describe trend: In California, diffuse knapweed primarily occurs as single plants or small patches and is under eradication through biological control in most areas (1). Stable to declining/decreasing statewide because of treatment efforts (2, 3). Once the seed production has been controlled, infestation levels drop rapidly (3). Sources of information: 1. Joley, D. B., and D. M. Woods. 1996. Biological control of diffuse knapweed, Centaurea diffusa. California Department of Food and Agriculture, Biological Control Program Annual Summary, 1996. Sacramento, CA 2. Carri Pirosko, California Dept. of Food and Agriculture. e-mail 11/29/05 3. Marla Knight, botanist, Klamath National Forest. e-mail 11/29/05
Question 2.4 Innate reproductive potential? Assess the innate reproductive potential of this species. Worksheet A is provided for computing the score.	A Reviewed Scientific Publication
Describe key reproductive characteristics: Typically biennial, sometimes annual or short-lived perennial. Obligate outcrosser. Flowers June-September. Seeds can persist in soil for many years. Purple-flowered plants were shown to set significantly more seed than white-flowered plants, event though white-flowered plants out-numbered purple-flowered plants and major insect visitors did not discriminate according to color. Seed production in Washington averaged 11,200 to 48, 100 seeds per square meter. Sources of information: DiTomaso, J.M., and E. Healy. 2006. Weeds of California and Other Western States Harrod, R. J., and R. J. Taylor. 1995. Reproduction and pollination biology of Centaurea and Acroptilon species, with emphasis on C. diffusa. Northwest Science. 69:97-105.
Question 2.5 Potential for human-caused dispersal? Assess whether this species is currently spread: or has high potential to be spread: by direct or indirect human activity. Such activity may enable the species to overcome natural barriers to dispersal that would not be crossed otherwise, or it may simply increase the natural dispersal of the species. Possible mechanisms for dispersal include: - commercial sales for use in agriculture, ornamental horticulture, or aquariums; - use as forage, erosion control, or revegetation; - presence as a contaminant (seeds or propagules) in bulk seed, hay, feed, soil, packing materials, etc.; - spread along transportation corridors such as highways, railroads, trails, or canals; - transport on boats or boat trailers. Select the one letter below that best describes human-caused dispersal and spread: A. High: there are numerous opportunities for dispersal to new areas. B. Moderate: human dispersal occurs, but not at a high level. C. Low: human dispersal is infrequent or inefficient. D. Does not occur. U. Unknown.	A Reviewed Scientific Publication
Identify dispersal mechanisms: Can be spread by hitchhiking in the frames of vehicles (1). Was originally introduced to the United States in contaminated alfalfa, and spread further in alfalfa and hay (2). Sources of information: 1. Roche and Roche 1999 2. Sheley et al. 1998
Question 2.6 Potential for natural long-distance dispersal? We have chosen 1 km as the threshold of "long-distance." Assess whether this species is frequently spread, or has high potential to be spread, by animals or abiotic mechanisms that can move seed, roots, stems, or other propagules this far. The following are examples of such natural long-distance dispersal mechanisms: - the species' fruit or seed is commonly consumed by birds or other animals that travel long distances; - the species' fruits or seeds are sticky or burred and cling to feathers or hair of animals; - the species has buoyant fruits, seeds, or other propagules that are dispersed by flowing water; - the species has light propagules that promote long-distance wind dispersal; - The species, or parts of it, can detach and disperse seeds as they are blown long distances (e.g., tumbleweed). Select the one letter below that best describes natural long-distance dispersal and spread: A. Frequent long-distance dispersal by animals or abiotic mechanisms. B. Occasional long-distance dispersal by animals or abiotic mechanisms. C. Rare dispersal more than 1 km by animals or abiotic mechanisms. D. No dispersal of more than 1 km by animals or abiotic mechanisms. U. Unknown.	B Other Published Material
Identify dispersal mechanisms: Ball-shaped plants can be spread by the wind, similar to tumbleweed. Seeds are lost gradually, allowing them to spread long distances. Plants can also be carried in rivers and irrigation systems. Sources of information: 1. Roche and Roche 1999
Question 2.7 Other regions invaded? Assess whether this species has invaded ecological types in other states or countries outside its native range that are analogous to ecological types not yet invaded in your state (see Worksheets B, C, and D for California, Arizona, and Nevada, respectively, in Part IV for lists of ecological types). This information is useful in predicting the likelihood of further spread within your state. Select the one letter below that best describes the species' invasiveness in other states or countries, outside its native range. A. This species has invaded 3 or more ecological types elsewhere that exist in your state and are as yet not invaded by this species (e.g. it has invaded Mediterranean grasslands, savanna, and maquis in southern Europe, which are analogous to California grasslands, savanna, and chaparral, respectively). B. Invades 1 or 2 ecological types that exist but are not yet invaded in your state. C. Invades elsewhere but only in ecological types that it has already invaded in the state. D. Not known as an escape anywhere else. U. Unknown.	C Other Published Material
Identify other regions: Native to Eurasia. Occurs in all western states, some central and eastern states, especially Illinois and surrounding states. Listed as a noxious weed in several states (1). Sources of information: 1. DiTomaso and Healy 2006
Section 3: Distribution
Question 3.1 Ecological amplitude/Range? Refer to Worksheet C and select the one letter below that indicates the number of different ecological types that this species invades. A. Widespread: the species invades at least three major types or at least six minor types. B. Moderate: the species invades two major types or five minor types. C. Limited: the species invades only one major type and two to four minor types. D. Narrow: the species invades only one minor type. U. Unknown.	A Other Published Material
Occurs in the North Coast, North Coast Ranges (Del Norte, Humboldt, Mendocino Cos.), Klamath Ranges (Trinity Co.), Cascade Ranges (Siskiyou, Shasta Cos.), north and central Sierra Nevada (Plumas, Nevada, sc Placer, e El Dorado, e Amador Cos), northern Sacramento Valley (c&e Tehama, sc Glenn, s Sutter, n Sacramento Cos), Modoc Plateau (Modoc, Lassen Cos), southern San Francisco Bay region (ne Santa Clara Co.), South Coast Ranges (se Monterey Co.), South Coast (Los Angeles, San Diego Cos), to 2300m. A population that has been eradicated occurred in the central area of the border between Mariposa and Madera counties (1). Habitats invaded (based on eradication projects listed in NRPI database): oak woodlands, blue oak-foothill pine, pasture, mixed evergreen forest, Great Basin scrub, coastal prairie, north coast coniferous forest, riparian forest and woodland, valley and foothill grassland (2) Mainly invades roadsides, waste areas, and rangelands. Found in some dry/seasonal creek beds in Siskiyou County. Mostly dry areas rather than true riparian areas (3). Sources of information: 1. DiTomaso and Healy 2006 2. Natural Resources Projects Inventory. Database of conservation and monitoring projects in California. Available: http://www.ice.ucdavis.edu/nrpi/. Accessed 11/28/05 3. Carri Pirosko, California Dept. of Food and Agriculture. e-mail 11/29/05.
Question 3.2 Distribution/Peak frequency? To assess distribution, record the letter that corresponds to the highest percent infested score entered in Worksheet C for any ecological type.	D Observational
Describe distribution: Mostly small infestations/patches; many single to small plants/patches that are eradicated. Sources of information: Carri Pirosko, California Dept. of Food and Agriculture. e-mail 11/29/05

Worksheet A - Innate reproductive potential

Reaches reproductive maturity in 2 years or less	Yes
Dense infestations produce >1,000 viable seed per square meter	Yes
Populations of this species produce seeds every year.	Yes
Seed production sustained over 3 or more months within a population annually	Yes
Seeds remain viable in soil for three or more years	Yes
Viable seed produced with both self-pollination and cross-pollination	No
Has quickly spreading vegetative structures (rhizomes, roots, etc.) that may root at nodes	No
Fragments easily and fragments can become established elsewhere	Yes
Resprouts readily when cut, grazed, or burned	Unknown
Total points:	9
Total unknowns:	1
Total score:	A? Scoring Criteria for Worksheet A A. High reproductive potential (6 or more points). B. Moderate reproductive potential (4-5 points). C. Low reproductive potential (3 points or less and less than 3 Unknowns). U. Unknown (3 or fewer points and 3 or more Unknowns).

Related traits:

Worksheet B - Arizona Ecological Types is not included here

Worksheet C - California Ecological Types

(sensu Holland 1986)

Major Ecological Types	Minor Ecological Types	Code? A means >50% of type occurrences are invaded; B means 20% to 50%; C means 5% to 20%; D means present but <5%; U means unknown (unable to estimate percentage of occurrences invaded)
Marine Systems	marine systems
Freshwater and Estuarine	lakes, ponds, reservoirs
Aquatic Systems	rivers, streams, canals
Aquatic Systems	estuaries
Dunes	coastal
	desert
	interior
Scrub and Chaparral	coastal bluff scrub
	coastal scrub
	Sonoran desert scrub
	Mojavean desert scrub (incl. Joshua tree woodland)
	Great Basin scrub	D, < 5%
	chenopod scrub
	montane dwarf scrub
	Upper Sonoran subshrub scrub
	chaparral
Grasslands, Vernal Pools, Meadows, and other Herb Communities	coastal prairie	D, < 5%
	valley and foothill grassland
	Great Basin grassland
	vernal pool
	meadow and seep
	alkali playa
	pebble plain
Bog and Marsh	bog and fen
Bog and Marsh	marsh and swamp
Riparian and Bottomland habitat	riparian forest	D, < 5%
	riparian woodland	D, < 5%
	riparian scrub (incl.desert washes)
Woodland	cismontane woodland	D, < 5%
	piñon and juniper woodland
	Sonoran thorn woodland
Forest	broadleaved upland forest
	North Coast coniferous forest	D, < 5%
	closed cone coniferous forest
	lower montane coniferous forest
	upper montane coniferous forest
	subalpine coniferous forest
Alpine Habitats	alpine boulder and rock field
Alpine Habitats	alpine dwarf scrub
	Amplitude (breadth):	A
	Distribution (highest score):	D

Infested Jepson Regions

Click here for a map of Jepson regions

Cascade Range
Central West
Great Valley
Northwest
Sierra Nevada
Southwest
Great Basin Province
Modoc Plateau
Sierra Nevada East