Fire Effects Information System (FEIS)

FEIS Home Page

Cirsium vulgare

• INTRODUCTORY
• DISTRIBUTION AND OCCURRENCE
• BOTANICAL AND ECOLOGICAL CHARACTERISTICS
• FIRE ECOLOGY
• FIRE EFFECTS
• MANAGEMENT CONSIDERATIONS
• REFERENCES

INTRODUCTORY

AUTHORSHIP AND CITATION FEIS ABBREVIATION SYNONYMS NRCS PLANT CODE COMMON NAMES TAXONOMY LIFE FORM FEDERAL LEGAL STATUS OTHER STATUS

Photo ©John M. Randall/The Nature Conservancy

Bull thistle exhibits variation in several morphological characteristics that have been described as subspecies by some authors; however, the Flora Europaea does not recognize these taxa because they lack sufficient morphological or geographical delimitation [50]. Several hybrids of bull thistle have been described in Europe, and 1 suggested in California. See reviews by Klinkhamer and deJong [80] and Forcella and Randall [50] for more information.

DISTRIBUTION AND OCCURRENCE

• GENERAL DISTRIBUTION
• ECOSYSTEMS
• STATES
• BLM PHYSIOGRAPHIC REGIONS
• KUCHLER PLANT ASSOCIATIONS
• SAF COVER TYPES
• SRM (RANGELAND) COVER TYPES
• HABITAT TYPES AND PLANT COMMUNITIES

Although bull thistle has been reported in all 50 states and most Canadian provinces [75,153], it usually is not considered as problematic as musk thistle (Carduus nutans) or Scotch thistle (Onopordum acanthium) [15]. Bull thistle is most troublesome in recently or repeatedly disturbed areas such as pastures, overgrazed rangelands, forest clearcuts, and waste places; and along roads, ditches, and fences. It is also a problem in some natural areas such as Yosemite National Park, California [50].

The following lists reflect ecosystems and cover types in which bull thistle may be invasive. Because it is so widespread and has broad ecological tolerances, it is difficult to exclude many ecosystems as potential hosts of bull thistle plants or populations.

In the Pacific Northwest, bull thistle occurs on foothills and in dry meadows [69]. It occurs in riparian areas, clearcuts, and alder (Alnus spp.) flats in the western hemlock-Sitka spruce (Tsuga heterophylla-Picea sitchensis) zones on Olympic Peninsula in Washington [35]. Bull thistle also occurs in riparian areas and in ponderosa pine (Pinus ponderosa) communities on preserves in northeastern Oregon [129]. 

In the Intermountain region, bull thistle occurs primarily in disturbed habitats and in seepage areas or along streams [28]. It often establishes after disturbance (logging with and without burning) in grand fir (Abies grandis), Douglas-fir (Pseudotsuga menziesii), and ponderosa pine forests in Idaho [57,141], Oregon [27,45,104], and Montana [4,5]. Bull thistle often establishes after timber harvest with and without burning in western hemlock, western redcedar (Thuja plicata), and grand fir forests in northern Idaho [72]; and in grand fir and subalpine fir (Abies lasiocarpa) forests in western Montana [2,92,143]. Bull thistle is also invasive in grasslands on Thousand Springs Preserve in Idaho [129]. 

Bull thistle is found on tallgrass prairie sites on preserves in South Dakota [129]. In Michigan, bull thistle may establish after logging in oak-hickory (Quercus-Carya spp.) and maple (Acer spp.)-oak forests [29,85]. Here it may also establish after fire or ground disturbance such as bulldozing, and can also invade natural communities adjacent to disturbed habitats [157]. In Minnesota, bull thistle established after full-tree logging  in a balsam fir-paper birch (Abies balsamea-Betula papyrifera) community [113].

BOTANICAL AND ECOLOGICAL CHARACTERISTICS

• GENERAL BOTANICAL CHARACTERISTICS
• RAUNKIAER LIFE FORM
• REGENERATION PROCESSES
• SITE CHARACTERISTICS
• SUCCESSIONAL STATUS
• SEASONAL DEVELOPMENT

Bull thistle is a biennial, and sometimes annual or monocarpic perennial, forb. In the juvenile phase, individual bull thistle plants form a single rosette with a taproot up to 28 inches (70 cm) long. Rosettes may develop up to 3.3 feet (1 m) in diameter. The taproot does not spread, but develops several smaller lateral roots. Stems have spiny wings and grow 1 to 6.6 feet (0.3 to 2 m) tall, with many spreading branches, and sometimes a single stem. Bull thistle stem leaves are more or less lance-shaped and 3 to 12 inches (7.6-30 cm) long, prickly hairy on the top and very hairy underneath. Lobes on leaves are tipped with stout spines. Bull thistle flowerheads are 1.5 to 2 inches (3.8 to 5 cm) in diameter, 1 to 2 inches (2.5-5 cm) long, usually solitary, and more or less clustered at the ends of shoots and branches. Flowers are subtended by narrow, spine-tipped bracts. Bull thistle fruits are achenes, 1/16th-inch (0.15 cm) long, with a long, hairy plume that is easily detached.

The litter of Cirsium species is said to inhibit the growth of other plants. In bull thistle, this is probably a result of the immobilization of nutrients during the process of litter breakdown [80]. Descriptions of mycorrhizal associations in bull thistle [17,62] and their positive effects on its growth [163] are available.

Breeding system: Bull thistle reproduces and spreads entirely from seeds [80]. Bull thistle flowers are bisexual [80,122]. While there is some evidence of self-pollination, selfing may result only in hollow seeds; therefore, bull thistle may require cross-pollination to set fertile seed. Only those plants that flower during the main flowering period, or where plants are growing in sufficient density, will contribute substantially to the following generation [103]. Klinkhamer and de Jong [80] report that self-pollinated plants produce a smaller number of large seeds than do cross-pollinated individuals. A review by Forcella and Randall [50], however, indicates that heavy seeds may be produced through self-pollination, and that these seeds can establish at high rates and enable isolated plants to begin new populations.

Pollination: Bull thistle flowers produce abundant nectar [122] and require pollinators for effective pollination [103]. A wide variety of insects pollinate bull thistle [106]. Pollinators of bull thistle in New Zealand include honey bees, bumble bees, flower flies and various other adult Lepidoptera, Thysanoptera, and Hymenoptera [103].

Seed production: Bull thistle plants produce about 100 to 300 or more seeds per flowerhead under favorable conditions, and anywhere from 1 to over 400 flowerheads per plant [50,79,80,103,122]. Variability in production of seeds per flowerhead and flowerheads per plant yields a wide range in number of seeds produced per plant. Bull thistle seed production can also vary considerably among years and within populations [79]. Size of mature bull thistle plants, timing of flowering and environmental conditions can influence seed production.

The number of viable seeds produced by a bull thistle plant varies with its overall size [50,103], which is, in turn, influenced by competition, site conditions, and herbivory [50]. Seed production and seedling establishment are often enhanced under disturbed conditions, which create open, habitable sites for invasive species [15]. A comparison of bull thistle demography in grazed and ungrazed pastures in Australia found that plants produced nearly 3 times as much seed on average in heavily grazed pasture (33 flowerheads per plant and 198 seeds per flowerhead) compared to ungrazed pasture (19 flowerheads per plant and 149 seeds per flowerhead) when averaged over 3 years [49]. Bull thistle seed production on Dutch coastal dunes was much lower than that recorded in an experimental garden, and was positively correlated with July rainfall. Seed and stem predation contributed to losses of 95% to over 99% in seed production on coastal dunes [79,80].

Time of flowering also affects total seed production in bull thistle. Seed production is highest in flowerheads that bloom during the peak of flowering at a particular location. At this time, more flowers are available for cross-pollination, and pollinators are most active [49,103]. Forcella and Randall [50] suggest that a lack of sufficient photosynthate and nutrients during aging can decrease seed production substantially.

Seed dispersal: Bull thistle seeds are equipped with a feathery pappus that is suited to wind dispersal, although it is unclear how effective this dispersal mechanism is. Several researchers (e.g. [1,92,95,97,137,143,150]) cite instances where bull thistle established after disturbance, possibly from wind-dispersed seed, unless bull thistle plants established from seed carried by animals or human activities or from seed stored in soil (see "Seed banking" section below).

Michaux [103] notes that the pappus readily detaches from bull thistle seed at maturity, so a majority of seeds (91%) fall within a distance of 1.5 times the height of the parent plant. This explains the dense pattern of seedlings near the parent plant often observed in the field [103]. On coastal dunes in the Netherlands, seeds landing within 3, 7, and 106 feet (1, 2, and 32 m) of parent plants represented 50, 66, and 90% of those observed. The remaining 10% reached higher air levels and dispersed to unknown distances greater than 106 feet (32 m) [79]. Based on weight, size, shape, fall speed, and lateral movement of bull thistle achenes in still air, the estimated lateral dispersal distance in a 6-mile-per-hour (10 km/hour) breeze is 38 feet (11.6 m) [94]. These studies suggest that wind dispersal is an inefficient mechanism for the majority of bull thistle seed, even under ideal conditions. However, up to 10% of seeds produced may travel distances of more than 90 feet (27 m), even on days with little wind [122], thus providing opportunities for establishment of new populations.

Rapid migration of bull thistle across large geographical regions is probably the result of human activities including movement of livestock, vehicles, farm machines, and plant products (such as seed and hay) [15,50,103]. Reviews by Mitich [106] and Beck [15] also suggest that bull thistle seeds may be carried by water and animals.

Seed banking: Evidence for seed banking in bull thistle varies. Numerous examples of bull thistle establishment following disturbance suggest either long-distance seed dispersal or seed stored in the soil. Reviews by Doucet and Cavers [41], Michaux [103], and Forcella and Randall [50] indicate that bull thistle is characterized as having either a transient or a very small persistent seed bank. Doucet and Cavers [41] note that studies concluding that bull thistle has a short-lived seed bank (e.g. [79]) only consider seeds located on or near the soil surface, and that seeds buried at least 6 inches (15 cm) may have over 50% viability 3 years after burial. Bull thistle seeds at or near the soil surface either germinate or are destroyed by rodents, insects, or microbes [79]. Those buried at greater depths appear to experience an induced dormancy, and decay more slowly with increasing depth [38,80,103]. A seed bank at 6 inches (15 cm) or deeper will not maintain a bull thistle population from year to year, but it could provide seeds that would re-establish the population after major physical disturbance of the soil [41]. In noncultivated areas, however, bull thistle seeds are not usually buried to great depths.

To form a persistent seed bank, bull thistle seeds would have to exhibit some form of dormancy that would enable them to remain viable at or near the surface of the soil without germinating [41]. Klinkhamer and de Jong [80] state that maintaining bull thistle seeds in an imbibed state in darkness induces dormancy. This suggests that in densely vegetated habitats it is possible that bull thistle seeds may become dormant and remain viable without germinating. 

Bull thistle seeds from 2 populations in Ontario established a persistent seed bank when buried 6 inches (15 cm). After storage on the soil surface or at 1-inch (3 cm) depth, bull thistle seeds in sandy soil on an open site did not persist beyond 6 months. In contrast, 2 to 14% of bull thistle seeds stored in shaded conditions in a clay loam soil maintained viability for 30 months at the surface and at 1 inch below the surface, with no decline in the number of viable seeds over time, suggesting an induced dormancy [41]. Induced dormancy might also explain why, although bull thistle was not present in the vegetation of 40- to 60-year-old, closed canopy forest understories in low-elevation forest on the Olympic Peninsula, it germinated from both litter and soil samples when placed in a greenhouse. This result suggests that bull thistle will establish on these sites following overstory removal [61].

Evidence provided by Doucet and Cavers [41,42], and by greenhouse studies in which bull thistle emerged from litter and soil samples from forested sites with little to no bull thistle cover [81,118,146], supports the possibility of a small persistent seed bank in bull thistle. There are also numerous examples of bull thistle's early seral dominance after disturbances such as harvesting [4,5,27,29,57,60,72,113,128,131], burning [10,16], or both [4,5,45,85,104,141], although it is not clear from these studies whether bull thistle established from buried seed or from seed dispersed from an off-site source.

Germination: Bull thistle seed viability is generally high, and may vary between 60 and 90% [80,103] or more [122]. Reviews by Michaux [103] and Klinkhamer and de Jong [80] indicate that bull thistle seeds have little innate dormancy and germinate rapidly after imbibition, while a review by Beck [15] indicates that 60 to 75% of bull thistle seeds may be dormant at maturity, but up to 90% may germinate within a year. A review by Forcella and Randall [50] indicates that the timing of emergence of bull thistle seedlings results from the interaction of dormancy mechanisms, soil temperature, and rainfall patterns and will, therefore, vary by site and regional characteristics. Cavers and others [24] discuss the pattern of germination in bull thistle over time. Germination of bull thistle seeds is affected by moisture, light availability, gap size, and temperature.

Germination of bull thistle seeds typically occurs in spring or fall in response to adequate soil moisture [49,103,106,122]. Bull thistle seed germination is less sensitive to low water potential than that of several other thistle species. The relative germination of bull thistle seeds decreased linearly from 100% at 0 MPa to 10% at -0.75 MPa [58]. In Dutch sand dunes the number of emerging seedlings in spring was related to both soil moisture and soil nitrate levels, implying that, combined with temperature, these 2 factors help to regulate seed germination [33]. Downs and Cavers [43] found that germination rate in bull thistle was reduced after exposure to 2 or more cycles of wetting and drying, and that total percent germination was reduced after exposure to 8 cycles of wetting and drying. This evidence supports the idea that bull thistle seeds may acquire an induced dormancy through exposure to cycles of wetting and drying such as can be experienced in the uppermost layers of soil. 

Germination rate of bull thistle seeds tends to decrease as light decreases [80,103,118]. Doucet and Cavers [42] report that fresh bull thistle seeds are capable of germinating in either alternating light and dark or constant dark conditions under favorable diurnal temperatures of 77/50 degrees Fahrenheit (25/10 °C). In a laboratory study, seeds were stored over winter at 41 degrees Fahrenheit (5 °C) in either alternating light and dark, or in constant darkness. Seeds treated with alternating light and dark did not require light for germination when placed under optimal temperatures, whereas seeds treated with constant darkness did require additional light for germination. In the field, seeds that do not germinate in the fall and then spend the winter in darkness (e.g. in deep shade, under leaves, or buried by ants, earthworms or other animals) can acquire this induced dormancy and be prevented from germinating. Such seeds have the potential of forming a persistent seed bank [41,42]. Light requirement for germination of bull thistle seeds is also evidenced by higher germination rates in large (4-8 inches (10-20 cm)) gaps than in smaller gaps (Silvertown and Smith 1989, as cited by [80]). Bull thistle is dependent on canopy gaps for seedling emergence and establishment [20].

Bull thistle seeds germinate well over a wide range of temperatures [50]. Germination of bull thistle seeds is reduced if the temperature is outside the range of 50 to 86 degrees Fahrenheit (10-30 °C). Fresh seeds have higher optimum temperature for germination than stored seeds [80,103].

Seedling establishment/growth: Bull thistle seedling establishment is favored by soil disturbance and seedling growth is favored by vegetation disturbance. The absolute growth of bull thistle seedlings is very low for 2 months after sowing, even under ideal conditions [50]. Transition from seedlings to rosettes is when the greatest attrition in bull thistle populations typically occurs [49,80,121]. Bull thistle seedlings have higher survival rates under high nutrient conditions [11,49].

Bull thistle establishes better in grazed versus ungrazed pasture. About 15% and 10% of seeds from grazed and ungrazed pastures, respectively, produced seedlings, and the average survival of seedlings in grazed and ungrazed pastures was 1% and 0.2% respectively. Fifty percent of rosettes in both pasture types survived and grew into adults [49]. An annual census of 2 bull thistle-infested meadows in Yosemite National Park found that seedlings accounted for about 85% of deaths observed, 13% of mortality was rosettes, and less than 2% of mortality was due to individuals that died after flowering [121]. On Dutch coastal dunes, bull thistle seedling mortality is related to soil moisture content and tends to be high, with only 23 to 47% of seedlings surviving from spring to autumn. Yearly death of rosettes varies between 10 and 69%, with the chance of dying inversely related to rosette size [80]. Bull thistle rosettes that are top-killed under dry or cold stress in the 1st season can grow again from the root crown during late winter or spring [106].

The distribution of bull thistle in Eurasia is closely linked to that of cultivated land. In the U.S., bull thistle is common in regions with intensive cattle commerce. In Canada, it is present in agricultural areas but absent from prairies [80]. In California, bull thistle is widespread and most common in coastal grasslands, along edges of fresh and brackish marshes, in meadows, and in mesic forest openings in the mountains [122]. Landscape patch types where bull thistle was found on the Olympic Peninsula in Washington include riparian cobble bars, riparian shrub communities, and clearcuts [35].

Bull thistle is found on dry and wet soils, but is most common on soils with intermediate moisture [80,120]. It is largely absent from deeply shaded and waterlogged habitats. Bull thistle tolerates a wide range of pH values, though it is rare on soils of pH <4.8-5.0 [80]. It proliferates in pastures subject to nitrogen fertilization [15,80], but has no apparent relationship with potassium or phosphorus content. Bull thistle is less common in sand and on soils of >30% humus content, and almost absent from pure clay [80].

Bull thistle is found as far north as 67°50' N latitude in Scandinavia. In shade, bull thistle is restricted to south-facing slopes. In dry habitats such as coastal dunes it is confined to north-facing slopes [80]. It has been suggested that bull thistle plants are not very invasive in the southern part of its North American range. This has been attributed to a peculiarity in their dispersal biology, where the achenes remain enclosed inside the seedheads after they are fully mature and are released only at the end of the season when the whole plant eventually falls over. Numerous achenes then germinate in situ in a cluster, but apparently only 1 of them survives to maturity [112]. 

Infestations of bull thistle in North America have been reported as high as 9,200 feet (2,800 m) [37]. Bull thistle was found on alpine sites in Utah [65], and on a subalpine riparian site in Montana [91]. The following table provides some elevational ranges for bull thistle by state:

Populations of bull thistle tend to be short lived, establishing after disturbance, dominating for a few years, and then declining as other vegetation recovers [27,33,41,95,96,155,167]. Few bull thistle plants can be found in undisturbed clearcuts and plantations older than 8 years [128], although some plants may remain for longer periods. After clearcutting of subalpine fir in western Montana, bull thistle cover peaked after 3 years, was still present after 17 years, but was not present on undisturbed sites or 1-year-old cuts [92]. Bull thistle was present 7 to 16 years after clearcutting in grand fir in western Montana, but absent from adjacent uncut forest [2]. Bull thistle was the most frequent species observed 6 to 9 years after clearcutting in Sierra Nevada mixed conifer [1]. The  peak distribution of bull thistle in German old fields is 3 to 4 years following disturbance [80]. Specific patterns of succession are described for Douglas-fir and ponderosa pine forests in California and Oregon [98] and for burned Douglas-fir clearcuts in the Coast range of western Oregon, where bull thistle is the dominant species the 2nd year after clearcutting [131].

True biennials are uncommon or absent in late successional plant communities because they often need abundant light for establishment [50]. Doucet and Cavers [41] note that bull thistle is absent from densely shaded areas. A review by Klinkhamer and de Jong [80] indicates that bull thistle is almost absent if light is reduced to less than 40% of full sunlight. Bull thistle invasion is enhanced in pastures with decreased vegetative cover [49]. In a greenhouse experiment, bull thistle germination was not suppressed by sedge (Carex spp.) cover, but subsequent survival of seedlings was reduced and the percentage of seeds that germinated and survived decreased exponentially with increasing cover [121]. In Dutch coastal dunes, however, bull thistle was more restricted to shaded sites [34].

Seed dispersal immediately follows maturation [49]. Individual flowers last for only a few days before the floral tissue wilts and the involucre bracts turn brown and open out -- the time taken depends on relative humidity. The pappus is thus exposed to air currents, and the innermost seeds are dispersed 1st as the receptacle arches upwards. A single plant may remain in a flowering state anywhere from 1 to 6 weeks, depending on the number of flowers. A population of bull thistle in New Zealand flowered for a total of 5 months [103]. Bull thistle plants are monocarpic, dying after they set seed [50]. Dead plants can remain standing for 1 or 2 years.

Bull thistle requires a vernalization period before bolting. There is no size requirement for vernalization; however, plants need to attain a certain size after vernalization to bolt and flower. Some plants (about 1%) may flower without vernalization, and this may vary with location [80]. At Yosemite National Park, 2 individuals less than a year old flowered in September [121]. Evidence presented by Wesselingh and others [162] suggests a geographical variation in the vernalization requirement among European populations of bull thistle, based on observations of a latitudinal gradient in vernalization requirement in other biennials. In a common garden experiment, bull thistle plants from southern Europe tended to exhibit an annual habit, bolting and flowering without vernalization. Those from northern populations acted as biennials, flowering after vernalization in their 2nd year. It is unknown if these plants exhibit annual or biennial life cycles at their place of origin. Klinkhamer and others [79] indicate that bull thistle plants may behave as winter annuals if they germinate in fall, are vernalized over winter, and grow luxuriantly in early spring. Plants behave as monocarpic perennials when growth was stunted because of conditions such as infertile soils and inclement weather [50].

Typical flowering dates are reported by area as follows: 

FIRE ECOLOGY

• FIRE ECOLOGY OR ADAPTATIONS
• POSTFIRE REGENERATION STRATEGY

Bull thistle is the most common and widespread thistle of pastures and rangelands in western North America, so it occurs in a large number of ecosystems with different fire regimes. Introduced species can alter the rate of fire spread, the probability of fire occurrence, and the intensity of fire in an ecosystem [30]. It is unclear how the presence of bull thistle alters the fire regime of a given site, and it is unclear how a historical fire regime might affect the presence or abundance of bull thistle at a given site. Dominant species of forest communities in which bull thistle has been noted as a primary or secondary colonizer after disturbance are described in Habitat Types and Plant Communities. Bull thistle also occurs in tallgrass prairie ecosystems, where fire can stimulate flowering of native grasses [31]. In Kansas, frequent burning of tallgrass prairie is said to be effective in keeping out exotic plants on sites where prairie grasses are vigorous [70]. Bull thistle did not occur in any of these communities at the time in which historic fire regimes were functioning, but has established since fire exclusion began. It is unclear how the presence of bull thistle might affect fire regimes in these communities.

Because it is so widespread and has broad ecological tolerances, it is difficult to exclude many ecosystems as potential hosts of bull thistle plants or populations. The following table provides fire return intervals for several plant communities in which bull thistle may be found. Find further fire regime information for the plant communities in which this species may occur by entering the species name in the FEIS home page under "Find Fire Regimes".

FIRE EFFECTS

• IMMEDIATE FIRE EFFECT ON PLANT
• DISCUSSION AND QUALIFICATION OF FIRE EFFECT
• PLANT RESPONSE TO FIRE
• DISCUSSION AND QUALIFICATION OF PLANT RESPONSE
• FIRE MANAGEMENT CONSIDERATIONS

It is also unclear what effects fire has on bull thistle seeds in the soil. Incidents of rapid colonization after fire [10,16,102] suggest that either bull thistle seeds were present in the soil at the time of the fire and survived to germinate after the overstory was removed, or that bull thistle seeds were dispersed after fire from off-site seed sources. However, when experimental heat treatments including 6 combinations of temperature, duration, and soil moisture were applied to bull thistle seeds from an old-growth Douglas-fir forest seed bank, researchers concluded that even low-severity fire could kill bull thistle seeds. Seed survival was lower in wet soil than in dry soil. In wet soil, 35% of the bull thistle seeds tested survived 122 degrees Fahrenheit (50 °C) for 60 minutes, and 0 seeds survived 167 degrees Fahrenheit (75 °C) or 212 degrees Fahrenheit (100 °C) for 15 minutes. In dry soil, 44% survived 122 degrees Fahrenheit (50 °C) for 60 minutes, 32% survived 167 degrees Fahrenheit (75 °C) for 15 minutes, and 6% survived 212 degrees Fahrenheit (100 °C) for 15 minutes [25].

Fire can create conditions that are favorable to the establishment of bull thistle such as an open canopy and areas of bare soil, so if bull thistle seeds are present in or dispersed into the burned area, it is likely to establish in the postfire community. Bull thistle densities increased dramatically after a prescribed burn in Yosemite Valley, leading managers to believe that burning promotes thistle populations. It is unclear, however, whether prescribed burning alone caused the increase in bull thistle cover [121].

Observations in tallgrass prairie sites in South Dakota indicate that late spring prescribed burning on a 4- to 5-year rotation encourages the growth of native plants and discourages the growth of Canada thistle (Cirsium arvense), musk thistle, and bull thistle [31,129]. Additionally, Hulbert [70] suggests that late spring burning in these ecosystems results in fewer forbs but greater grass production than fall or early spring burning.

A spring prescribed fire following clearcutting in 1968 on Miller Creek in western Montana (when the lower half of the duff was still wet from snowmelt and rain) left a continuous, intact duff mantle as a seedbed and killed the aerial portions of understory herbs and shrubs. Forest succession then began with regrowth of heartleaf arnica (Arnica cordifolia) and beargrass (Xerophyllum tenax) and establishment of the offsite colonizers fireweed (Epilobium angustifolium) and bull thistle. Other sites in the area that were harvested during the same time period but either unburned or burned in summer or fall did not have bull thistle in the postdisturbance plant community [137].

Spring prescribed burning in a basin big sagebrush community in east-central Oregon had no significant effect on bull thistle frequency in postfire year 1 [134]. See the Research Project Summary of this work for more information on fire effects on bull thistle and 60 additional forb, grass, and woody plant species.

Lyon's Research Paper and the Research Project Summary Vegetation response to restoration treatments in ponderosa pine-Douglas-fir forests of western Montana also provide information on prescribed fire and postfire response of bull thistle and other plant community species.

More research is needed on short- and long-term secondary effects of fire on bull thistle. See "Postfire colonization potential" below for more details.

Postfire colonization potential: General precautions should be followed to prevent bull thistle establishment after fire. The USDA Forest Service's "Guide to noxious weed prevention practices" [152] provides several fire management considerations for weed prevention in general that can be applied to bull thistle. Fire managers might consider including weed prevention education and providing weed identification aids during fire training; avoiding known weed infestations when locating firelines, monitoring camps, staging areas, and helibases to be sure they are kept weed free; taking care that equipment is weed free; incorporating cost of weed prevention and management into fire rehabilitation plans; and acquiring restoration funding [152]. Careful postfire vigilance to identify and record the establishment of new populations is critical. About 1 month after fire, survey for signs of new or resprouting weeds. Repeated surveys will be needed, with the frequency and intensity guided by local conditions [9].

MANAGEMENT CONSIDERATIONS

• IMPORTANCE TO LIVESTOCK AND WILDLIFE
• OTHER USES
• IMPACTS AND CONTROL

Studies in the Netherlands indicate that rabbits eat bull thistle leaves, especially in winter and early spring. The flowering stem may be attacked by a variety of herbivores, which can result in a reduced seed output. If the main stem is severed or damaged by herbivores, secondary flowering stems can form. Recovery of bull thistle after herbivore damage is dependent on moisture availability. Bull thistle seeds are eaten by mice and voles. Birds sometimes eat seeds in the Dutch coastal dune area [80]. Goldfinches eat bull thistle seeds and use the thistledown to build their nests [106]. Bull thistle is included in a list of known grizzly bear food plants [32]. Bull thistle is eaten by Mazama pocket gophers in south-central Oregon [22], and high bull thistle densities were observed in Yosemite National Park in sites of intense pocket gopher digging. Pocket gophers consume taproots from below, and their digging provides sites for further thistle establishment, so that they are effectively "farming" thistles [121]. Bull thistle flower nectar is a favorite of bees and butterflies [84].

Palatability/nutritional value: No information

Cover value: No information

Bull thistle often dominates recently clearcut forest areas in the Sierra Nevada of California, and infestations may limit growth of replanted tree seedlings. Work in a replanted Sierra clearcut forest indicated that stem growth of ponderosa pine saplings was negatively correlated with density of thistles within about 7 feet (2 m) of pines [123]. Bull thistle also colonizes and maintains high population densities for up to 6 years in clearcuts in redwood and mixed evergreen forests in northwestern California [122].  

Control: Bull thistle should be accurately identified before attempting any control measures, since several native species of thistles have a similar appearance. See Klinkhamer and de Jong [80] and General Botanical Characteristics for information on proper identification.

The key to successful management of bull thistle is to prevent seed production. Combining control methods into an integrated management system will result in the best long-term population decreases. Control data suggest that viable seed production by biennial thistles must be eliminated to achieve long-term population decreases, although zero seed production may not be a realistic goal. The transition from seedling to rosette in bull thistle may be the most precarious stage in its life cycle. Seedling and rosette growth stages are the most logical to target for control efforts in biennial thistles. 

Desirable plant competition to deter establishment of bull thistle seedlings is a critical part of any biennial thistle management strategy. Recovery of infested areas should not be considered complete until a diverse population of  desirable plants has replaced invasive biennial thistles, and bull thistles are a minor to nonexistent component of the plant community. Always monitor and evaluate weed management programs to determine whether and when to repeat and/or modify control treatments [15].

Prevention: Prevention is the most effective method for managing invasive species, including bull thistle [15,139]. Preventing or dramatically reducing seed production will help decrease the spread of infestations. This is accomplished by cleaning mowers, vehicles, and tillage equipment after operation in an infested area. When seeding is necessary, use clean, certified weed-free seed and mulch to ensure that bull thistle or other weeds are not being sown. Preventing the establishment of weeds in natural areas is achieved by maintaining healthy natural communities and by conducting aggressive monitoring several times each year. Monitoring efforts are best concentrated on the most disturbed areas in a site, particularly along roadsides, parking lots, fencelines, and waterways. When an infestation is found, the location can be recorded and the surrounding area surveyed to determine the size and extent of the infestation, so these sites can be revisited on follow-up surveys. For more on monitoring see Johnson [74]. Place a priority on controlling small infestations so they do not expand [15,74].

Good grazing management will stimulate grass growth and keep pastures and rangelands healthy. Healthy pastures and rangeland may be more resistant to biennial thistle invasion. Bare spots caused by overgrazing are prime habitable sites for biennial thistles. In many instances, grazing lands will have to be rested from grazing for grasses to recover. This should be coupled with precipitation cycles, so adequate soil moisture will be available to stimulate grass growth. Grazed pastures that are managed carefully may enhance grass competition and deter thistle survival from seedlings to rosettes [15].

Weed prevention and control can be incorporated into all types of management plans, including logging and site preparation, management of grazing allotments, recreation management, research projects, road building and maintenance, and fire management [152]. See the "Guide to noxious weed prevention practices" [152] for specific guidelines in preventing the spread of weed seeds and propagules under different management conditions.

Integrated management: The goal of any management plan should be not only controlling invasive plants, but also improving the affected community, maximizing forage quality and quantity and/or preserving ecosystem integrity, and preventing reinvasion or invasion by other invasive species, in a way that is complementary to the ecology and economics of the site [40,73]. Effective long-term control requires that invasive plants be removed and replaced by more desirable and weed-resistant plant communities [73]. Once the desired plant community has been determined, an integrated weed management strategy can be developed to direct succession toward that plant community by identifying key mechanisms and processes directing plant community dynamics (site availability, species availability, and species performance) and predicting plant community response to control measures [138]. This requires a long-term integrated management plan [15].

Most often, a single method is not effective for controlling an invasive plant, and many possible combinations of methods can achieve the desired objectives. Methods selected for removal or control of bull thistle on a specific site will be determined by land use objectives, desired plant community, extent and nature of the infestation(s), environmental factors (nontarget vegetation, soil types, climatic conditions, important water resources), economics, and effectiveness and limitations of available control techniques [126]. Killing thistles and decreasing weed populations must be followed by the establishment of desirable vegetation in the newly opened niches; herbicide applications in spring followed by dormant seeding of competitive perennial grasses in the fall is an example of an effective management system for biennial thistles in the western U.S. Similarly, integrating herbicides and biological control agents is likely to be more effective than insects alone [15] (see "Biological control" below, for more information). For information on integrated weed management without herbicides, see the Bio-Integral Resource Center (BIRC) website.

Some examples of combined approaches and considerations for managing bull thistle infestations are presented within the following sections. Managers are encouraged to use combinations of control techniques in a manner that is appropriate to the site objectives, desired plant community, available resources, and timing of applications.

Physical/mechanical: Any mechanical or physical method that severs the root below the soil surface will kill bull thistle plants. However, it is essential to re-vegetate the site with desirable plants to compete with bull thistle that may reinvade from seeds left in the soil. Tillage, hoeing, and hand pulling may provide effective control, providing these operations are done before the reproductive growth stages to prevent seed production. Mowing alone is not an effective control measure for biennial thistles, because some seed will still be produced. Mechanical methods may not be practical on rangeland and natural areas, but could be useful in improved pastures or roadsides [15]. The long duration of flowering in bull thistle increases the importance of timely control operations and may make repeated treatments necessary [49,122].

A single mowing will not control a bull or musk thistle infestation, because infestations often consist of plants of various ages, and stands therefore have nonuniform development and flowering. Bull thistle plants mowed just before seed dispersal do not produce seed or recover well [50,80,122]. If mowed too early, bull thistle plants resprout and flower. About 4% of bull thistles cut 2 to 4 inches (5-10 cm) above the soil surface a month before flowering resprout [121,122].

Bull thistle will not withstand cultivation; however, tillage is not appropriate in wildlands and rangelands since it can damage important desirable species, increase erosion, alter soil structure, and expose the soil for rapid reinfestation by bull thistle and other invasive species [88]. Slicing off the root crown of bull thistle plants is time consuming, but very effective [129]. At Yosemite National Park, less than 5% of adult bull thistles cut at the soil surface resprouted, while over 80% of adult bull thistles in control plots survived and flowered [120,121]. Of the bull thistle plants that resprouted, mean height and number of inflorescences were lower (25 inches or 63 cm and 3.7 flowerheads ) than for adults in control plots (33 inches or 85 cm and 15.8 flowerheads) [121,122].  Plants that were cut at the root crown a few days after their 1st flowers appeared and then laid on the ground produced abundant viable seed, so removing cut stems from areas being cleared may be important [50,122].

Even if bull thistle plants resprout after mechanical control, populations may be reduced by limiting seed production [122]. Removal of adult bull thistle plants must be repeated annually for 4 years or more, since some plants will stay in the rosette form for up to 5 years [121]. Mechanical control may be labor intensive; however, sometimes volunteer groups are available. The Salmon River Restoration Council (SRRC) provides an example of watershed-scale weed control using primarily mechanical control methods.

Fire: See Fire Management Considerations.

Biological: Biological control of invasive species has a long history, and there are many important considerations to be made before the implementation of a biological control program. The reader is referred to other sources [126,164] and the Weed Control Methods Handbook [151] for background information on biological control. Additionally, Cornell University, Texas A & M University, and NAPIS websites offer information on biological control.

In its native range, number of viable seeds produced by bull thistle plants can be greatly reduced by insects feeding on the stem, flowerheads, or seeds [80]. Several agents have been considered and tested for bull thistle control, and those in the following table have been introduced in North America:

Rhinocyllus conicus was introduced from Europe to Montana and Virginia in 1969 to control musk thistle, but it also uses bull thistle. Rhinocyllus conicus will use Carduus, Cirsium, Silybum, and Onopordum genera as hosts but prefers the musk thistle group [127]. In areas where the plant and insect life cycles are synchronized, R. conicus is extremely effective in reducing seed production in musk thistle [125]. It is unclear if it is as effective on bull thistle. Several strains of R. conicus have been identified and they vary in their utilization of various thistle species. At least 1 of these strains does attack some native Cirsium species [89,125], and reviews by Randall [122], Beck [15] and Wilson and McCaffrey [164] indicate that it is known to attack native and rare thistles. Therefore, before releasing insects in a new area containing native Cirsium species, investigate whether any of the local species may be attacked [125]. A detailed discussion of the biology of R. conicus is given by Harris and Shorthouse [63].

Urophora stylata feeds on developing seeds in bull thistle flowerheads and decreases seed production up to 60% [26]. Trichosirocalus horridus was introduced to the U.S. in 1974. This weevil uses thistles of the subtribe Carduinae, including bull thistle, musk thistle, plumeless thistle (Carduus acanthoides), Italian thistle (C. pycnocephalus), Canada thistle, and Scotch thistle. Reports of suppression vary from slight to great. Trichosirocalus horridus is more effective when used in conjunction with R. conicus [125]. In areas of Missouri where R. conicus and T. horridus have been present for over 15 years, an 80 to 90% reduction in thistle populations has occurred [142].

Chemical: Herbicides are effective in gaining initial control of a new invasion or a severe infestation, but are rarely a complete or long-term solution to weed management [23]. Herbicides are more effective on large infestations when incorporated into long-term management plans that include replacement of weeds with desirable species, careful land use management, and prevention of new infestations. Control with herbicides is temporary, as it does not change conditions that allow infestations to occur [169]. See the Weed Control Methods Handbook for considerations on the use of herbicides in natural areas and detailed information on specific chemicals. 

Chemical control of bull thistle is reviewed by Beck [15], Forcella and Randall [50], and Randall [122]. Clopyralid, dicamba, MCPA, picloram, 2,4-D, metsulfuron, and chlorsulfuron will all kill bull and musk thistles. Timing of application is important. Autumn is a good time to control biennial thistles with herbicides because all live plants will be seedlings or rosettes, and plants are easiest to control in the seedling and rosette stages. Plants are, however, more difficult to locate at this stage, and cold weather may decrease the effectiveness of some chemicals. Herbicide choice and rates are influenced by growth stage, stand density, and environmental conditions (e.g. drought or cold temperatures). Check with state or county weed specialists for appropriate local use rates and timing. Bull thistle is less aggressive and easier to control than other biennial thistles [14].

In pastures and range containing appreciable quantities of broadleaf forage species, application of any of the herbicides listed above may damage valuable plants and reduce forage production and livestock weight gain as much as that caused by thistle interference [64], so it is important to prevent these and other non-target effects of chemical control.

Cultural: Bull thistle germination and establishment is favored in open areas and by disturbance [15]. No matter what method is used to kill weeds, reestablishment of competitive, desirable plant cover is imperative for long-term control. Fertilization and reseeding with competitive, adapted species is often necessary in areas without a residual understory of desirable plants [126]. 

Revegetation with aggressive desirable species has been shown to inhibit reinvasion of bull thistle, especially with the help of effective biological control agents and carefully prescribed grazing practices. Promoting desirable competitors is important both after weed control and before weed establishment. Choice of species to sow will depend upon climate, location, and management objectives. The Natural Resource Conservation Service and land grant universities are good sources of information about appropriate perennial grass species for a particular locale. Management that allows grasses to grow taller in spring to shade bull thistle seedlings may decrease seedling establishment and growth [15]. 

Cirsium vulgare: REFERENCES

1. Allen, Barbara H.; Bartolome, James W. 1989. Cattle grazing effects on understory cover and tree growth in mixed conifer clearcuts. Northwest Science. 63(5): 214-220. [10932]

2. Antos, Joseph A.; Shearer, Raymond C. 1980. Vegetation development on disturbed grand fir sites, Swan Valley, northwestern Montana. Res. Pap. INT-251. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station. 26 p. [7269]

3. Arno, Stephen F. 1980. Forest fire history in the Northern Rockies. Journal of Forestry. 78(8): 460-465. [11990]

4. Arno, Stephen F. 1996. Percent coverage for selected plant species at Lick Creek under different prescribed fire treatments. Unpublished data on file at: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Lab, Missoula, MT. 1 p. [37323]

5. Arno, Stephen F. 1999. Undergrowth response, shelterwood cutting unit. In: Smith, Helen Y., Arno, Stephen F., eds. Eighty-eight years of change in a managed ponderosa pine forest. Gen. Tech. Rep. RMRS-GTR-23. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 36-37. [+ Appendix C: Summary of vegetation changes in shelterwood cutting unit]. [38264]

6. Arno, Stephen F. 2000. Fire in western forest ecosystems. In: Brown, James K.; Smith, Jane Kapler, eds. Wildland fire in ecosystems: Effects of fire on flora. Gen. Tech. Rep. RMRS-GTR-42-vol. 2. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 97-120. [36984]

7. Arno, Stephen F.; Gruell, George E. 1983. Fire history at the forest-grassland ecotone in southwestern Montana. Journal of Range Management. 36(3): 332-336. [342]

8. Arno, Stephen F.; Wilson, Andrew E. 1986. Dating past fires in curlleaf mountain-mahogany communities. Journal of Range Management. 39(3): 241-243. [350]

9. Asher, Jerry E.; Dewey, Steve; Johnson, Curt; Olivarez, Jim. 2001. Reducing the spread of invasive exotic plants following fire in western forests, deserts, and grasslands. In: Galley, Krista E. M.; Wilson, Tyrone P., eds. Proceedings of the invasive species workshop: The role of fire in the control and spread of invasive species; Fire conference 2000: the first national congress on fire ecology, prevention, and management; 2000 November 27 - December 1; San Diego, CA. Misc. Publ. No. 11. Tallahassee, FL: Tall Timbers Research Station: 102-103. Abstract. [40681]

10. Ashton, D. H. 1981. Fire in tall open-forests (wet sclerophyll forests). In: Gill, A. M.; Groves, R. H.; Noble, I. R., eds. Fire and the Australian biota. Canberra City, ACT: The Australian Academy of Science: 339-366. [21566]

11. Austin, M. P.; Groves, R. H.; Fresco, L. M. F.; Kaye, P. E. 1985. Relative growth of six thistle species along a nutrient gradient with multispecies competition. Journal of Ecology. 73: 667-684. [41053]

12. Baisan, Christopher H.; Swetnam, Thomas W. 1990. Fire history on a desert mountain range: Rincon Mountain Wilderness, Arizona, U.S.A. Canadian Journal of Forest Research. 20: 1559-1569. [14986]

13. Bare, Janet E. 1979. Wildflowers and weeds of Kansas. Lawrence, KS: The Regents Press of Kansas. 509 p. [3801]

14. Beck, K. George. 1991. Biennial thistle control with herbicides. In: James, Lynn F.; Evans, John O.; Ralphs, Michael H.; Child, R. Dennis, eds. Noxious range weeds. Westview Special Studies in Agricultural Science and Policy. Boulder, CO: Westview Press: 254-259. [23553]

15. Beck, K. George. 1999. Biennial thistles. In: Sheley, Roger L.; Petroff, Janet K., eds. Biology and management of noxious rangeland weeds. Corvallis, OR: Oregon State University Press: 145-161. [35718]

16. Benson, Nathan C.; Kurth, Laurie L. 1995. Vegetation establishment on rehabilitated bulldozer lines after the 1988 Red Bench Fire in Glacier National Park. In: Brown, James K.; Mutch, Robert W.; Spoon, Charles W.; Wakimoto, Ronald H., technical coordinators. Proceedings: symposium on fire in wilderness and park management; 1993 March 30 - April 1; Missoula, MT. Gen. Tech. Rep. INT-GTR-320. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station: 164-167. [26216]

17. Berch, Shannon M.; Gamiet, Sharmin; Deom, Elisabeth. 1988. Mycorrhizal status of some plants of southwestern British Columbia. Canadian Journal of Botany. 66: 1924-1928. [8841]

18. Bernard, Stephen R.; Brown, Kenneth F. 1977. Distribution of mammals, reptiles, and amphibians by BLM physiographic regions and A.W. Kuchler's associations for the eleven western states. Tech. Note 301. Denver, CO: U.S. Department of the Interior, Bureau of Land Management. 169 p. [434]

19. Buchholz, Kenneth; Good, Ralph E. 1982. Density, age structure, biomass and net annual aboveground productivity of dwarfed Pinus rigida Moll. from the New Jersey Pine Barren Plains. Bulletin of the Torrey Botanical Club. 109(1): 24-34. [8639]

20. Bullock, J. M.; Hill, Clear B.; Dale, M. P.; Silvertown, J. 1994. An experimental study of the effect of sheep grazing on vegetation change in a species-poor grassland and the role of seedling recruitment into gaps. Journal of Applied Ecology. 31: 493-507. [24351]

21. Burkhardt, Wayne J.; Tisdale, E. W. 1976. Causes of juniper invasion in southwestern Idaho. Ecology. 57: 472-484. [565]

22. Burton, Douglas H.; Black, Hugh C. 1978. Feeding habits of Mazama pocket gophers in south-central Oregon. Journal of Wildlife Management. 42(2): 383-390. [15818]

23. Bussan, Alvin J.; Dyer, William E. 1999. Herbicides and rangeland. In: Sheley, Roger L.; Petroff, Janet K., eds. Biology and management of noxious rangeland weeds. Corvallis, OR: Oregon State University Press: 116-132. [35716]

24. Cavers, Paul B.; Qaderi, Mirwais M.; Downs, Michael P.; [and others]. 1998. The thistles: a spectrum of seed banks. In: In: Weed seedbanks: determination, dynamics and manipulation: Proceedings; 1998 March 23-24; Oxford, UK. Aspects of Applied Biology 51. [Place of publication unknown]: [Publisher unknown]: 135-141. [41129]

25. Clark, Deborah L.; Wilson, Mark V. 1994. Heat-treatment effects on seed bank species of an old-growth Douglas-fir forest. Northwest Science. 68(1): 1-5. [25904]

26. Coombs, E. M.; Piper, G. L.; Rees, N. E. 1996. Bull thistle: Cirsium vulgare. In: Rees, Norman E.; Quimby, Paul C., Jr.; Piper, Gary L.; [and others], eds. Biological control of weeds in the West. Bozeman, MT: Western Society of Weed Science. In cooperation with: U.S. Department of Agriculture, Agricultural Research Service; Montana Department of Agriculture; Montana State University: Section II. [40872]

27. Cox, Stephen William. 1970. Microsite selection of resident and invading plant species following logging and slash burning on Douglas fir clear-cuts in the Oregon Coast Range. Corvallis, OR: Oregon State University. 49 p. Thesis. [29736]

28. Cronquist, Arthur; Holmgren, Arthur H.; Holmgren, Noel H.; Reveal, James L.; Holmgren, Patricia K. 1994. Intermountain flora: Vascular plants of the Intermountain West, U.S.A. Vol. 5: Asterales. New York: The New York Botanical Garden. 496 p. [28653]

29. Crow, T. R.; Mroz, G. D.; Gale, M. R. 1991. Regrowth and nutrient accumulations following whole-tree harvesting of a maple-oak forest. Canadian Journal of Forest Research. 21: 1305-1315. [16600]

30. D'Antonio, Carla M. 2000. Fire, plant invasions, and global changes. In: Mooney, Harold A.; Hobbs, Richard J., eds. Invasive species in a changing world. Washington, DC: Island Press: 65-93. [37679]

31. Dailey, Ryan. 2001. Fire and thistles [Email to Kris Zouhar]. Sioux Falls, SD: The Nature Conservancy of the Dakotas, South Dakota. On file at: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory, Missoula, MT; RWU 4403 files. [38366]

32. Davis, Dan; Butterfield, Bart. 1991. The Bitterroot Grizzly Bear Evaluation Area: A report to the Bitterroot Technical Review Team. Unpublished report on file with: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory, Missoula, MT. 56 p. [30041]

33. de Jong, Tom J.; Klinkhamer, Peter G. L. 1988. Population ecology of the biennials Cirsium vulgare and Cynoglossum officinale in a coastal sand-dune area. Journal of Ecology. 76: 366-382. [5008]

34. de Jong, Tom J.; Klinkhamer, Peter G. L. 1988. Seedling establishment of the biennials Cirsium vulgare and Cynoglossum officinale in a sand-dune area: the implications of water for differential survival and growth. Journal of Ecology. 767: 393-402. [3517]

35. DeFerrari, Collette M.; Naiman, Robert J. 1994. A multi-scale assessment of the occurrence of exotic plants on the Olympic Peninsula, Washington. Journal of Vegetation Science. 5: 247-258. [23698]

36. DeLoach, C. Jack. 1997. Biological control of weeds in the United States and Canada. In: Luken, James O.; Thieret, John W., eds. Assessment and management of plant invasions. New York: Springer-Verlag: 172-194. [38164]

37. Dewey, Steven A. 1991. Weedy thistles of the western United States. In: James, Lynn F.; Evans, John O.; Ralphs, Michael H.; Child, R. Dennis, eds. Noxious range weeds. Westview Special Studies in Agricultural Science and Policy. Boulder, CO: Westview Press: 247-253. [23552]

38. Dickie, J. B.; Gajjar, Kamini H.; Birch, P.; Harris, J. A. 1988. The survival of viable seeds in stored topsoil from opencast coal workings and its implications for site restoration. Biological Conservation. 43: 257-265. [16671]

39. Diggs, George M., Jr.; Lipscomb, Barney L.; O'Kennon, Robert J. 1999. Illustrated flora of north-central Texas. Sida Botanical Miscellany, No. 16. Fort Worth, TX: Botanical Research Institute of Texas. 1626 p. [35698]

40. DiTomaso, Joe. 2001. Element stewardship abstract: Centaurea solstitialis L. In: Weeds on the web: The Nature Conservancy wildland invasive species program, [Online]. Available: http://tncweeds.ucdavis.edu/esadocs/documnts/centsols.html [2001, December 19]. [40416]

41. Doucet, Colleen; Cavers, Paul B. 1996. A persistent seed bank of the bull thistle Cirsium vulgare. Canadian Journal of Botany. 74: 1386-1391. [27089]

42. Doucet, Colleen; Cavers, Paul B. 1997. Induced dormancy and colour polymorphism in seeds of the bull thistle Cirsium vulgare (Savi) Ten. Seed Science Research. 7(4): 399-407. [41134]

43. Downs, Michael P.; Cavers, Paul B. 2000. Effects of wetting and drying on seed germination and seedling emergence of bull thistle, Cirsium vulgare (Savi) Ten. Canadian Journal of Botany. 78(12): 1545-1551. [41135]

44. Duchesne, Luc C.; Hawkes, Brad C. 2000. Fire in northern ecosystems. In: Brown, James K.; Smith, Jane Kapler, eds. Wildland fire in ecosystems: Effects of fire on flora. Gen. Tech. Rep. RMRS-GTR-42-vol. 2. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 35-51. [36982]

45. Edgerton, Paul J. 1987. Influence of ungulates on the development of the shrub understory of an upper slope mixed conifer forest. In: Provenza, Frederick D.; Flinders, Jerran T.; McArthur, E. Durant, compilers. Proceedings--symposium on plant-herbivore interactions; 1985 August 7-9; Snowbird, UT. Gen. Tech. Rep. INT-222. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station: 162-167. [7411]

46. Elias, Thomas S.; Dykeman, Peter A. 1982. Field guide to North American edible wild plants. New York: Outdoor Life Books. 286 p. [21103]

47. Eyre, F. H., ed. 1980. Forest cover types of the United States and Canada. Washington, DC: Society of American Foresters. 148 p. [905]

48. Finney, Mark A.; Martin, Robert E. 1989. Fire history in a Sequoia sempervirens forest at Salt Point State Park, California. Canadian Journal of Forest Research. 19: 1451-1457. [9845]

49. Forcella, F.; Wood, Helen. 1986. Demography and control of Cirsium vulgare (Savi) Ten. in relation to grazing. Weed Research. 26(3): 199-206. [41127]

50. Forcella, Frank; Randall, John M. 1994. Biology of bull thistle, Cirsium vulgare (Savi) Tenore. Review of Weed Science. 6: 29-50. [41130]

51. Frissell, Sidney S., Jr. 1968. A fire chronology for Itasca State Park, Minnesota. Minnesota Forestry Research Notes No. 196. Minneapolis, MN: University of Minnesota. 2 p. [34527]

52. Garrison, George A.; Bjugstad, Ardell J.; Duncan, Don A.; Lewis, Mont E.; Smith, Dixie R. 1977. Vegetation and environmental features of forest and range ecosystems. Agric. Handb. 475. Washington, DC: U.S. Department of Agriculture, Forest Service. 68 p. [998]

53. Gleason, Henry A.; Cronquist, Arthur. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. 2nd ed. New York: New York Botanical Garden. 910 p. [20329]

54. Goodwin, Kim M.; Sheley, Roger L. 2001. What to do when fires fuel weeds: A step-by-step guide for managing invasive plants after a wildfire. Rangelands. 23(6): 15-21. [40399]

55. Gourley, Mark; Vomocil, Marc; Newton, Michael. 1990. Forest weeding reduces the effect of deer-browsing on Douglas-fir. Forest Ecology and Management. 36: 177-185. [13064]

56. Great Plains Flora Association. 1986. Flora of the Great Plains. Lawrence, KS: University Press of Kansas. 1392 p. [1603]

57. Green, Pat; Jensen, Mark. 1991. Plant succession within managed grand fir forests of northern Idaho. In: Harvey, Alan E.; Neuenschwander, Leon F., compilers. Proceedings--management and productivity of western-montane forest soils; 1990 April 10-12; Boise, ID. Gen. Tech. Rep. INT-280. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station: 232-236. [15987]

58. Groves, R. H.; Kaye, P. E. 1989. Germination and phenology of seven introduced thistle species in southern Australia. Australian Journal of Botany. 37(4): 351-359. [41079]

59. Gruell, G. E.; Loope, L. L. 1974. Relationships among aspen, fire, and ungulate browsing in Jackson Hole, Wyoming. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station. 33 p. In cooperation with: U.S. Department of the Interior, National Park Service, Rocky Mountain Region. [3862]

60. Hall, Frederick C. 1974. Prediction of plant community development and its use in management. In: Black, Hugh C., ed. Wildlife and forest management in the Pacific Northwest: Proceedings of a symposium; 1973 September 11-12; Corvallis, OR. Corvallis, OR: Oregon State University, School of Forestry, Forest Research Laboratory: 113-119. [7998]

61. Halpern, Charles B.; Frenzen, Peter M.; Means, Joseph E.; Franklin, Jerry F. 1990. Plant succession in areas of scorched and blown-down forest after the 1980 eruption of Mount St. Helens, Washington. Journal of Vegetation Science. 1: 181-194. [13191]

62. Harris, P.; Clapperton, M. J. 1997. An exploratory study on the influence of vesicular-arbuscular mycorrhizal fungi on the success of weed biological control with insects. Biocontrol Science and Technology. 7(2): 193-201. [38345]

63. Harris, P.; Shorthouse, J. D. 1996. Effectiveness of gall inducers in weed biological control. The Canadian Entomologist. 128(6): 1021-1055. [37288]

64. Hartley, M. J. 1983. Effect of Scotch thistles on sheep growth rates. Proceedings, 36th New Zealand Weed and Pest Control Conference. 36: 86-88. [41133]

65. Hayward, C. Lynn. 1952. Alpine biotic communities of the Uinta Mountains, Utah. Ecological Monographs. 22(2): 93-120. [11657]

66. Heinselman, Miron L. 1970. The natural role of fire in northern conifer forests. In: The role of fire in the Intermountain West: Symposium proceedings; 1970 October 27-29; Missoula, MT. Missoula, MT: Intermountain Fire Research Council: 30-41. In cooperation with: University of Montana, School of Forestry. [15735]

67. Hendrickson, William H. 1972. Perspective on fire and ecosystems in the United States. In: Fire in the environment: Symposium proceedings; 1972 May 1-5; Denver, CO. FS-276. [Washington, DC]: U.S. Department of Agriculture, Forest Service: 29-33. In cooperation with: Fire Services of Canada, Mexico, and the United States; Members of the Fire Management Study Group; North American Forestry Commission; FAO. [17276]

68. Hickman, James C., ed. 1993. The Jepson manual: Higher plants of California. Berkeley, CA: University of California Press. 1400 p. [21992]

69. Hitchcock, C. Leo; Cronquist, Arthur. 1973. Flora of the Pacific Northwest. Seattle, WA: University of Washington Press. 730 p. [1168]

70. Hulbert, Lloyd C. 1986. Fire effects on tallgrass prairie. In: Clambey, Gary K.; Pemble, Richard H., eds. The prairie: past, present and future: Proceedings of the 9th North American Prairie Conference; 1984 July 29 - August 1; Moorhead, MN. Fargo, ND: Tri-College University Center for Environmental Studies: 138-142. [3550]

71. Hulten, Eric. 1968. Flora of Alaska and neighboring territories. Stanford, CA: Stanford University Press. 1008 p. [13403]

72. Irwin, Larry L. 1976. Effects of intensive silviculture on big game forage sources in northern Idaho. In: Hieb, S., ed. Proceedings, elk-logging roads symposium. Moscow, ID: University of Idaho: 135-142. [16146]

73. Jacobs, James S.; Carpinelli, Michael F.; Sheley, Roger L. 1999. Revegetating noxious weed-infested rangeland. In: Sheley, Roger L.; Petroff, Janet K., eds. Biology and management of noxious rangeland weeds. Corvallis, OR: Oregon State University Press: 133-141. [35717]

74. Johnson, Douglas E. 1999. Surveying, mapping, and monitoring noxious weeds on rangelands. In: Sheley, Roger L.; Petroff, Janet K., eds. Biology and management of noxious rangeland weeds. Corvallis, OR: Oregon State University Press: 19-36. [35707]

75. Kartesz, John T. 1999. A synonymized checklist and atlas with biological attributes for the vascular flora of the United States, Canada, and Greenland. 1st ed. In: Kartesz, John T.; Meacham, Christopher A. Synthesis of the North American flora (Windows Version 1.0), [CD-ROM]. Chapel Hill, NC: North Carolina Botanical Garden (Producer). In cooperation with: The Nature Conservancy; U.S. Department of Agriculture, Natural Resources Conservation Service; U.S. Department of the Interior, Fish and Wildlife Service. [36715]

76. Kearney, Thomas H.; Peebles, Robert H.; Howell, John Thomas; McClintock, Elizabeth. 1960. Arizona flora. 2nd ed. Berkeley, CA: University of California Press. 1085 p. [6563]

77. Kendall, Deborah M. 1999. Biological controls of thistles. Annual report: May 1, 1998 - May 1, 1999. Cooperative Agreement CA 1268-1-9016. Project Number MEVE-R92-0197. Task Agreement Number FLC-24. Washington, DC: U.S. Department of the Interior, National Park Service, Mesa Verde National Park. 46 p. Unpublished manuscript on file with: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory, Missoula, MT. [34573]

78. Kirt, Russell R. 1996. A nine-year assessment of successional trends in prairie plantings using seed broadcast and seedling transplant methods. In: Warwick, Charles, ed. 15th North American prairie conference: Proceedings; 1996 October 23-26; St. Charles, IL. Bend, OR: The Natural Areas Association: 144-153. [30259]

79. Klinkamer, Peter G. L.; de Jong, Tom J.; Van Der Meijden, Ed. 1988. Production, dispersal and predation of seeds in the biennial Cirsium vulgare. Journal of Ecology. 76: 403-414. [5675]

80. Klinkhamer, Peter G. L.; De Jong, Tom J. 1993. Cirsium vulgare (Savi) Ten.: (Carduus lanceolatus L., Cirsium lanceolatum (L.) Scop., non Hill). Journal of Ecology. 81: 177-191. [20980]

81. Kramer, Neal B.; Johnson, Frederic D. 1987. Mature forest seed banks of three habitat types in central Idaho. Canadian Journal of Botany. 65: 1961-1966. [3961]

82. Kucera, Clair L. 1981. Grasslands and fire. In: Mooney, H. A.; Bonnicksen, T. M.; Christensen, N. L.; Lotan, J. E.; Reiners, W. A., tech. coords. Fire regimes and ecosystem properties: Proceedings of the conference; 1978 December 11-15; Honolulu, HI. Gen. Tech. Rep. WO-26. Washington, DC: U.S. Department of Agriculture, Forest Service: 90-111. [4389]

83. Kuchler, A. W. 1964. United States [Potential natural vegetation of the conterminous United States]. Special Publication No. 36. New York: American Geographical Society. 1:3,168,000; colored. [3455]

84. Lackschewitz, Klaus. 1991. Vascular plants of west-central Montana--identification guidebook. Gen. Tech. Rep. INT-227. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Research Station. 648 p. [13798]

85. Lantagne, Douglas O. 1991. Tree shelters increase heights of planted northern red oaks. In: McCormick, Larry H.; Gottschalk, Kurt W., eds. Proceedings, 8th central hardwood forest conference; 1991 March 4-6; University Park, PA. Gen. Tech. Rep. NE-148. Radnor, PA: U.S. Department of Agriculture, Forest Service, Northeastern Forest Experiment Station: 291-298. [15318]

86. Laven, R. D.; Omi, P. N.; Wyant, J. G.; Pinkerton, A. S. 1980. Interpretation of fire scar data from a ponderosa pine ecosystem in the central Rocky Mountains, Colorado. In: Stokes, Marvin A.; Dieterich, John H., tech. coords. Proceedings of the fire history workshop; 1980 October 20-24; Tucson, AZ. Gen. Tech. Rep. RM-81. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station: 46-49. [7183]

87. Ledgard, N. J.; Davis, M. R.; Platt, K. 1987. Reforestation after fire in Canterbury beech forests. What's New in Forest Research No. 149. Rotorua, New Zealand: Ministry of Forestry, Forestry Research Centre, Forest Research Institute. 4 p. [19486]

88. Leininger, Wayne C. 1988. Non-chemical alternatives for managing selected plant species in the western United States. XCM-118. Fort Collins, CO: Colorado State University, Cooperative Extension. In cooperation with: U.S. Department of the Interior, Fish and Wildlife Service. 47 p. [13038]

89. Louda, S. M.; Kendall, D.; Connor, J.; Simberloff, D. 1997. Ecological effects of an insect introduced for the biological control of weeds. Science. 277: 1088-1090. [41080]

90. Lyon, L. Jack. 1971. Vegetal development following prescribed burning of Douglas-fir in south-central Idaho. Res. Pap. INT-105. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station. 30 p. [1495]

91. Malanson, George P.; Butler, David R. 1991. Floristic variation among gravel bars in a subalpine river, Montana, U.S.A. Arctic and Alpine Research. 23(3): 273-278. [16470]

92. Marcum, Les. 1971. Vegetal development on montane fir clearcuts in western Montana. Missoula, MT: University of Montana. 122 p. Thesis. [36494]

93. Martin, William C.; Hutchins, Charles R. 1981. A flora of New Mexico. Volume 2. Germany: J. Cramer. 2589 p. [37176]

94. Matlack, Glenn R. 1987. Diaspore size, shape, and fall behavior in wind-dispersed plant species. American Journal of Botany. 74(8): 1150-1160. [28]

95. McDonald, Philip M. 1999. Diversity, density, and development of early vegetation in a small clear-cut environment. Res. Pap. PSW-RP-239. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. 22 p. [36204]

96. McDonald, Philip M.; Fiddler, Gary O. 1995. Development of a mixed shrub - ponderosa pine community in a natural and treated condition. Res. Pap. PSW-RP-224. Berkeley, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. 18 p. [34998]

97. McDonald, Philip M.; Fiddler, Gary O. 1996. Development of a mixed shrub-tanoak-Douglas-fir community in a treated and untreated condition. Res. Pap. PSW-RP-225. Albany, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. 16 p. [27731]

98. McDonald, Philip M.; Helgerson, Ole T. 1990. Mulches aid in regenerating California and Oregon forests: past, present, and future. Gen. Tech. Rep. PSW-123. Berkeley, CA: U.S. Department of Agriculture, Forest Service, Pacific Southwest Research Station. 19 p. [15105]

99. McIntire, Patrick W. 1984. Fungus consumption by the Siskiyou chipmunk within a variously treated forest. Ecology. 65(1): 137-146. [8456]

100. McPherson, Guy R. 1995. The role of fire in the desert grasslands. In: McClaran, Mitchel P.; Van Devender, Thomas R., eds. The desert grassland. Tucson, AZ: The University of Arizona Press: 130-151. [26576]

101. Meinecke, E. P. 1929. Quaking aspen: A study in applied forest pathology. Tech. Bull. No. 155. Washington, DC: U.S. Department of Agriculture. 34 p. [26669]

102. Messinger, Richard Duane. 1974. Effects of controlled burning on waterfowl nesting habitat in northwest Iowa. Ames, IA: Iowa State University. 49 p. Thesis. [20673]

103. Michaux, B. 1989. Reproductive and vegetative biology of Cirsium vulgare (Savi) Ten. (Compositae: Cynareae). New Zealand Journal of Botany. 27(3): 401-414. [41131]

104. Miller, Richard F.; Krueger, William C.; Vavra, Martin. 1986. Twelve years of plant succession on a seeded clearcut under grazing and protection from cattle. In: Special Report 773. 1986 Progress report...research in rangeland management. Corvallis, OR: Oregon State University, Agricultural Experiment Station: 4-10. In cooperation with: U.S. Department of Agriculture, Agricultural Research Service. [3650]

105. Miller, Richard F.; Rose, Jeffery A. 1995. Historic expansion of Juniperus occidentalis (western juniper) in southeastern Oregon. The Great Basin Naturalist. 55(1): 37-45. [25666]

106. Mitich, Larry W. 1998. Bull thistle, Cirsium vulgare. Weed Technology. 12(4): 761-763. [41067]

107. Mohlenbrock, Robert H. 1986. [Revised edition]. Guide to the vascular flora of Illinois. Carbondale, IL: Southern Illinois University Press. 507 p. [17383]

108. Moir, William H. 1982. A fire history of the High Chisos, Big Bend National Park, Texas. The Southwestern Naturalist. 27(1): 87-98. [5916]

109. Morrison, Peter H.; Swanson, Frederick J. 1990. Fire history and pattern in a Cascade Range landscape. Gen. Tech. Rep. PNW-GTR-254. Portland, OR: U.S. Department of Agriculture, Forest Service, Pacific Northwest Research Station. 77 p. [13074]

110. Myers, Ronald L. 2000. Fire in tropical and subtropical ecosystems. In: Brown, James K.; Smith, Jane Kapler, eds. Wildland fire in ecosystems: Effects of fire on flora. Gen. Tech. Rep. RMRS-GTR-42-vol. 2. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 161-173. [36985]

111. Nyboer, Randy W. 1981. Grazing as a factor in the decline of Illinois hill prairies. In: Stuckey, Ronald L.; Reese, Karen J., eds. The Prairie Peninsula--in the "shadow" of Transeau: Proceedings, 6th North American prairie conference; 1978 August 12-17; Columbus, OH. Ohio Biological Survey Biological Notes No. 15. Columbus, OH: Ohio State University, College of Biological Sciences: 209-211. [3408]

112. O'Kennon, Bob; Nesom, Guy. 1988. First report of Cirsium vulgare (Asteraceae) from Texas. SIDA. 13(1): 115-116. [41132]

113. Outcalt, Kenneth Wayne; White, Edwin H. 1981. Phytosociological changes in understory vegetation following timber harvest in northern Minnesota. Canadian Journal of Forest Research. 11: 175-183. [16301]

114. Parendes, Laurie A.; Jones, Julia A. 2000. Role of light availability and dispersal in exotic plant invasion along roads and streams in the H. J. Andrews Experimental Forest, Oregon. Conservation Biology. 14(1): 64-75. [36371]

115. Paysen, Timothy E.; Ansley, R. James; Brown, James K.; Gottfried, Gerald J.; Haase, Sally M.; Harrington, Michael G.; Narog, Marcia G.; Sackett, Stephen S.; Wilson, Ruth C. 2000. Fire in western shrubland, woodland, and grassland ecosystems. In: Brown, James K.; Smith, Jane Kapler, eds. Wildland fire in ecosystems: Effects of fire on flora. Gen. Tech. Rep. RMRS-GTR-42-vol. 2. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 121-159. [36978]

116. Popay, A. I.; Medd, R. W. 1990. The biology of Australian weeds 21. Carduus nutans L. spp nutans. Plant Protection Quarterly. 5(1): 3-13. [23673]

117. Powell, David C. 1994. Effects of the 1980's western spruce budworm outbreak on the Malheur National Forest in northeastern Oregon. Tech. Pub. R6-FI&D-TP-12-94. Portland, OR: U.S. Department of Agriculture, Forest Service, Natural Resources Staff, Forest Insects and Diseases Group. 176 p. [29717]

118. Pratt, David W.; Black, R. Alan; Zamora, B. A. 1984. Buried viable seed in a ponderosa pine community. Canadian Journal of Botany. 62: 44-52. [16219]

119. Radford, Albert E.; Ahles, Harry E.; Bell, C. Ritchie. 1968. Manual of the vascular flora of the Carolinas. Chapel Hill, NC: The University of North Carolina Press. 1183 p. [7606]

120. Randall, John M. 1990. Establishment and control of bull thistle (Cirsium vulgare) in Yosemite Valley. In: van Riper, Charles, III; Stohlgren, Thomas J.; Veirs, Stephen D., Jr.; Hillyer, Silvia Castillo, eds. Examples of resource inventory and monitoring in National Parks of California: proceedings of the 3rd biennial conference on research in California's national parks; 1988 September 13-15; Davis, CA. Transactions and Proceedings Series No. 8. Washington, DC: U.S. Department of the Interior, National Park Service: 177-193. [61416]

121. Randall, John M. 1991. Population dynamics and control of bull thistle, Cirsium vulgare, in Yosemite Valley. In: Center, Ted D.; Doren, Robert F.; Hofstetter, Ronald L.; [and others], eds. Proceedings of the symposium on exotic pest plants; 1988 November 2-4; Miami, FL. Tech. Rep. NPS/NREVER/NRTR-91/06. Washington, DC: U.S. Department of the Interior, National Park Service: 261-281. [17871]

122. Randall, John M. 2000. Cirsium vulgare (Savi) Tenore. In: Bossard, Carla C.; Randall, John M.; Hoshovsky, Marc C., eds. Invasive plants of California's wildlands. Berkeley, CA: University of California Press: 112-119. [41140]

123. Randall, John M.; Rejmanek, Marcel. 1993. Interference of bull thistle (Cirsium vulgare) with growth of ponderosa pine (Pinus ponderosa) seedlings in a forest plantation. Canadian Journal of Forest Research. 23(8): 1507-1513. [22286]

124. Raunkiaer, C. 1934. The life forms of plants and statistical plant geography. Oxford: Clarendon Press. 632 p. [2843]

125. Rees, N. E.; Littlefield, J. L.; Bruckart, W. L.; Baudoin, A. 1996. Musk thistle: Carduus nutans (group). In: Rees, Norman E.; Quimby, Paul C., Jr.; Piper, Gary L.; [and others], eds. Biological control of weeds in the West. Bozeman, MT: Western Society of Weed Science. In cooperation with: U.S. Department of Agriculture, Agricultural Research Service; Montana Department of Agriculture; Montana State University: Section II. [40873]

126. Rees, N. E.; Quimby, P. C., Jr.; Mullin, B. H. 1996. Section I. Biological control of weeds. In: Rees, Norman E.; Quimby, Paul C., Jr.; Piper, Gary L.; [and others], eds. Biological control of weeds in the West. Bozeman, MT: Western Society of Weed Science. In cooperation with: U.S. Department of Agriculture, Agricultural Research Service; Montana Department of Agriculture; Montana State University: 3-24. [38273]

127. Rees, Norman E. 1991. Biological control of thistles. In: James, Lynn F.; Evans, John O.; Ralphs, Michael H.; Child, R. Dennis, eds. Noxious range weeds. Westview Special Studies in Agricultural Science and Policy. Boulder, CO: Westview Press: 264-273. [23554]

128. Rejmanek, Marcel; Leps, Jan. 1996. Negative associations can reveal interspecific competition and reversal of competitive hierarchies during succession. Oikos. 76(1): 161-168. [41136]

129. Rice, Barry Meyers; Randall, John M., compilers. 2001. Weed report: Cirsium vulgare--bull thistle. In: Wildland weeds management and research: 1998-99 weed survey. Davis, CA: The Nature Conservancy, Wildland Invasive Species Program. 5 p. On file with: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory, Missoula, MT. [41139]

130. Ripple, William J. 1994. Historic spatial patterns of old forests in western Oregon. Journal of Forestry. 92(11): 45-49. [33881]

131. Robinson, Myles C.; Chilcote, W. W. 1962. Temperature microenvironments associated with early stages in plant succession on Douglas-fir clear-cuts in the Oregon Coast Range. Bulletin of the Ecological Society of America. 43(3): 74. [41138]

132. Romme, William H. 1982. Fire and landscape diversity in subalpine forests of Yellowstone National Park. Ecological Monographs. 52(2): 199-221. [9696]

133. Rutledge, Chris R.; McLendon, Terry. 2002. Cirsium vulgare. In: An assessment of exotic plant species of Rocky Mountain National Park, [Online]. Fort Collins, CO: Colorado State University, Department of Rangeland and Ecosystem Science. Northern Prairie Wildlife Research Center Home Page. Jamestown, ND: U.S. Geological Survey, (Producer). Available: http://www.npwrc.usgs.gov/resource/othrdata/Explant/cirsvulg.htm [2002 April 22]. [41141]

134. Sapsis, David B. 1990. Ecological effects of spring and fall prescribed burning on basin big sagebrush/Idaho fescue--bluebunch wheatgrass communities. Corvallis, OR: Oregon State University. 105 p. Thesis. [16579]

135. Schultz, Brad W. 1987. Ecology of curlleaf mountain mahogany (Cercocarpus ledifolius) in western and central Nevada: population structure and dynamics. Reno, NV: University of Nevada. 111 p. Thesis. [7064]

136. Seymour, Frank Conkling. 1982. The flora of New England. 2nd ed. Phytologia Memoirs 5. Plainfield, NJ: Harold N. Moldenke and Alma L. Moldenke. 611 p. [7604]

137. Shearer, Raymond C.; Stickney, Peter F. 1991. Natural revegetation of burned and unburned clearcuts in western larch forests of northwest Montana. In: Nodvin, Stephen C.; Waldrop, Thomas A., eds. Fire and the environment: ecological and cultural perspectives: Proceedings of an international symposium; 1990 March 20-24; Knoxville, TN. Gen. Tech. Rep. SE-69. Asheville, NC: U.S. Department of Agriculture, Forest Service, Southeastern Forest Experiment Station: 66-74. [16635]

138. Sheley, Roger L. 2001. Ecological principles for managing knapweed. In: Smith, Lincoln, ed. Proceedings, 1st international knapweed symposium of the 21st century; 2001 March 15-16; Coeur d'Alene, ID. Albany, CA: U.S. Department of Agriculture, Agricultural Research Service: 62. Abstract. [37834]

139. Sheley, Roger; Manoukian, Mark; Marks, Gerald. 1999. Preventing noxious weed invasion. In: Sheley, Roger L.; Petroff, Janet K., eds. Biology and management of noxious rangeland weeds. Corvallis, OR: Oregon State University Press: 69-72. [35711]

140. Shiflet, Thomas N., ed. 1994. Rangeland cover types of the United States. Denver, CO: Society for Range Management. 152 p. [23362]

141. Simmerman, Dennis G.; Arno, Stephen F.; Harrington, Michael G.; Graham, Russell T. 1991. A comparison of dry and moist fuel underburns in ponderosa pine shelterwood units in Idaho. In: Andrews, Patricia L.; Potts, Donald F., eds. Proceedings, 11th annual conference on fire and forest meteorology; 1991 April 16-19; Missoula, MT. SAF Publication 91-04. Bethesda, MD: Society of American Foresters: 387-397. [16186]

142. Smith, Tim E., ed. 2001. Vegetation management guideline: Musk thistle (Carduus nutans L.), [Online]. In: Missouri vegetation management guideline. Jefferson City, MO: Missouri Department of Conservation (Producer). Available: http://conservation.state.mo.us/nathis/exotic/vegman/eighteen.htm [2002, March 11]. [41083]

143. Stickney, Peter F. 1980. Data base for post-fire succession, first 6 to 9 years, in Montana larch-fir forests. Gen. Tech. Rep. INT-62. Ogden, UT: U.S. Department of Agriculture, Forest Service, Intermountain Forest and Range Experiment Station. 133 p. [6583]

144. Stickney, Peter F. 1989. Seral origin of species comprising secondary plant succession in Northern Rocky Mountain forests. FEIS workshop: Postfire regeneration. Unpublished draft on file at: U.S. Department of Agriculture, Forest Service, Intermountain Research Station, Fire Sciences Laboratory, Missoula, MT. 10 p. [20090]

145. Strausbaugh, P. D.; Core, Earl L. 1977. Flora of West Virginia. 2nd ed. Morgantown, WV: Seneca Books, Inc. 1079 p. [23213]

146. Strickler, Gerald S.; Edgerton, Paul J. 1976. Emergent seedlings from coniferous litter and soil in eastern Oregon. Ecology. 57: 801-807. [2039]

147. Stuart, John D. 1987. Fire history of an old-growth forest of Sequoia sempervirens (Taxodiaceae) forest in Humboldt Redwoods State Park, California. Madrono. 34(2): 128-141. [7277]

148. Swetnam, Thomas W.; Baisan, Christopher H.; Caprio, Anthony C.; Brown, Peter M. 1992. Fire history in a Mexican oak-pine woodland and adjacent montane conifer gallery forest in southeastern Arizona. In: Ffolliott, Peter F.; Gottfried, Gerald J.; Bennett, Duane A.; Hernandez C., Victor Manuel; Ortega-Rubio, Alfred; Hamre, R. H., tech. coords. Ecology and management of oak and associated woodlands: perspectives in the southwestern United States and northern Mexico: Proceedings; 1992 April 27-30; Sierra Vista, AZ. Gen. Tech. Rep. RM-218. Fort Collins, CO: U.S. Department of Agriculture, Forest Service, Rocky Mountain Forest and Range Experiment Station: 165-173. [19759]

149. Tappeiner, John C., II; McDonald, Philip M.; Roy, Douglass F. 1990. Lithocarpus densiflorus (Hook. & Arn.) Rehd. tanoak. In: Burns, Russell M.; Honkala, Barbara H., technical coordinators. Silvics of North America. Volume 2. Hardwoods. Agric. Handb. 654. Washington, DC: U.S. Department of Agriculture, Forest Service: 417-425. [13969]

150. Titus, Jonathan H.; Moore, Scott; Arnot, Mildred; Titus, Priscilla J. 1998. Inventory of the vascular flora of the blast zone, Mount St. Helens, Washington. Madrono. 45(2): 146-161. [30322]

151. Tu, Mandy; Hurd, Callie; Randall, John M., eds. 2001. Weed control methods handbook: tools and techniques for use in natural areas. Davis, CA: The Nature Conservancy. 194 p. [37787]

152. U.S. Department of Agriculture, Forest Service. 2001. Guide to noxious weed prevention practices. Washington, DC: U.S. Department of Agriculture, Forest Service. 25 p. Available online: https://www.fs.usda.gov /rangelands/ftp/invasives/documents/GuidetoNoxWeedPrevPractices_07052001.pdf [2005, October 25]. [37889]

153. U.S. Department of Agriculture, Natural Resources Conservation Service. 2008. PLANTS Database, [Online]. Available: https://plants.usda.gov /. [34262]

154. University of Montana, Division of Biological Sciences. 2001. INVADERS Database System, [Online]. Available: http://invader.dbs.umt.edu/ [2001, June 27]. [37489]

155. Uresk, Daniel W.; Severson, Kieth E. 1998. Response of understory species to changes in ponderosa pine stocking levels in the Black Hills. The Great Basin Naturalist. 58(4): 312-327. [29413]

156. Vincent, Dwain W. 1992. The sagebrush/grasslands of the upper Rio Puerco area, New Mexico. Rangelands. 14(5): 268-271. [19698]

157. Voss, Edward G. 1996. Michigan flora. Part III: Dicots (Pyrolaceae--Compositae). Bulletin 61: Cranbrook Institute of Science; University of Michigan Herbarium. Ann Arbor, MI: The Regents of the University of Michigan. 622 p. [30401]

158. Wade, Dale D.; Brock, Brent L.; Brose, Patrick H.; Grace, James B.; Hoch, Greg A.; Patterson, William A., III. 2000. Fire in eastern ecosystems. In: Brown, James K.; Smith, Jane Kapler, eds. Wildland fire in ecosystems: Effects of fire on flora. Gen. Tech. Rep. RMRS-GTR-42-vol. 2. Ogden, UT: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station: 53-96. [36983]

159. Weaver, J. E. 1968. Prairie plants and their environment: A fifty-year study in the Midwest. Lincoln, NE: University of Nebraska Press. 276 p. [17547]

160. Weber, William A.; Wittmann, Ronald C. 1996. Colorado flora: eastern slope. 2nd ed. Niwot, CO: University Press of Colorado. 524 p. [27572]

161. Welsh, Stanley L.; Atwood, N. Duane; Goodrich, Sherel; Higgins, Larry C., eds. 1987. A Utah flora. The Great Basin Naturalist Memoir No. 9. Provo, UT: Brigham Young University. 894 p. [2944]

162. Wesselingh, Renate A.; Klinkhammer, Peter G. L.; de Jong, Tom J.; Schlatmann, Els G. M. 1994. A latitudinal cline in vernalization requirements in Cirsium vulgare. Ecography. 17(3): 272-277. [41137]

163. Wilson, Gail W. T.; Hartnett, David C. 1998. Interspecific variation in plant responses to mycorrhizal colonization in tallgrass prairie. American Journal of Botany. 85(12): 1732-1738. [30311]

164. Wilson, Linda M.; McCaffrey, Joseph P. 1999. Biological control of noxious rangeland weeds. In: Sheley, Roger L.; Petroff, Janet K., eds. Biology and management of noxious rangeland weeds. Corvallis, OR: Oregon State University Press: 97-115. [35715]

165. Wofford, B. Eugene. 1989. Guide to the vascular plants of the Blue Ridge. Athens, GA: The University of Georgia Press. 384 p. [12908]

166. Wunderlin, Richard P. 1998. Guide to the vascular plants of Florida. Gainesville, FL: University Press of Florida. 806 p. [28655]

167. Young, J. A.; Hedrick, D. W.; Keniston, R. F. 1967. Forest cover and logging--herbage and browse production in the mixed coniferous forest of northeastern Oregon. Journal of Forestry. 65: 807-813. [16290]

168. Young, James A.; Evans, Raymond A. 1981. Demography and fire history of a western juniper stand. Journal of Range Management. 34(6): 501-505. [2659]

169. Youtie, Berta; Soll, Jonathan. 1990. Diffuse knapweed control on the Tom McCall Preserve and Mayer State Park. Unpublished report (prepared for the Mazama Research Committee) on file at: U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory, Missoula, MT. 18 p. [38353]