Centaurea Diffusa Research Articles

The complete chloroplast genome of Tianshan Snow Lotus (Saussurea involucrata), a famous traditional Chinese medicinal plant of the family Asteraceae

abtractWe decoded the complete chloroplast DNA (cpDNA) sequence of the Tianshan Snow Lotus (Saussurea involucrata), a famous traditional Chinese medicinal plant of the family Asteraceae, by using next-generation sequencing technology. The genome consists of 152 490 bp containing a pair of inverted repeats (IRs) of 25 202 bp, which was separated by a large single-copy region and a small single-copy region of 83 446 bp and 18 639 bp, respectively. The genic regions account for 57.7% of whole cpDNA, and the GC content of the cpDNA was 37.7%. The S. involucrata cpDNA encodes 114 unigenes (82 protein-coding genes, 4 rRNA genes, and 28 tRNA genes). There are eight protein-coding genes (atpF, ndhA, ndhB, rpl2, rpoC1, rps16, clpP, and ycf3) and five tRNA genes (trnA-UGC, trnI-GAU, trnK-UUU, trnL-UAA, and trnV-UAC) containing introns. A phylogenetic analysis of the 11 complete cpDNA from Asteracease showed that S. involucrata is closely related to Centaurea diffusa (Diffuse Knapweed). The complete cpDNA of S. involucrata provides essential and important DNA molecular data for further phylogenetic and evolutionary analysis for Asteraceae.

Adaptive plasticity and niche expansion in an invasive thistle.

Phenotypic differentiation in size and fecundity between native and invasive populations of a species has been suggested as a causal driver of invasion in plants. Local adaptation to novel environmental conditions through a micro-evolutionary response to natural selection may lead to phenotypic differentiation and fitness advantages in the invaded range. Local adaptation may occur along a stress tolerance trade-off, favoring individuals that, in benign conditions, shift resource allocation from stress tolerance to increased vigor and fecundity and, therefore, invasiveness. Alternately, the typically disturbed invaded range may select for a plastic, generalist strategy, making phenotypic plasticity the main driver of invasion success. To distinguish between these hypotheses, we performed a field common garden and tested for genetically based phenotypic differentiation, resource allocation shifts in response to water limitation, and local adaptation to the environmental gradient which describes the source locations for native and invasive populations of diffuse knapweed (Centaurea diffusa). Plants were grown in an experimental field in France (naturalized range) under water addition and limitation conditions. After accounting for phenotypic variation arising from environmental differences among collection locations, we found evidence of genetic variation between the invasive and native populations for most morphological and life-history traits under study. Invasive C. diffusa populations produced larger, later maturing, and therefore potentially fitter individuals than native populations. Evidence for local adaptation along a resource allocation trade-off for water limitation tolerance is equivocal. However, native populations do show evidence of local adaptation to an environmental gradient, a relationship which is typically not observed in the invaded range. Broader analysis of the climatic niche inhabited by the species in both ranges suggests that the physiological tolerances of C. diffusa may have expanded in the invaded range. This observation could be due to selection for plastic, “general-purpose” genotypes with broad environmental tolerances.

New records for the flora of Lebanon

Seven species not previously reported for Lebanon and one species re-located after 183 years are reported here after their discovery during an ex situ seed conservation project in the country (Bombycilaena erecta (L.) Smoljan., Centaurea diffusa Lam.; Hypochaeris radicata L.; Gypsophila perfoliata L.; Euphorbia cheiradenia Boiss. & Hohen.; Stachys setifera C.A.Mey.; Stachys zoharyana Eig and Plantago maritima L.).

Increased seed consumption by biological control weevil tempers positive CO2 effect on invasive plant (Centaurea diffusa) fitness

Predicted increases in atmospheric CO2 and temperature may benefit some invasive plants, increasing the need for effective invasive plant management. Biological control can be an effective means of managing invasive plants, but the anticipated range in responses of plant–insect interactions to climate change make it difficult to predict how effective biological control will be in the future. Field experiments that manipulate climate within biological control systems could improve predictive power, but are challenging to implement and therefore rare to date. Here, we show that free air CO2 enrichment in the field increased the fitness of Centaurea diffusa Lam., a problematic invader in much of the western United States. However, CO2 enrichment also increased the impact of the biological control agent Larinus minutus (Coleoptera: Curculionidae) on C. diffusa fitness. C. diffusa plants flowered earlier and seed heads developed faster with both elevated CO2 and increased temperature. Natural dispersal of L. minutus into the experimental plots provided a unique opportunity to examine weevil preference for and effects on C. diffusa grown under the different climate change treatments. Elevated CO2 increased both the proportion of seed heads infested by L. minutus and, correspondingly, the amount of seed removed by weevils. Warming had no detectable effect on weevil utilization of plants. Higher weevil densities on elevated CO2 plants reduced, but did not eliminate, the positive effects of CO2 on C. diffusa fitness. Correlations between plant development time and weevil infestation suggest that climate change increased weevil infestation by hastening plant phenology. Phenological mismatches are anticipated with climate change in many plant–insect systems, but in the case of L. minutus and C. diffusa in mixed-grass prairie, a better phenological match may make the biological control agent more effective as CO2 levels rise.

Iron in complex with the alleged phytosiderophore 8-hydroxyquinoline induces functional iron deficiency and non-autolytic programmed cell death in rapeseed plants

The alkaloid 8-hydroxyquinoline (HQ) was pointed out as potential phytosiderophore that is taken up unspecifically by iron acquisition strategy I and II plants. HQ in root exudate of the knapweed Centaurea diffusa even was considered as a factor contributing to the invasive success of this species. This study compares the iron supply efficiencies of the Fe–HQ complex and Fe–EDTA in hydroponic cultures of rapeseed (Brassica napus) as a plant model system to explore the proposed function. Iron (FeCl3) was supplied in 2 and 10μM concentrations in a 1:1 ratio with the ligand (HQ). After 20 days, Fe content, lipid peroxidation, superoxide anion, hydrogen peroxide, and ascorbic acid concentrations, various enzyme activities associated with antioxidant defences and programmed cell death (PCD), and nuclear condensation were determined. Iron supply in the form of a Fe–HQ complex clearly was less efficient. These plants developed chlorosis and showed symptoms of non-autolytic PCD similar to those that had been subjected to Fe deprivation. Reactive oxygen species (ROS) concentration levels and enzyme activities (superoxide dismutase, catalase, alkaline protease, caspase-3-like and deoxyribonuclease) resembled more those observed in plants which suffered from Fe deprivation, than those of Fe-sufficient Fe–EDTA supplied plants. The results do not support the putative phytosiderophore function alleged to HQ in the studied concentration range (2–10μM) but instead corroborate the one hypothesis explaining HQ toxicity: Fe in complex with HQ is not released efficiently enough from the complex to be available for metalloenzymes that require it as a co-factor.

Rapid evolution of an invasive weed

Trade-offs between performance and the ability to tolerate abiotic and biotic stress have been suggested to explain both the success of invasive species and phenotypic differentiation between native and invasive populations. It is critical to sample broadly across both ranges and to account for latitudinal clines and maternal effects when testing this premise. Wild-collected Centaurea diffusa seeds were grown in benign and stressful conditions (drought, flooding, nutrient stress and simulated herbivory), to evaluate whether native and introduced individuals differ in performance or life history phenotypes. A second experiment used glasshouse-grown seeds to evaluate whether patterns remain comparable when maternal environment is consistent. Many traits differed between ranges, and in all cases but one, invasive individuals grew larger, performed better, or matured later. No trade-off in performance with herbivore defense was evident. Invasive populations may have been released from a trade-off between growth and drought tolerance apparent in the native range. Larger individuals with delayed maturity and greater reproductive potential have evolved in invasive populations, a pattern evident across broad population sampling, and after latitude and maternal environment were considered. Release from abiotic stress tolerance trade-offs may be important for the invasion success of Centaurea diffusa.

Testing biological control agent compatibility: Cyphocleonus achates and Larinus minutus on diffuse knapweed

While weed biological control success is typically achieved with one agent, multiple agents are invariably introduced. Biological control agents that share a host–plant may interact either directly or indirectly through changes in host–plant quality. Negative interactions could reduce the impacts of the agents on the density of their host–plant while positive interactions (facilitation) could improve biological control success.In the Okanagan Valley of British Columbia, Canada, initial declines in the invasive rangeland weed, diffuse knapweed (Centaurea diffusa) were attributed to the introduction of the weevil Larinus minutus. A second weevil, Cyphocleonus achates has recently become common on diffuse knapweed. We sought to determine if the recent increase of C. achates could threaten the success of L. minutus. We considered whether L. minutus colonisation or performance remained the same when C. achates was present, and whether the two agents acted independently to reduce plant performance.Neither changes in colonisation rates nor competitive interactions were apparent between C. achates and L. minutus. Both insects reduced plant performance and, for all metrics, the reduction in plant performance by one species was independent of the second. The two agents appear to be compatible and both should contribute to the control of diffuse knapweed. To assess how biological control agents interact requires understanding both their competitive interactions and their joint effects on the shared host.

Influences of two life‐history stages of the weevil, Larinus minutus, on its host plant Centaurea diffusa

1. We hypothesised that, for weed biological control, using species that feed as both adults and larvae would be advantageous. Here, we test the impacts of adult and larval feeding of Larinus minutus (Col.: Curculionidae), a successful biological control agent of diffuse knapweed, Centaurea diffusa (Asteraceae), in British Columbia, Canada.2. At one site, the observations of the present study showed that the intensity of adult weevil feeding did not predict the level of larval attack. Experiments found no evidence for plant‐mediated competition between the life‐history stages.3. At two sites and in two years, experimental manipulations of adult and/or larval feeding damage were conducted and plant performance measured. Neither antagonistic nor synergistic interactions occurred, but at each of the two sites a different life‐history stage was responsible for reduction in the number of seeds produced by the plants.4. Although one of the two different feeding modes was redundant at each site, the ability of adults and larvae to reduce plant performance in different areas makes the species effective in a wider range of environments.

Impact of Biological Control on Two Knapweed Species in British Columbia

Diffuse and spotted knapweed (Centaurea diffusa Lam and C. stoebe L.) are two closely related invasives found in many parts of British Columbia’s Southern Interior, causing substantial economic losses in rangelands. Beginning in 1970, the provincial government initiated a long-term biological control effort against the knapweeds, introducing 10 different insect agents from 1970 to 1987. In an effort to evaluate the efficacy of the program, archival (1983–2008) data was amassed from 19 vegetation monitoring sites that contained knapweed. In 2010, these sites were relocated and re-monitored and cover values were analyzed. Diffuse knapweed showed significant declines at 14 of 15 sites; spotted knapweed declined at three of four sites. Possible alternative explanations for the decline are discussed. Evidence strongly points to a suite of biocontrol agents (seed feeders and root feeders) as the primary drivers of knapweed decline in British Columbia’s Southern Interior.

Resource concentration by insects and implications for plant populations

Summary1. The distribution of herbivores among plant patches may be an important factor determining plant population persistence. The resource concentration hypothesis proposes that herbivores are more abundant per unit plant at higher host plant densities and this has been found to occur in many systems. However, the opposite pattern, resource dilution, in which the herbivores are more abundant in low‐density patches and situations in which the number of insect herbivores per unit plant remains constant, also occurs.2. We developed a simulation model to explore how the distribution of insects per plant affects plant population decline and persistence. We varied the numbers of plants per patch and the distribution pattern, i.e. whether insects were found in a resource concentration distribution, a resource dilution distribution or a distribution in which insect abundance increased linearly with plant density.3. Resource concentration resulted in longer persistence of plant populations. Plant populations declined more rapidly with either weak resource dilution or directly proportional insect distribution patterns. As the intensity of resource concentration increased, the decline in plant population density was reduced, and plant persistence increased because of increasing variance in insect load. Under strong resource dilution, increasing variance in the insect load also led to a reduction in plant population decline and an increase in plant persistence.4. We complement our model with field data from the diffuse knapweed, Centaurea diffusa biocontrol system. We compared the relationship with plant density of a successful biocontrol agent, Larinus minutus, and an unsuccessful one, Urophora affinis. Larinus minutus density was directly proportional to plant density, while U. affinis showed a resource concentration pattern with higher rates of attack in high‐density patches.5. Synthesis: Patterns of insect distribution with host plant density will alter the extent to which patches of differing plant densities decline or persist. Resource concentration promotes persistence of the insect–plant system because increased herbivore pressure in high‐density patches leads to negative density‐dependent plant growth. Weak resource dilution and a distribution of insects that is directly proportionate to plant density can accelerate plant population decline. Strong resource dilution leads to positive density dependence with higher population growth in large patches. Our simulation model and field data demonstrate that the relationship between insect distribution and plant densities can influence plant population dynamics and has implications for choices of weed biological control agents.

Impact of Cyphocleonus achates on diffuse knapweed and its interaction with Larinus minutus

The root weevil Cyphocleonus achates (Fahr.), and the seed-head weevil, Larinus minutus Gyll. were introduced into North America as part of a classical biological control program against spotted (Centaurea stoebe L. ssp. micranthos (Gugler) Hayek) and diffuse knapweed (Centaurea diffusa Lamarck). The impact of C. achates alone, or in combination with L. minutus, has not been assessed on diffuse knapweed. In a field study, densities of C. achates were initially manipulated to measure its impact on the density, size, shape, and reproductive potential of diffuse knapweed. Subsequently L. minutus was released at a constant rate across all experimental treatments to determine how attack by this species was affected by previous impacts of C. achates. C. achates reduced plant density by 58%, height by 29%, seed-head diameter by 4.6% as well as the number of seed heads per plant through its effect on plant height. Rosettes attacked by C. achates also produced more flowering stems than unattacked plants and these plants produced fewer seed heads than unattacked plants after accounting for plant size. The reductions in seed-head size had a small negative effect on attack rates by L. minutus but this was generally balanced by a positive effect of reduced seed-head numbers on L. minutus attack rates. After accounting for direct impacts of root herbivory, attack rates by L. minutus were still significantly lower in C. achates plots. We conclude that the negative effects of C. achates on diffuse knapweed outweigh the potential negative interactions with L. minutus and that the two species will likely have additive effects when released in combination at diffuse knapweed sites.

Laboratory and field experimental evaluation of host plant specificity of Aceria solstitialis, a prospective biological control agent of yellow starthistle

Centaurea solstitialis (yellow starthistle, Asteraceae) is an invasive annual weed in the western USA that is native to the Mediterranean Region and is a target for classical biological control. Aceria solstitialis is an eriophyid mite that has been found exclusively in association with Ce. solstitialis in Italy, Greece, Turkey and Bulgaria. The mite feeds on leaf tissue and damages bolting plants, causing stunting, witch's broom and incomplete flower development. Field experiments and laboratory no-choice and two-way choice experiments were conducted to assess host plant specificity of the mite in Bulgaria. Mites showed the highest degree of host specificity in the field and lowest in the no-choice experiments. In the field, highest densities of mites occurred on Ce. solstitialis and Ce. cyanus (bachelor's button), and either no mites or trace numbers occurred on the other test plants: Ce. diffusa (diffuse knapweed), Carthamus tinctorius (safflower) and Cynara scolymus (artichoke). In no-choice experiments, mites persisted for 60 days on Ce. diffusa, Ce. cyanus, Ce. solstitialis, Ca. tinctorius and Cy. scolymus, whereas in two-way choice experiments mites persisted on 25% of Cy. scolymus plants for 60 days and did not persist on Ca. tinctorius beyond 40 days. The eight other species of plants that were tested in the laboratory were less suitable for the mite. These results suggest that although A. solstitialis can persist on some nontarget plants for as long as 60 days in the laboratory, it appears to be much more specific under natural conditions, and warrants further evaluation as a prospective biological control agent.

Hybridization and invasion: an experimental test with diffuse knapweed (Centaurea diffusa Lam.).

A number of studies have suggested a link between hybridization and invasion. In this study, we experimentally test the potential for hybridization to influence invasion through a greenhouse common garden study. Diffuse knapweed (DK) (Centaurea diffusa Lam.) was introduced to North America with admixture from spotted knapweed (SK) (Centaurea stoebe subsp. stoebe L.). Comparisons between North American DK (including hybrid phenotypes) and native (European) DK in a common garden did not reveal enhanced performance or increased phenotypic variance, suggesting that pre-introduction hybridization or, more generally, post-introduction evolutionary change has not significantly contributed to the invasion of DK. In contrast, early generation hybrids [artificially created Backcross 1 (BC1) plants] exhibited increased variance for eight of the examined traits, and greater leaf and reproductive shoot production when compared to North American DK. Individual BC1 lines differed for several traits, suggesting the importance of the cross for drawing conclusions from such comparisons. When compared to the parental species (DK and SK), the BC1 plants were not transgressive for any of the measured traits. Overall, these findings suggest that if diploid SK is introduced to North America, interspecific hybridization has the potential to result in even more aggressive invaders.

Polyploidy in Phenotypic Space and Invasion Context: A Morphometric Study ofCentaurea stoebes.l.

The taxonomy of the Centaurea stoebe complex is controversial. Diploid and tetraploid plants occur in its native European range, but to date only tetraploids have been recorded from its introduced range in North America. We examined morphological differentiation of C. stoebe using multivariate and univariate approaches to clarify the taxonomic status of the known cytotypes. We measured more than 40 morphological traits on plants originating from 78 populations, grown from seed under uniform glasshouse conditions. The ploidy of almost 300 plants from 2 native and 20 introduced populations from Canada was assessed to test for the absence of diploids from North America. Finally, we explored whether postintroduction processes have resulted in phenotypic changes in introduced plants which may have contributed to the invasion success of C. stoebe. Morphometric analyses showed a clear separation of 2x and 4x plants and thus supported recognition of both cytotypes as separate taxa. Differences in the life cycle, the number of florets, the shape of capitula, and the shape of young rosette leaves were the best discriminant characters. Only minor differences were found between native and introduced tetraploids. All plants from the introduced range except for one hexaploid were found to be tetraploid. Rare diploids from Canada were identified as Centaurea diffusa or Centaurea psamogenna.

Long-term plant growth legacies overwhelm short-term plant growth effects on soil microbial community structure

Plant–soil feedbacks are gaining attention for their ability to determine plant community development. Plant–soil feedback models and research assume that plant–soil interactions occur within days to weeks, yet, little is known about how quickly and to what extent plants change soil community composition. We grew a dominant native plant ( Pseudoroegneria spicata ) and a dominant non-native plant ( Centaurea diffusa ) separately in both native- and non-native-cultivated field soils to test if these species could overcome soil legacies and create new soil communities in the short-term. Soil community composition before and after plant growth was assessed in bulk and rhizosphere soils using phospholipid fatty acid analyses. Nematode abundance and mycorrhizal colonization were also measured following plant growth. Field-collected, native-cultivated soils showed greater bacterial, Gram (−), fungal, and arbuscular mycorrhizal PLFA abundance and greater PLFA diversity than field-collected, non-native-cultivated soils. Both plant species grew larger in native- than non-native-cultivated soils, but neither plant affected microbial composition in the bulk or rhizosphere soils after two months. Plants also failed to change nematode abundance or mycorrhizal colonization. Plants, therefore, appear able to create microbial legacies that affect subsequent plant growth, but contrary to common assumptions, the species in this study are likely to require years to create these legacies. Our results are consistent with other studies that demonstrate long-term legacies in soil microbial communities and suggest that the development of plant–soil feedbacks should be viewed in this longer-term context. ► Plants appear to require several years to cultivate typical microbial communities. ► Short-term plant growth fails to alter microbial structure. ► Rhizosphere soils were more similar to historic than current plant growth. ► Plant–soil feedback processes likely to be slower than thought.

Phenolics and Plant Allelopathy

Phenolic compounds arise from the shikimic and acetic acid (polyketide) metabolic pathways in plants. They are but one category of the many secondary metabolites implicated in plant allelopathy. Phenolic allelochemicals have been observed in both natural and managed ecosystems, where they cause a number of ecological and economic problems, such as declines in crop yield due to soil sickness, regeneration failure of natural forests, and replanting problems in orchards. Phenolic allelochemical structures and modes of action are diverse and may offer potential lead compounds for the development of future herbicides or pesticides. This article reviews allelopathic effects, analysis methods, and allelopathic mechanisms underlying the activity of plant phenolic compounds. Additionally, the currently debated topic in plant allelopathy of whether catechin and 8-hydroxyquinoline play an important role in Centaurea maculata and Centaurea diffusa invasion success is discussed. Overall, the main purpose of this review is to highlight the allelopacthic potential of phenolic compounds to provide us with methods to solve various ecology problems, especially in regard to the sustainable development of agriculture, forestry, nature resources and environment conservation.

Diffuse Knapweed (Centaurea diffusa) Seed Germination

The objective of this study was to define further the environmental requirements for safe sites for germination of diffuse knapweed achenes (seeds). Germination temperature profiles were developed for diffuse knapweed seeds collected from sites in the Great Basin and Colorado. Each profile consisted of seeds germinated at 55 constant or alternating temperatures from 0 through 40 C. The resulting germination was used to develop quadratic response surfaces with regression analysis. Some germination occurred from 71 to 96% of the temperature regimes, depending on the accession being tested. Maximum observed germination ranged from 85 to 98%. Optimum germination, defined as the maximum observed minus one half the confidence interval at the 0.01 level of probability, occurred at a wide range of temperatures from cold periods of 0 through 20 C, alternating with warm periods of 10 through 35 C. The temperature regimes that most frequently supported optimum germination were 5/25 C (5 C for 16 h and 25 C for 8 h in each 24-h period) and 10/25 C. Germination of diffuse knapweed seeds was generally higher at alternating than constant temperatures.

Tumbling: Use of Diffuse Knapweed (Centaurea diffusa) to Examine an Understudied Dispersal Mechanism

AbstractDispersal is a critically important process in the spread of invasive plants. Although knowledge of dispersal will be crucial to preventing the spread of invasive plants, little research has been performed within this context. Many important invasive or agricultural weeds disperse their seeds via tumbling, yet only one previously published paper investigated this dispersal mechanism. Field and wind tunnel experiments were conducted to quantify and model tumbling dispersal. We developed competing models for diffuse knapweed seed dispersal from wind tunnel experiments and compared predictions to data collected from a field site in Colorado. Seeds were retained in plants that had traveled hundreds to as much as 1,039 m (3,408 ft). Although neither model accurately predicted dispersal when compared with independent field data, surprisingly, seed retention with distance was somewhat better described as a linear process than as exponential decay. Wind tunnel trials showed no evidence that the number of seeds deposited per meter depended on plant size. Thus, fecundity might be a key factor determining seed dispersal distances; plants with higher fecundity might disperse seeds over longer distances than those with fewer seeds.

Rangeland Invasive Plant Management

lands provide a valuable resource for wildlife and recreational activities, as well as livestock grazing. The establishment and spread of invasive plants on rangeland are often symptomatic of long-term cultural practices.3 Invasion of many rangeland sites by non-native plants has been facilitated by multiple factors including overgrazing by domestic livestock, purposeful introduction of non-native plants for agricultural purposes, unintentional introduction of non-native plant seeds as a contaminate in crop seed, changes in fire frequency and intensity, and climatic changes that converge to cause shifts in plant community composition. There are over 300 rangeland weeds in the United States, but perhaps 30 have significant negative impact in the western United States.2 Of these, some of the most problematic include downy brome or cheatgrass {Bromus tectorum), medusahead {Taeniatherum caput-medusae), leafy spurge {Euphorbia esula)> perennial pepperweed {Lepidium latifolium), Dalmatian toadflax {Linaria dalmatica), and several members of the Asteraceae, including yellow starthistle {Centaurea solstitialis), diffuse knapweed (C. diffusa), spotted knapweed {C. stoebe subsp. micranthos), musk thistle {Carduus nutans), Canada thistle {Cirsium arvense), hawkweeds {Hieracium spp.), and Russian knapweed {Acroptilon repens). These and other invasive plants can have adverse effects on rangeland and pastures by disrupting vegetation dynamics and reducing ecosystem capacity to recover after disturbance.3 The financial impacts of invasive plants in rangelands are substantial and have been estimated to cost ranchers US$5 billion annually just for control.4 Other impacts to the livestock industry not included in the estimate are the negative effects of invasive plants on yield and quality of forage, livestock poisoning, interference with grazing, supplemental costs associated with managing and producing livestock, and land values. In addition to the livestock industry, invasive rangeland species can decrease wildlife habitat and forage, deplete soil and water resources, and reduce plant and animal diversity. Mechanical, cultural, biological, and chemical methods have been developed to manage rangeland invasive plants. These methods include mowing, prescribed burning, appropriate grazing management, herbicides, biocontrol agents, and revegetation with desirable plants.1 The merits of each control measure and potential for complementary or synergistic interactions when applied in appropriate sequences and combinations should be considered when developing integrated weed management programs. For example, summer burning resulted in a dramatic increase in yellow starthistle germination the following fall and winter.5 Thus, using an herbicide treatment in the year following a prescribed burn gave far better control of yellow starthistle compared with an herbicide treatment followed by prescribed burning. Successful integrated strategies require careful planning that incorporates a long-term approach consisting of prevention programs, education activities, and management approaches. These are designed to diminish negative impacts of invasive plants by improving the invaded plant community to increase its ecosystem function, niche occupation, and competitiveness. These qualities can increase the communities' resistance to reinvasion by other invasive plants.3

Hybridization and invasion: one of North America's most devastating invasive plants shows evidence for a history of interspecific hybridization.

Hybridization has been hypothesized to influence invasion through the generation of novel phenotypes and/or increased levels of genetic variance. Based on morphology, hybrids between diffuse knapweed and spotted knapweed, two invasive plants in North America, are present in the invaded range. Some individuals within most diffuse knapweed sites in North America exhibit intermediate diffuse × spotted floral morphology. We examined hybridization at the molecular level, using amplified fragment length polymorphisms. Approximately a quarter of the assayed North American diffuse knapweed individuals exhibited evidence of introgression from spotted knapweed. However, plants with intermediate morphology did not show evidence of mixed ancestry more often than the plants with typical diffuse knapweed morphology. The high proportion of hybrid individuals in North American diffuse knapweed sites found here, combined with evidence from recent studies, suggests that diffuse knapweed was likely introduced with admixed individuals, and the hybrids are not newly created postintroduction. A century of backcrossing with diffuse knapweed has likely decoupled the relationship between morphology and admixture at the molecular level. In contrast to the scenario encountered in North America, in the native range where diploid diffuse and spotted knapweed overlap, hybrid swarms are common. In such sites, the floral phenotype aligns more closely with the genotype.