Multiple introductions of two invasive Centaurea taxa inferred from cpDNA haplotypes

Knowing the origin of invasive taxa, whether multiple introductions have occurred, and levels of genetic variation relative to the native range, is vital to conducting rigorous tests of hypotheses to explain biological invasions. We explore phylogeographical relationships of two Eurasian knapweed taxa that are invasive in North America, Centaurea diffusa and C. stoebe micranthos (Asteraceae), using chloroplast DNA intron sequences. We also gathered data from C. stoebe stoebe , hybrids between C. diffusa and C. stoebe stoebe ( C. x psammogena ), and three other species in the genus. We sequenced 213 individuals from Eurasia and North America, and found 22 haplotypes. Centaurea diffusa has lower haplotype diversity and allelic richness in the introduced range relative to the native range. Even with reduced variation, the data suggest at least two introductions of C. diffusa . There is a trend towards reduced variation in C. stoebe micranthos , but it is not significant. One of the haplotypes found in North American C. stoebe micranthos matches a haplotype from a taxon other than C. stoebe micranthos in Europe. This suggests introgression of the chloroplast between taxa, or possibly the invasion of another Centaurea taxon into North America. Additionally, C. diffusa , C. stoebe micranthos , and C. stoebe stoebe share several haplotypes, including their most common haplotype. This suggests ongoing hybridization between the species or incomplete segregation. These data can guide further exploration for the origins of these species, and point out locations within the introduced range with unique and diverse genetic makeup.     

Effects of native species diversity and resource additions on invader impact

Theory and empirical work have demonstrated that diverse communities can inhibit invasion. Yet, it is unclear how diversity influences invader impact, how impact varies among exotics, and what the relative importance of diversity is versus extrinsic factors that themselves can influence invasion. To address these issues, we established plant assemblages that varied in native species and functional richness and crossed this gradient in diversity with resource (water) addition. Identical assemblages were either uninvaded or invaded with one of three exotic forbs: spotted knapweed (Centaurea maculosa), dalmatian toadflax (Linaria dalmatica), or sulfur cinquefoil (Potentilla recta). To determine impacts, we measured the effects of exotics on native biomass and, for spotted knapweed, on soil moisture and nitrogen levels. Assemblages with high species richness were less invaded and less impacted than less diverse assemblages. Impact scaled with exotic biomass; spotted knapweed had the largest impact on native biomass compared with the other exotics. Although invasion depressed native biomass, the net result was to increase total community yield. Water addition increased invasibility (for knapweed only) but had no effect on invader impact. Together, these results suggest that diversity inhibits invasion and reduces impact more than resource additions facilitate invasion or impact.     

How do biological control and hybridization affect enemy escape?

Two mechanisms often linked with plant invasions are escape from enemies and hybridization. Classical biological control aims to reverse enemy escape and impose top-down population control. However, hybridization has the potential to alter interactions with enemies and thus affect biological control. We examined how introductions of biological control agents affect enemy escape by comparing specialist enemy loads between the native and introduced ranges of two noxious weeds (spotted and diffuse knapweed; Centaurea stoebe L. and C. diffusa Lam.) that have been the targets of an extensive biological control program. Hybrids between spotted and diffuse knapweed are often found within diffuse knapweed sites in North America, so we also compared enemy loads on plants that appeared morphologically like diffuse knapweed and hybrids. Finally, we tested the preference for diffuse knapweed, hybrids, and spotted knapweed of one of the agents thought to be instrumental in control of diffuse knapweed (Larinus minutus; Curculionidae). In North America spotted knapweed has largely escaped its root herbivores, while seedhead herbivore loads are comparable in the introduced and native ranges. Diffuse knapweed exhibited seedhead herbivore loads five times higher in the introduced compared to native range. While this pattern of seedhead herbivory is expected with successful biological control, increased loads of specialist insect herbivores in the introduced range have rarely been reported in the literature. This finding may partially explain the better population control of diffuse vs. spotted knapweed. Within North American diffuse knapweed sites, typical diffuse knapweed and hybrid plants carried similar herbivore loads. However, in paired feedings trials, the specialist L. minutus demonstrated a preference for newly created artificial hybrids over North American diffuse knapweed and for European diploid spotted knapweed over North American tetraploid spotted knapweed. Overall though, hybridization does not appear to disrupt biological control in this system.     

Effect of summer drought relief on the impact of the root weevil Cyphocleonus achates on spotted knapweed

A recent decline in spotted knapweed, Centaurea stoebe L. subsp. micranthos (Asteraceae), has been observed in parts of western Montana. The release of the biological control agent Cyphocleonus achates (Fahraeus) is thought to contribute to the decline, but persistent drought since at least 1999 may be an additional factor. We conducted outdoor plot experiments to test the relative impacts of C. achates weevils and summer drought relief on spotted knapweed survival and growth. Groups of spotted knapweed transplants were assigned to one of four weekly water addition treatments (no added water, and 0.25, 0.5 or full recovery of plant water deficit, where "deficit" refers to potential evapotranspiration minus rainfall) in May to August 2004 and June to August 2005 and to either exposure to or protection from C. achates. In June of each subsequent year (2005 and 2006), plants were harvested and growth attributes that reflect plant vigor were measured. Drought indices showed that throughout the time of the study until January 2006, western Montana was in drought alert or severe drought. Summer drought relief had no effect on aboveground biomass and plant height of knapweed plants in subsequent years, but feeding by C. achates larvae reduced these two measures of plant vigor. Knapweed plants resuming growth after the drought ended in spring 2006 were significantly larger than those resuming growth under drought conditions in spring 2005. Spring drought may reduce knapweed growth, but C. achates reduced knapweed growth regardless of drought conditions.     

Predicting current and future biological invasions: both native and invaded ranges matter

The classical approach to predicting the geographical extent of species invasions consists of training models in the native range and projecting them in distinct, potentially invasible areas. However, recent studies have demonstrated that this approach could be hampered by a change of the realized climatic niche, allowing invasive species to spread into habitats in the invaded ranges that are climatically distinct from those occupied in the native range. We propose an alternative approach that involves fitting models with pooled data from all ranges. We show that this pooled approach improves prediction of the extent of invasion of spotted knapweed (Centaurea maculosa) in North America on models based solely on the European native range. Furthermore, it performs equally well on models based on the invaded range, while ensuring the inclusion of areas with similar climate to the European niche, where the species is likely to spread further. We then compare projections from these models for 2080 under a severe climate warming scenario. Projections from the pooled models show fewer areas of intermediate climatic suitability than projections from the native or invaded range models, suggesting a better consensus among modelling techniques and reduced uncertainty.     

Linking field based studies with greenhouse experiments: the impact of <Emphasis Type="Italic">Centaurea stoebe</Emphasis> (=<Emphasis Type="Italic">C. maculosa</Emphasis>) in British Columbia grasslands

The grassland community of Lac du Bois Provincial Park in the interior of British Columbia has become increasingly invaded by Centaurea stoebe (=Centaurea maculosa; spotted knapweed). Allelopathy, through the production of the phytotoxin (±)-catechin by C. stoebe roots is believed to be partly responsible for knapweed’s invasive success. We used field sampling and greenhouse experiments to examine two questions: (1) Is increasing abundance of C. stoebe negatively associated with the abundance of specific native grassland species? (2) Do species that exhibit a negative correlation with C. stoebe abundance in the field demonstrate increased levels of susceptibility to application of (±)-catechin during germination? Thirty-eight plots were sampled in the grassland, encompassing areas of low—high knapweed abundance. Seeds from eight native species, exhibiting positive, neutral or negative correlation with knapweed abundance, were treated with three concentrations (0, 0.5, 2.0 mg/mL) of (±)-catechin. Root growth and percent germination were measured over a 6-week period. The results indicate that C. stoebe abundance is negatively correlated with native plant species abundance and may alter plant community composition. Moreover root radical growth was significantly negatively affected by treatment with (±)-catechin in all four native plant species that exhibited a negative correlation with knapweed abundance in the field. Past studies have failed to conclusively link greenhouse results with plant community patterns. Here, we provide a correlative link between plant community composition and tolerance to a phytotoxin.     

Allopolyploid origin of highly invasive Centaurea stoebe s.l. (Asteraceae)

Spotted knapweed (Centaurea stoebe) occurs from Western Asia to Western Europe both as diploid and tetraploid cytotypes, predominantly in single-cytotype populations with higher frequency of diploid populations. Interestingly, only tetraploids have been recorded so far from its introduced range in North America where they became highly invasive. We performed phylogenetic and network analyses of more than 40 accessions of the C. stoebe and C. paniculata groups and other related taxa using cloned internal transcribed spacer (ITS) and sequences of the chloroplast trnT-trnL and atpBrbcL regions to (i) assess the evolutionary origin of tetraploid C. stoebe s.l., and (ii) uncover the phylogeny of the C. stoebe group. Both issues have not been studied so far and thus remained controversial. Cloned ITS sequences showed the presence of two slightly divergent ribotypes occurring in tetraploid cytotype, while only one major ribotype was present in diploid C. stoebe s.str. This pattern suggests an allopolyploid origin of tetraploids with contribution of the diploid C. stoebe s.str. genome. Although we were not able to detect the second parental taxon, we hypothesize that hybridization might have triggered important changes in morphology and life history traits, which in turn may explain the colonization success of the tetraploid taxon. Bayesian relaxed clock estimations indicate a relatively recent--Pleistocene origin of the tetraploid C. stoebe s.l. Furthermore, our analyses showed a deep split between the C. paniculata and C. stoebe groups, and a young diversification of the taxa within the C. stoebe group. In contrast to nrDNA analyses, the observed pattern based on two cpDNA regions was inconclusive with respect to the origin and phylogeny of the studied taxa, most likely due to shared ancient polymorphism and frequent homoplasies.     

Soil pathogen communities associated with native and non‐native Phragmites australis populations in freshwater wetlands

Soil pathogens are believed to be major contributors to negative plant–soil feedbacks that regulate plant community dynamics and plant invasions. While the theoretical basis for pathogen regulation of plant communities is well established within the plant–soil feedback framework, direct experimental evidence for pathogen community responses to plants has been limited, often relying largely on indirect evidence based on above‐ground plant responses. As a result, specific soil pathogen responses accompanying above‐ground plant community dynamics are largely unknown. Here, we examine the oomycete pathogens in soils conditioned by established populations of native noninvasive and non‐native invasive haplotypes of Phragmites australis (European common reed). Our aim was to assess whether populations of invasive plants harbor unique communities of pathogens that differ from those associated with noninvasive populations and whether the distribution of taxa within these communities may help to explain invasive success. We compared the composition and abundance of pathogenic and saprobic oomycete species over a 2‐year period. Despite a diversity of oomycete taxa detected in soils from both native and non‐native populations, pathogen communities from both invaded and noninvaded soils were dominated by species of Pythium. Pathogen species that contributed the most to the differences observed between invaded and noninvaded soils were distributed between invaded and noninvaded soils. However, the specific taxa in invaded soils responsible for community differences were distinct from those in noninvaded soils that contributed to community differences. Our results indicate that, despite the phylogenetic relatedness of native and non‐native P. australis haplotypes, pathogen communities associated with the dominant non‐native haplotype are distinct from those of the rare native haplotype. Pathogen taxa that dominate either noninvaded or invaded soils suggest different potential mechanisms of invasion facilitation. These findings are consistent with the hypothesis that non‐native plant species that dominate landscapes may “cultivate” a different soil pathogen community to their rhizosphere than those of rarer native species.     

Influence of seed head-attacking biological control agents on spotted knapweed reproductive potential in western Montana over a 30-year period

Five insect biological control agents that attack flower heads of spotted knapweed, Centaurea stoebe L. subsp. micranthos (Gugler) Hayek, became established in western Montana between 1973 and 1992. In a controlled field experiment in 2006, seed-head insects reduced spotted knapweed seed production per seed head by 84.4%. The seed production at two sites in western Montana where these biological control agents were well established was 91.6-93.8% lower in 2004-2005 than 1974-1975, whereas the number of seed heads per square meter was 70.7% lower, and the reproductive potential (seeds/m(2)) was 95.9-99.0% lower. The average seed bank in 2005 at four sites containing robust spotted knapweed populations was 281 seeds/m(2) compared with 19 seeds/m(2) at four sites where knapweed density has declined. Seed bank densities were much higher at sites in central Montana (4,218 seeds/m(2)), where the insects have been established for a shorter period. Urophora affinis Frauenfeld was the most abundant species at eight study sites, infesting 66.7% of the seed heads, followed by a 47.3% infestation by Larinus minutus Gyllenhal and L. obtusus Gyllenhal. From 1974 to 1985, Urophora spp. apparently reduced the number of seeds per seed head by 34.5-46.9%; the addition of Larinus spp. further reduced seed numbers 84.2-90.5% by 2005. Path analysis indicated that both Larinus spp. and U. affinis contributed significantly to reduction of seed production over the 30-yr period. Spotted knapweed density may not decrease significantly until the seed bank falls below a critical threshold.     

Influence of Nutrient Availability on the Interaction Between Spotted Knapweed and Bluebunch Wheatgrass

Centaurea maculosa (Lam.) (spotted knapweed) reduces wildlife and livestock habitat biodiversity and increases erosion. Nutrient availability to plants may be used to accelerate succession away from spotted knapweed. Early‐successional plant communities often have high nutrient availability, whereas late‐successional communities are often found on lower nutrient soils. We hypothesized that removal of nutrients would change the competitive advantage from spotted knapweed to Pseudoroegneria spicatum (bluebunch wheatgrass) (late seral). In two addition series matrices, background densities of Secale cereale (annual rye) and Elymus elimoides (bottlebrush squirreltail) (3,000 seeds/m²) were used to remove nutrients from the soil. In another set of addition series matrices, nitrogen (33 kg/ha) or phosphorus (33 kg/ha) were added to the soil. Nutrient analysis of soil and vegetation indicated that annual rye and bottlebrush squirreltail reduced nutrient availability in soils. In another matrix, neither a background density nor nutrients were added. Data were fit into Watkinson's curvilinear model to determine the competitive relationship between bluebunch wheatgrass and spotted knapweed. This allowed comparison of the equivalence ratios (C) generated from each addition series. The C parameters are the per‐plant equivalent of bluebunch wheatgrass or spotted knapweed and can be interpreted as the ratio of intra‐to‐interspecific competition. The C parameters are also the equivalence ratio of the number of spotted knapweed it takes to have equivalent effect on bluebunch wheatgrass or the number of bluebunch wheatgrass having the equivalent effect on spotted knapweed. Without nutrient manipulation, spotted knapweed was more competitive than bluebunch wheatgrass. The C for bluebunch wheatgrass was 0.17, indicating that 0.17 knapweed plants were competitively equivalent to one wheatgrass. Annual rye changed the competitive balance in favor of bluebunch wheatgrass (C = 9.9). Addition of nitrogen, phosphorus, or the mid‐seral species did not change the competitive relationship between the two species. This preliminary study suggests that succession from spotted knapweed to late‐seral bluebunch wheatgrass community may be accelerated by altering resource availability.     

Do invasive plants structure microbial communities to accelerate decomposition in intermountain grasslands?

Invasive plants are often associated with greater productivity and soil nutrient availabilities, but whether invasive plants with dissimilar traits change decomposer communities and decomposition rates in consistent ways is little known. We compared decomposition rates and the fungal and bacterial communities associated with the litter of three problematic invaders in intermountain grasslands; cheatgrass (Bromus tectorum), spotted knapweed (Centaurea stoebe) and leafy spurge (Euphorbia esula), as well as the native bluebunch wheatgrass (Pseudoroegneria spicata). Shoot and root litter from each plant was placed in cheatgrass, spotted knapweed, and leafy spurge invasions as well as remnant native communities in a fully reciprocal design for 6 months to see whether decomposer communities were species‐specific, and whether litter decomposed fastest when placed in a community composed of its own species (referred to hereafter as home‐field advantage–HFA). Overall, litter from the two invasive forbs, spotted knapweed and leafy spurge, decomposed faster than the native and invasive grasses, regardless of the plant community of incubation. Thus, we found no evidence of HFA. T‐RFLP profiles indicated that both fungal and bacterial communities differed between roots and shoots and among plant species, and that fungal communities also differed among plant community types. Synthesis. These results show that litter from three common invaders to intermountain grasslands decomposes at different rates and cultures microbial communities that are species‐specific, widespread, and persistent through the dramatic shifts in plant communities associated with invasions.     

Invasion of an exotic forb impacts reproductive success and site fidelity of a migratory songbird

Although exotic plant invasions threaten natural systems worldwide, we know little about the specific ecological impacts of invaders, including the magnitude of effects and underlying mechanisms. Exotic plants are likely to impact higher trophic levels when they overrun native plant communities, affecting habitat quality for breeding songbirds by altering food availability and/or nest predation levels. We studied chipping sparrows (Spizella passerina) breeding in savannas that were either dominated by native vegetation or invaded by spotted knapweed (Centaurea maculosa), an exotic forb that substantially reduces diversity and abundance of native herbaceous plant species. Chipping sparrows primarily nest in trees but forage on the ground, consuming seeds and arthropods. We found that predation rates did not differ between nests at knapweed and native sites. However, initiation of first nests was delayed at knapweed versus native sites, an effect frequently associated with low food availability. Our seasonal fecundity model indicated that breeding delays could translate to diminished fecundity, including dramatic declines in the incidence of double brooding. Site fidelity of breeding adults was also substantially reduced in knapweed compared to native habitats, as measured by return rates and shifts in territory locations between years. Declines in reproductive success and site fidelity were greater for yearling versus older birds, and knapweed invasion appeared to exacerbate differences between age classes. In addition, grasshoppers, which represent an important prey resource, were substantially reduced in knapweed versus native habitats. Our results strongly suggest that knapweed invasion can impact chipping sparrow populations by reducing food availability. Food chain effects may be an important mechanism by which strong plant invaders impact songbirds and other consumers.     

A knapweed biological control perspective

Diffuse and spotted knapweed (Centaurea diffusa Lam. and C. stoebe micranthos (Gugler) Hayek) are Eurasian plants that devastate dry and mesic North American grasslands. They have a mutualistic association with arbuscular mycorrhizal fungal (AMF) phylotypes with hyphal links to nearby plants and a nutrient flux to the strongest sink, usually knapweed. They displace many AMF beneficial to grass and affect knapweed nutrient allocation, biology, knapweed insects and probably root necrosis and emergence of ant buried seed. AMF determined nutrient root or shoot allocation determines nutrient shoot and root allocation and the benefit to root or seed-head insect species and whether C. diffusa is an annual–biannual or a semelparous perennial needing 5 or more years to flower. Both knapweeds do well without its AMF phylotypes without competition in fertile soil. In grass in Eurasia, they have a community of seven seed-head species segregated by head development stage. Prolonged seed dormancy buffered knapweed decline that resulted in release of a surfeit seed-head species. The presence of an eliasome on the seed and vigorous seedling clumps suggests burial by myrmecochorous ants with AMF supplied carbon supporting their growth. The root species community is segregated by habitat, climate, root part, and size. With larval induced compensatory growth and AMF nutrient sharing, the growth of plants with and without a larva was the same. On feeding completion, a nutrient out flux from the attacked plants reduced growth; but without killing. This needs a dual species or a repeated single species attack. Root species packing increases knapweed utilization; but the four approved species are insufficient for maximum utilization. Two additions are suggested. The aim of the paper is to provide enough understanding of the AMF and its plant and insect interactions to facilitate knapweed biological control and avoid past mistakes.     

Is spotted knapweed (<Emphasis Type="Italic">Centaurea stoebe</Emphasis> L.) patch size related to the effect on soil and vegetation properties?

Spotted knapweed (Centaurea stoebe L. subsp. Micranthos (Gugler) Hayek) was first introduced in the 1890s from Europe into western North America, where it now occupies over three million hectares of rangeland and pasture in 14 states and two Canadian provinces, reducing forage production and causing economic damage. Despite many reported effects spotted knapweed can have on soils and native vegetation, it is not known whether patch size is correlated with these ecosystem-level effects. The objective of our study was to determine whether the effects of spotted knapweed on plant composition and soil properties was related to spotted knapweed patch size. We asked the following questions: (1) Are there differences in plant species richness and diversity between small and large knapweed patches? and (2) Do soil water and soil mineral nutrient properties change depending on knapweed patch size? Twenty-four knapweed patches, and paired natural grassland plots, were randomly selected within Lac du Bois Provincial Park, British Columbia, Canada. Knapweed patch size ranged from 6 to 366 m². Sampling and analysis revealed a significant effect of knapweed patch size on soil and vegetation properties. Soil P, soil temperature, and total dry plant biomass (g/0.25 m²) increased, while soil N, soil C, and soil moisture decreased with patch size. Since our results show that spotted knapweed patch size is related to degree of soil alteration, it is important to consider size of patch when modeling the impact of spotted knapweed in North America. Since large patches of spotted knapweed seem to have a proportionately greater effect on soil chemistry properties, large patches may move the system further away from a point where it is possible to restore the site to pre-invasion conditions.     

Geographic locality greatly influences fungal endophyte communities in Cephalotaxus harringtonia

Although endophytes of conifers have been extensively studied, few data are available on Cephalotaxaceae. We examined foliar and stem endophytes of Cephalotaxus harringtonia, within its natural range in Japan and outside its natural range in France to study the effect of geography on endophyte community composition. In Japan, rapidly growing endophytes were dominant and may have masked the real diversity, in comparison to France where most endophytes were growing slowly. Analyses of ITS rDNA revealed 104 different Blast Groups among 554 isolates. Almost no overlap between endophyte assemblages of C. harringtonia from the two countries was observed. It seems that Japanese C. harringtonia trees, which should be well adapted to their native site, would host a specific, endemic endophyte community, while trees that have been introduced recently to a foreign site, in France, should have captured existing cosmopolitan and more generalist taxa. In Japan the majority of xylariaceous taxa, which dominated the communities, were unknown and, although closely related to Asian taxa, may be new to science. Dothideomycetes were more prevalent in France. Locally, urban environment, particularly in Japan, may have introduced some perturbations in the native endophyte community of C. harringtonia, with an abundance of generalist fungi such as Nigrospora and Colletotrichum.     

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.     

Impact of clear-cutting and prescribed burning on microbial diversity and community structure in a Jack pine (Pinus banksiana Lamb.) clear-cut using Biolog Gram-negative microplates

Three isolates ofFusarium avenaceum are pathogenic on spotted knapweed(Centaurea maculosa), a major weed plant of pasturelands and rangelands of the Pacific Northwestern USA. One isolate (no. 1) obtained from the European centre of origin of knapweed and isolate no. 365 native to Montana, did not significantly affect knapweed seed germination. However,F. avenaceum no. 1003, another Montana native isolate, caused a 100% decrease in seed germination and hence, no seedling emergence. When formulated, isolate no. 1003, could be recovered from treated soils after 7 days and caused a significant reduction in seedling emergence or seedling dry weight. This organism had no effect on the germination ofTriticum aestivum orMedicago sativa, but did affect the germination of other plant species.F. avenaceum appears to be a candidate for the biocontrol of spotted knapweed, however, a native isolate is potentially more effective than an isolate obtained from the centre of origin ofC. maculosa.     

Soil physical properties associated with the invasive spotted knapweed and native grasses are similar

Centaurea maculosa Lam. (spotted knapweed), a Eurasian perennial forb, has invaded disturbed and undisturbed semiarid grasslands in the western United States. In the past, success in controlling C. maculosa and restoring invaded areas has been limited. Most research has addressed chemical aspects of invasive species interactions with soils, while potential impacts of altered soil physical properties on C. maculosa's success has not been studied. We hypothesized that the persistence of C. maculosa in semiarid rangelands might reflect an ability to alter site conditions. The objective of this study was to compare selected soil physical properties under C. maculosa-dominated and native perennial grass-dominated areas on semiarid grassland. We used six field sites in western Montana containing adjacent plots dominated by C. maculosa and by native perennial grasses. Soil physical properties including particle size fractions, bulk density, and hydraulic and thermal properties, as well as total organic carbon content, of near-surface soils were measured for each vegetation type. Soil physical properties seldom differed between C. maculosa- and native grass-dominated areas. When soil physical properties differed, the differences were inconsistent within and among sites. Presence of C. maculosa did not alter surface soil characteristics at our six sites, thus its persistence on these semi-arid grasslands cannot be explained by an ability to alter near-surface soil characteristics.     

Simulated nitrogen deposition influences the growth and competitive ability of Centaurea stoebe populations

Aims Soil nitrogen (N) availability is the most limiting factor for terrestrial plant growth, and global N deposition can improve the soil N availability. Fast growth may be a general trait of successful invaders, so learning how N addition affected the growth and competitive ability of three Centaurea stoebe populations is conductive to forecasting the plant invasion risk under N deposition. Methods We conducted an experiment simulating N deposition at Chengdu, in which three populations from the invasive forb C. stoebe and one native species Poa pratensis were subjected to two treatments: N addition and ambient. In our study, C. stoebe populations and P. pratensis were planted alone or together, and we determined plant height, leaf area and biomass. Important findings In the absence of competition, N addition promoted the growth of C. stoebe populations, thereby improving their invasive potential to a certain extent. So under the condition of competition, we found that N addition obviously enhanced the competitive effects of C. stoebe on P. pratensis, particularly interspecific root competition. The competitive ability of different populations performed similarly in response to N addition. These results preliminarily suggest that N deposition may increase the potential invasion risks of C. stoebe populations by improving their competitive ability.     

Invasion of Lepidium draba (Brassicaceae) in the western United States: distributions and origins of chloroplast DNA haplotypes

Advances in phylogeography are of great value for understanding the population structure and origins of invasive genotypes. Such insights provide constructive information for current or future biological control research efforts. In this study, we investigated a highly variable chloroplast DNA (cpDNA) marker for populations of the weed Lepidium draba (Brassicaceae) in its native Eurasian and invasive US ranges. We sequenced DNA from 684 individuals from Eurasia and the US and found 41 different haplotypes. Our comparative study between the native and invasive ranges showed a 33% reduction in allelic richness (A) and a 7% reduction in haplotype diversity (h) since introduction into the US. Most genetic variation in the native range was observed within geographical regions and populations, not between regions, and this result was similar for the invasive range. Assignment tests indicated the most likely origins of many invasive haplotypes. Some of these occurred in western Europe, supporting an expanded native range that had been proposed for the species. Exact locations were identified for a diverse set of invasive haplotypes which can be used in ongoing host-specificity tests of potential biological control agents.