Invasive Centaurea Stoebe Research Articles

Increased reproduction under disturbance is responsible for high population growth rate of invasive Centaurea stoebe

To further our understanding of biological invasions, it is important to evaluate hypothesized causal factors facilitating invasions using manipulative field experiments. Disturbance and density are frequently invoked as major factors operating during the process of plant invasions. Soil disturbance by mowing can increase nutrient and light availability while intraspecific competition can reduce potential invader success. Disturbance can therefore reduce the negative effects of density, however the interaction between these effects on invaders have rarely been tested. We studied the combined effect of intraspecific density and soil disturbance on the population dynamics of Centaurea stoebe, an invader in North America. We used an experimental field study of spotted knapweed established in Stony Brook, NY, to parameterize matrix models for four combined disturbance and density treatments. We conducted elasticity analysis to investigate the potential influence of demographic variables on population growth rate. We also used retrospective analysis to identify the actual influence of these variables on the population growth rate of plants in our experimental study. We found higher differences in λ between disturbance treatments compared to between density treatments. LTRE analysis showed that the increased λ of spotted knapweed in response to disturbance was due to substantial increases in survival of vegetative plants and reproduction. Prospective elasticity analysis revealed that λ was most sensitive to small changes in the transition of vegetative plants to reproductive plants, and the production of vegetative plants from reproductive plants. Our results show that the increased growth rate response of spotted knapweed populations to disturbance occurs irrespective of density, and emphasizes the importance of reducing reproduction in the management of spotted knapweed.

The influence of environmental factors on the distribution and density of invasive Centaurea stoebe across Northeastern USA

Centaurea stoebe is an emerging invader in northeast US, and is a major invasive plant in the northern Midwest and western USA. Although it has been present in New York State (NYS) for over 100 years, its apparent recent population increases and spread provide a rare opportunity to study a plant in the early stages of invasion. Using occurrence, density and change in density in species distribution models, we assessed the potential influence of environmental factors on the invasion of spotted knapweed in northeast US. Within different parts of C. stoebe‘s range, different factors explained its occurrence, density and change in density over 2 years. Across northeast US, climate and soil factors were the most influential predictors explaining C. stoebe‘s distribution, while within Long Island in southeastern NYS and the Adirondack Mountains in northern NYS, precipitation and disturbance respectively were the most important. Using density and change in density, we found that for both Long Island and the Adirondacks, tree cover, precipitation, and temperature were the most influential determinants. The comparatively minor influence of disturbance on C. stoebe density is likely because C. stoebe in the northeastern US is almost exclusively found in open disturbed sites, so this factor does not discriminate among densities. Our study is valuable for both basic ecological understanding and decision making, as control efforts may depend on whether management goals are to decrease the risk of spread to other locations, or decrease the density in locations in which invaders already occur.

Dynamics of an Invasion: The Spatial Interactions of Invasive Centaurea stoebe with Native Cirsium pitcheri and Tanacetum huronense in a Dune Environment

Abstract Centaurea stoebe is a major invader of sensitive dune habitats of the Great Lakes region in North America, home to many unique and increasingly rare species. We analyzed the beginning stages of invasion of Centaurea on a relatively isolated island in northern Lake Michigan and sought to determine whether Centaurea is negatively impacting two species of concern, Cirsium pitcheri and Tanacetum huronense. Using a nearest neighbor analysis of mapped individuals in a long term study plot, we determined Centaurea, Cirsium, and Tanacetum all have aggregated underlying spatial structures. Two-species analyses revealed highly statistically significant segregation between the species but no significant positive or negative association between the invader and either native species. Using abundance data in widespread smaller plots, we found no statistical indication that high abundances of Centaurea were correlated with low abundances of the two native species, despite marked increases in the invader. In the...

Flowering phenology influences bee community dynamics in old fields dominated by the invasive plant Centaurea stoebe

Wild and managed bees provide critical pollination services to both native and cultivated plants, and the invasion of exotic plants can have positive or negative effects on bee communities. In this study we investigated the influence of the exotic invasive Centaurea stoebe ssp. micranthos (spotted knapweed) on bee species diversity and abundance in old-fields (Michigan, USA). We conducted field observations in knapweed dominated fields and adjacent fields with greater forb diversity to determine whether the bee communities foraging in each field type differed in their composition, diversity, and abundance, and to determine how bees’ use of floral resources changed throughout the growing season. This was coupled with a common garden experiment that contrasted the attractiveness of C. stoebe to 12 native plant species, which occur in similar habitats. Both studies were conducted throughout the 2012–2013 growing seasons to examine the temporal effect of floral resource availability. C. stoebe was the most heavily-visited plant, in terms of total bee visitation and bee species richness in both studies. While C. stoebe-dominated old-fields had high floral resource levels during its peak bloom period, before and after this period these fields contained very few floral resources. In contrast, diverse fields had a number of flowering plant species that provided floral resources before, during, and after C. stoebe bloom. As a result, diverse fields contained significantly greater season-long floral resource availability and significantly greater season-total bee abundance, diversity and species richness. This greater species richness was driven at least in part by the ability of diverse flowering plant communities to support bee species that are active before and after the bloom period of C. stoebe. Our results suggest that efforts to manage C. stoebe should take into account the floral resources the plant is providing, and coincide with the restoration of diverse forb communities in order to enhance bee foraging habitat in old-fields.

The population genetics of the fundamental cytotype-shift in invasive Centaurea stoebe s.l.: genetic diversity, genetic differentiation and small-scale genetic structure differ between cytotypes but not between ranges

Polyploids are overrepresented in invasive species. Yet, the role of genetic diversity and drift in colonization success of polyploids remains unclear. Here, we investigate genetic diversity, genetic differentiation and small-scale genetic structure in our model system, the three geo-cytotypes of Centaurea stoebe: monocarpic diploids and polycarpic (allo)tetraploids coexist in the native range (Eurasia), but only tetraploids are reported from the invasive range (North America). For each geo-cytotype, we investigated 18–20 populations varying in size and habitat type (natural vs. ruderal). Population genetic analyses were conducted at eight microsatellite loci. Compared to diploids, tetraploids revealed higher genetic diversity and lower genetic differentiation, whereas both were comparable in tetraploids between both ranges. Within spatial distances of a few meters, diploid individuals were more strongly related to one another than tetraploids. In addition, expected heterozygosity in diploids increased with population size and was higher in natural than in ruderal habitats. However, neither relationship was found for tetraploids. The higher genetic diversity of tetraploid C. stoebe may have enhanced its colonization abilities, if genetic diversity is correlated with fitness and adaptive capabilities. Furthermore, the inheritance of a duplicated chromosome set as well as longevity and frequent gene flow reduces drift in tetraploids. This counteracts genetic depletion during initial introductions and in subsequent phases of small or fluctuating population sizes in ruderal habitats. Our findings advocate the importance of studying colonization genetic processes to gain a more mechanistic understanding of the role of polyploidy in invasion dynamics.

Dissecting impact of plant invaders: Do invaders behave differently in the new range?

Knowledge from basic plant ecology suggests that impact of one plant species on another is driven by either competition for the same limiting resources, or by unique plant traits. These processes might be context specific, explaining a differential impact of exotic plant invaders in the native vs. introduced range. With the help of a conceptual framework, we aimed at identifying the relationship between invader biomass and impact in the invasive Centaurea stoebe by conducting pairwise competition experiments with 15 European (old) and 15 North American (new) neighboring species. Old neighbors grew larger and could use available soil moisture more efficiently for growth than new neighbors. Interestingly, biomass of C. stoebe explained a substantial amount of the variation in biomass of the coevolved neighbors, but not of the new "naive" neighbors. Thus, impact in the home range appears to be driven by competition for the same limiting resources, but by other factors in the introduced range, possibly by exploitation of resources that are not used by the new neighbors or by interference competition. This distinction has important consequences for the management of invasive species, as in our study ecosystem recovery is less likely after simple biomass reduction.

Effects of soil fungi, disturbance and propagule pressure on exotic plant recruitment and establishment at home and abroad

SummaryBiogeographic experiments that test how multiple interacting factors influence exotic plant abundance in their home and recipient communities are remarkably rare. We examined the effects of soil fungi, disturbance and propagule pressure on seed germination, seedling recruitment and adult plant establishment of the invasiveCentaurea stoebein its native European and non‐native NorthAmerican ranges.Centaurea stoebecan establish virtual monocultures in parts of its non‐native range, but occurs at far lower abundances where it is native. We conducted parallel experiments at four European and four Montana (USA) grassland sites with all factorial combinations of ± suppression of soil fungi, ±disturbance and low versus high knapweed propagule pressure [100 or 300 knapweed seeds per 0.3 m × 0.3 m plot (1000 or 3000 per m2)]. We also measured germination in buried bags containing locally collected knapweed seeds that were either treated or not with fungicide.Disturbance and propagule pressure increased knapweed recruitment and establishment, but did so similarly in both ranges. Treating plots with fungicides had no effect on recruitment or establishment in either range. However, we found: (i) greater seedling recruitment and plant establishment in undisturbed plots in Montana compared to undisturbed plots in Europe and (ii) substantially greater germination of seeds in bags buried in Montana compared to Europe. Also, across all treatments, total plant establishment was greater in Montana than in Europe.Synthesis. Our results highlight the importance of simultaneously examining processes that could influence invasion in both ranges. They indicate that under ‘background’ undisturbed conditions, knapweed recruits and establishes at greater abundance in Montana than in Europe. However, our results do not support the importance of soil fungi or local disturbances as mechanisms for knapweed's differential success in North America versus Europe.

(±)-Catechin, A Root Exudate of the Invasive Centaurea Stoebe Lam. (Spotted Knapweed) Exhibits Bacteriostatic Activity Against Multiple Soil Bacterial Populations

Understanding the effects of allelopathic plant chemicals on soil microorganisms is critical to understanding their ecological roles and importance in exotic plant invasion. Centaurea stoebe Lam. (spotted knapweed), an aggressive invasive weed in North America, secretes a racemic mixture of (±)-catechin as a root exudate. This enantiomeric, polyphenolic compound has been reported to have allelopathic effects on surrounding flora and microflora. To better understand how catechin affects microbial communities in the root zone of spotted knapweed, we assessed its impact on the total culturable bacterial component and numerous individual bacterial populations from Romanian (native range) and Montana (invaded range) soils. Catechin suppressed total culturable count numbers from the bacterial community and inhibited growth of some, but not all, soil bacterial populations tested. The native soil bacterial community was significantly more resistant to inhibitory effects of catechin than either the invaded or non-invaded soils. We further show that the inhibitory effect of catechin on nine different soil bacterial strains from seven genera was reversible, demonstrating that it acts via a bacteriostatic rather than bactericidal mechanism. These findings suggest that catechin might affect bacterial community composition and activity in the root zone.