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Xavier Vekemans - One of the best experts on this subject based on the ideXlab platform.
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A morphometric study of populations of the Centaurea jacea complex (Asteraceae) in Belgium
Plant Biology, 2002Co-Authors: Sonia Vanderhoeven, Olivier J. Hardy, Xavier Vekemans, Claude Lefebvre, M De Loose, Jacques Lambinon, Pierre MeertsAbstract:The Centaurea jacea aggregate is a polymorphic polyploid complex whose taxonomic treatment is still controversial. A numerical taxonomic approach was applied to 394 individuals of known ploidy level, from 19 populations, based on the main diagnostic characters proposed in earlier revisions. Populations from xeric grasslands were not considered. Principal Component Analysis shows that variation within the complex is continuous. UPGMA Cluster Analysis based on population means supports the recognition of three groups of populations. However, the limits between these groups are blurred to a considerable extent due to extensive within-population polymorphism. It is argued that the Belgian populations of Centaurea jacea occurring in mesic grasslands should be treated as a single species, with three subspecies. The two extremes of the morphological gradient can be referred to as C. jacea subsp. jacea and C. jacea subsp. nigra, with C. jacea subsp. pratensis occupying an intermediate position. Most populations from Belgium are tetraploid, a diploid chromosome number being found only in populations of C. jacea subsp. nigra from the Ardennes massif. On average, diploids grow at higher altitude and on more acidic soils than tetraploids. Finally, a key to the three subspecies is provided.
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Allozyme segregation and inter-cytotype reproductive barriers in the polyploid complex Centaurea jacea
Heredity, 2001Co-Authors: Olivier J. Hardy, Marc De Loose, Xavier Vekemans, Pierre MeertsAbstract:In eastern Belgium, diploid and tetraploid knapweeds ( Centaurea jacea L. s. l. ) show a parapatric distribution with a contact zone. To compare the success of inter- and intra-cytotype crosses, and to investigate the pattern of allozyme segregation, controlled crosses involving plants from this contact zone were performed. Polysomic inheritance at two loci was observed in tetraploids, suggesting an autopolyploid origin. Two crosses allowed the detection of double reduction events in tetraploids at one locus, but no such event was detected among the 217 progenies. Null alleles were detected in tetraploids at two loci. Both cytotypes were highly self-incompatible. Inter-cytotype crosses were much less successful than intra-cytotype crosses in terms of seed set (five-fold reduction) and germination rates (three-fold reduction), suggesting a ‘triploid block’ effect. Progenies from inter-cytotype crosses most often had the maternal cytotype and resulted from selfing or pollen contamination. However, six triploids were detected. The significance of these results for the understanding of the geographical distribution of the cytotypes and inter-cytotype gene flow is discussed.
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Patterns of allozyme variation in diploid and tetraploid Centaurea jacea at different spatial scales.
Evolution; international journal of organic evolution, 2001Co-Authors: Olivier J. Hardy, Xavier VekemansAbstract:The extent and spatial patterns of genetic variation at allozyme markers were investigated within and between diploid and autotetraploid knapweeds (Centaurea jacea L. sensu lato, Asteraceae) at contrasted geographic scales: (1) among populations sampled from a diploid-tetraploid contact zone in the northeastern part of the Belgian Ardennes, and (2) within mixed populations from that zone where diploids and tetraploids coexist. Our data were also compared with a published dataset by Sommer (1990) describing allozyme variation in separate diploid and tetraploid knapweeds populations collected throughout Europe. Genetic diversity was higher in tetraploids. In the Belgian Ardennes and within the mixed populations, both cytotypes had similar levels of spatial genetic structure, they were genetically differentiated, and their distributions of allele frequencies were not spatially correlated. In contrast, at the European scale, diploids and tetraploids did not show differentiated gene pools and presented a strong correlation between their patterns of spatial genetic variation. Numerical simulations showed that the striking difference in patterns observed at small and large geographic scales could be accounted for by a combination of (1) isolation by distance within cytotypes; and (2) partial reproductive barriers between cytotypes and/or recurrent formation of tetraploids. We suggest that this may explain the difficulty of the taxonomic treatment of knapweeds and of polyploid complexes in general.
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Spatial autocorrelation of allozyme and quantitative markers within a natural population of Centaurea jacea (Asteraceae)
Journal of Evolutionary Biology, 2000Co-Authors: Olivier J. Hardy, Pierre Meerts, Sonia Vanderhoeven, Xavier VekemansAbstract:This paper compares the fine-scale genetic structure of quantitative traits and allozyme markers within a natural population of Centaurea jacea s.l. To that end, a spatial autocorrelation approach is developed based on pairwise correlation coefficients between individuals and using sib families. Statistical properties of the proposed statistics are investigated with numerical simulations. Our results show that most quantitative traits have a significant spatial structure for their genetic component. On average, allozyme markers and the genetic component of quantitative traits have similar patterns of spatial autocorrelation that are consistent with a neutral model of isolation by distance. We also show evidence that environmental heterogeneity generates a spatial structure for the environmental component of quantitative traits. Results are discussed in terms of mechanisms generating spatial structure and are compared with those obtained on a large geographical scale.
Olivier J. Hardy - One of the best experts on this subject based on the ideXlab platform.
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A morphometric study of populations of the Centaurea jacea complex (Asteraceae) in Belgium
Plant Biology, 2002Co-Authors: Sonia Vanderhoeven, Olivier J. Hardy, Xavier Vekemans, Claude Lefebvre, M De Loose, Jacques Lambinon, Pierre MeertsAbstract:The Centaurea jacea aggregate is a polymorphic polyploid complex whose taxonomic treatment is still controversial. A numerical taxonomic approach was applied to 394 individuals of known ploidy level, from 19 populations, based on the main diagnostic characters proposed in earlier revisions. Populations from xeric grasslands were not considered. Principal Component Analysis shows that variation within the complex is continuous. UPGMA Cluster Analysis based on population means supports the recognition of three groups of populations. However, the limits between these groups are blurred to a considerable extent due to extensive within-population polymorphism. It is argued that the Belgian populations of Centaurea jacea occurring in mesic grasslands should be treated as a single species, with three subspecies. The two extremes of the morphological gradient can be referred to as C. jacea subsp. jacea and C. jacea subsp. nigra, with C. jacea subsp. pratensis occupying an intermediate position. Most populations from Belgium are tetraploid, a diploid chromosome number being found only in populations of C. jacea subsp. nigra from the Ardennes massif. On average, diploids grow at higher altitude and on more acidic soils than tetraploids. Finally, a key to the three subspecies is provided.
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Allozyme segregation and inter-cytotype reproductive barriers in the polyploid complex Centaurea jacea
Heredity, 2001Co-Authors: Olivier J. Hardy, Marc De Loose, Xavier Vekemans, Pierre MeertsAbstract:In eastern Belgium, diploid and tetraploid knapweeds ( Centaurea jacea L. s. l. ) show a parapatric distribution with a contact zone. To compare the success of inter- and intra-cytotype crosses, and to investigate the pattern of allozyme segregation, controlled crosses involving plants from this contact zone were performed. Polysomic inheritance at two loci was observed in tetraploids, suggesting an autopolyploid origin. Two crosses allowed the detection of double reduction events in tetraploids at one locus, but no such event was detected among the 217 progenies. Null alleles were detected in tetraploids at two loci. Both cytotypes were highly self-incompatible. Inter-cytotype crosses were much less successful than intra-cytotype crosses in terms of seed set (five-fold reduction) and germination rates (three-fold reduction), suggesting a ‘triploid block’ effect. Progenies from inter-cytotype crosses most often had the maternal cytotype and resulted from selfing or pollen contamination. However, six triploids were detected. The significance of these results for the understanding of the geographical distribution of the cytotypes and inter-cytotype gene flow is discussed.
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Patterns of allozyme variation in diploid and tetraploid Centaurea jacea at different spatial scales.
Evolution; international journal of organic evolution, 2001Co-Authors: Olivier J. Hardy, Xavier VekemansAbstract:The extent and spatial patterns of genetic variation at allozyme markers were investigated within and between diploid and autotetraploid knapweeds (Centaurea jacea L. sensu lato, Asteraceae) at contrasted geographic scales: (1) among populations sampled from a diploid-tetraploid contact zone in the northeastern part of the Belgian Ardennes, and (2) within mixed populations from that zone where diploids and tetraploids coexist. Our data were also compared with a published dataset by Sommer (1990) describing allozyme variation in separate diploid and tetraploid knapweeds populations collected throughout Europe. Genetic diversity was higher in tetraploids. In the Belgian Ardennes and within the mixed populations, both cytotypes had similar levels of spatial genetic structure, they were genetically differentiated, and their distributions of allele frequencies were not spatially correlated. In contrast, at the European scale, diploids and tetraploids did not show differentiated gene pools and presented a strong correlation between their patterns of spatial genetic variation. Numerical simulations showed that the striking difference in patterns observed at small and large geographic scales could be accounted for by a combination of (1) isolation by distance within cytotypes; and (2) partial reproductive barriers between cytotypes and/or recurrent formation of tetraploids. We suggest that this may explain the difficulty of the taxonomic treatment of knapweeds and of polyploid complexes in general.
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Spatial autocorrelation of allozyme and quantitative markers within a natural population of Centaurea jacea (Asteraceae)
Journal of Evolutionary Biology, 2000Co-Authors: Olivier J. Hardy, Pierre Meerts, Sonia Vanderhoeven, Xavier VekemansAbstract:This paper compares the fine-scale genetic structure of quantitative traits and allozyme markers within a natural population of Centaurea jacea s.l. To that end, a spatial autocorrelation approach is developed based on pairwise correlation coefficients between individuals and using sib families. Statistical properties of the proposed statistics are investigated with numerical simulations. Our results show that most quantitative traits have a significant spatial structure for their genetic component. On average, allozyme markers and the genetic component of quantitative traits have similar patterns of spatial autocorrelation that are consistent with a neutral model of isolation by distance. We also show evidence that environmental heterogeneity generates a spatial structure for the environmental component of quantitative traits. Results are discussed in terms of mechanisms generating spatial structure and are compared with those obtained on a large geographical scale.
Eelke Jongejans - One of the best experts on this subject based on the ideXlab platform.
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Bottlenecks and spatiotemporal variation in the sexual
2011Co-Authors: Eelke Jongejans, Merel B Soons, Hans De KroonAbstract:19 Sexual reproduction is important for the growth of populations and the maintenance of 20 genetic diversity. Several steps are involved in the sexual reproduction pathway of plants: 21 the production of flowers, the production of seeds and the establishment of seedlings 22 from seeds. In this paper we quantify the relative importance and spatio-temporal 23 variability of these different steps for four grassland perennials: Centaurea jacea, 24 Cirsium dissectum, Hypochaeris radicata and Succisa pratensis. We compared 25 undisturbed meadows with meadows where the top soil layer had been removed as a 26 restoration measure. Data on the number of flower heads per flowering rosette, the 27 numbers of flowers and seeds per flower head, and the seedling establishment 28 probabilities per seed were collected by field observations and experiments in several 29 sites and years. Combination of these data shows that H. radicata and S. pratensis have 30 higher recruitment rates (1.9 and 3.3 seedlings per year per flowering rosette, 31 respectively) than the more clonal C. dissectum and C. jacea (0.027 and 0.23, 32 respectively). Seedling establishment is the major bottleneck for successful sexual 33 reproduction in all species. Large losses also occurred due to failing seed set in C. 34 dissectum. Comparison of the coefficients of variation per step in space and time revealed 35 that spatio-temporal variability was largest in seedling establishment, followed closely by 36 flower head production and seed set.
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the interplay between shifts in biomass allocation and costs of reproduction in four grassland perennials under simulated successional change
Oecologia, 2006Co-Authors: Hans De Kroon, Eelke Jongejans, Frank BerendseAbstract:When perennial herbs face the risk of being outcompeted in the course of succession, they are hypothesized to either increase their biomass allocation to flowers and seeds or to invest more in vegetative growth. We tested these hypotheses in a 3-year garden experiment with four perennials (Hypochaeris radicata, Cirsium dissectum, Succisa pratensis and Centaurea jacea) by growing them in the midst of a tall tussock-forming grass (Molinia caerulea) that may successionally replace them in their natural habitat. In all species except for the short-lived H. radicata, costs of sexual reproduction were significant over the 3 years, since continuous bud removal enhanced total biomass or rosette number. To mimic succession we added nutrients, which resulted in a tripled grass biomass and higher death rates in the shorter-lived species. The simulated succession resulted also in a number of coupled growth responses in the survivors: enhanced plant size as well as elevated seed production. The latter was partly due to larger plant sizes, but mostly due to higher reproductive allocation, which in turn could be partly explained by lower relative somatic costs and by lower root–shoot ratios in the high-nutrient plots. Our results suggest that perennial plants can increase both their persistence and their colonization ability by simultaneously increasing their vegetative size and reproductive allocation in response to enhanced competition and nutrient influxes. These responses can be very important for the survival of a species in a metapopulation context.
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Bottlenecks and spatiotemporal variation in the sexual reproduction pathway of perennial meadow plants
Basic and Applied Ecology, 2006Co-Authors: Eelke Jongejans, Merel B Soons, J.c.j.m. De KroonAbstract:Summary Sexual reproduction is important for the growth of populations and the maintenance of genetic diversity. Several steps are involved in the sexual reproduction pathway of plants: the production of flowers, the production of seeds and the establishment of seedlings from seeds. In this paper we quantify the relative importance and spatiotemporal variability of these different steps for four grassland perennials: Centaurea jacea , Cirsium dissectum , Hypochaeris radicata and Succisa pratensis . We compared undisturbed meadows with meadows where the top soil layer had been removed as a restoration measure. Data on the number of flower heads per flowering rosette, the numbers of flowers and seeds per flower head, and the seedling establishment probabilities per seed were collected by field observations and experiments in several sites and years. Combination of these data shows that H. radicata and S. pratensis have higher recruitment rates (1.9 and 3.3 seedlings per year per flowering rosette, respectively) than the more clonal C. dissectum and C. jacea (0.027 and 0.23, respectively). Seedling establishment is the major bottleneck for successful sexual reproduction in all species. Large losses also occurred due to failing seed set in C. dissectum . Comparison of the coefficients of variation per step in space and time revealed that spatiotemporal variability was largest in seedling establishment, followed closely by flower head production and seed set.
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Release thresholds strongly determine the range of seed dispersal by wind
Ecological Modelling, 2005Co-Authors: Peter Schippers, Eelke JongejansAbstract:Abstract The effect of the seed abscission process on the dispersal distance of seeds has never been studied explicitly and is often ignored in studies that aim to estimate the seed shadows of species. To examine the importance of the abscission process for the seed shadow we used a seed trajectory model that keeps track of the release threshold dynamics of the individual seeds on mother plant. We defined the release threshold as the critical wind speed that induces a mechanical force that is just large enough to release a seed from its mother plant. The model used real wind speed sequences and seed appearance over time on the mother plant. Several calculations were performed to investigate the effect of release thresholds dynamics on seed shadow of two herbaceous species with contrasting terminal velocity values ( V t ): Centaurea jacea ( V t = 4.1 m s −1 ) and Hypochaeris radicata ( V t = 0.49 m s −1 ). Release thresholds were responsible for a two-fold increase of median dispersal distances in both species. Tails of the seed shadows, the fraction of seeds that travel furthest, were even more sensitive and increased with a factor 4.5 for Centaurea and 7.0 for Hypochaeris . Our work indicates that the abscission process appears to be very important and suggests that dispersal distance of plants is currently severely underestimated, which, in turn, has major consequences for our current understanding of the distribution, metapopulation dynamics and survival of plant species.
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Space versus time variation in the population dynamics of three co‐occurring perennial herbs
Journal of Ecology, 2005Co-Authors: Eelke Jongejans, Hans De KroonAbstract:1 Many plant species are currently restricted to small and isolated populations as a result of habitat destruction and fragmentation. Lack of sufficient data often leads to spatial variation being substituted for temporal variation in models to evaluate management options, but variation in population growth rate () between sites or over years may be caused by variation in different life history components. 2 We studied the demography of three coexisting and related perennial herbs (the longlived perennial Succisa pratensis, the shorter-lived Hypochaeris radicata and the longlived, but more clonal Centaurea jacea ) in the same plots over 4 years (1999?2003). 3 Life table response experiment analysis revealed that temporal and spatial variation in the life history components of S. pratensis and H. radicata were qualitatively different: variation in fecundity contributed most to variation in between sites, but variation in growth contributed most to variation between years. 4 In years with a below average, most life history components of S. pratensis and H. radicata had lower transition probabilities, whereas the reasons for low population growth differed between the three poor sites. All major life history components of S. pratensis had lower probabilities in the most productive of these sites, but in the other two, negative contributions from some components (fecundity or growth) were partly compensated for by positive contributions from others (growth or stasis). 5 Centaurea jacea showed a different pattern: in poor years, negative contributions from some life history components were buffered by positive contributions from other components; however, this buffering did not occur in poor sites. 6 Our analysis shows that the population dynamics of perennial plants may respond differently to temporal and spatial variation in environmental conditions. Moreover, co-occurring species with similar life histories responded differently to the same spatiotemporal variation. We conclude that temporal and spatial dynamics cannot readily be interchanged in population viability analyses and management studies, as such substitution may lead to incorrect projections of a species? population dynamics in time. Key-words : buffering of variation, demography, life history components, population growth rate, variation decomposition.
Volkmar Wolters - One of the best experts on this subject based on the ideXlab platform.
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the influence of matrix type on flower visitors of Centaurea jacea l
Agriculture Ecosystems & Environment, 2003Co-Authors: Michaela Hirsch, Sabine Pfaff, Volkmar WoltersAbstract:The structure of invertebrate communities is impacted by landscape variables. Here we present a study on the influence of the surrounding matrix on pollinating flower visitors of the knapweed Centaurea jaceain a small-scale agricultural landscape (Central Hesse, Germany). The study was carried out in late summer 1998 by monitoring visits of 24 insect taxa at 15 C. jacea patches. The following matrix types were studied: (i) arable land only, (ii) arable land close to grassland ( <50 m), (iii) a mosaic of arable land, grassland and forests, and (iv) grassland only. More than half of the flower visitors were bees, with honeybees ( Apis mellifera) and bumblebees being the dominant taxa. The matrix type did not affect either the mean frequency of total flower visits or the mean richness of all taxa. The same applied to different size classes. Significant matrix effects were confined to five large taxa of the Apoidea: Bombus lapidarius, B. pascuorum, the B. terrestris group, large wild bees (other than bumblebees) and A. mellifera. A. mellifera significantly preferred patches surrounded by grassland, while large wild bees preferred patches surrounded by a mosaic of arable land, grassland and forests. The flower visiting frequency of all three bumblebee species was high in patches surrounded by grassland, but only the B. terrestris group showed a clear-cut preference for this matrix type. It is concluded that matrix effects on flower visitors of C. jacea are taxon- and body size-specific. Strong matrix effects on large pollinators suggest that these taxa are able to discriminate between patches. Considering the ecological services provided by pollinators, the preservation of large areas covered by interconnected grassland sites as well as by a mosaic of different land use forms should have high priority in future management strategies. © 2003 Elsevier Science B.V. All rights reserved.
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The influence of matrix type on flower visitors of Centaurea jacea L.
Agriculture Ecosystems & Environment, 2003Co-Authors: Michaela Hirsch, Sabine Pfaff, Volkmar WoltersAbstract:The structure of invertebrate communities is impacted by landscape variables. Here we present a study on the influence of the surrounding matrix on pollinating flower visitors of the knapweed Centaurea jaceain a small-scale agricultural landscape (Central Hesse, Germany). The study was carried out in late summer 1998 by monitoring visits of 24 insect taxa at 15 C. jacea patches. The following matrix types were studied: (i) arable land only, (ii) arable land close to grassland (
Stephanie Wight - One of the best experts on this subject based on the ideXlab platform.
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Big plants — Do they affect neighbourhood species richness and composition in herbaceous vegetation?
Acta Oecologica, 2014Co-Authors: Lonnie W. Aarssen, Brandon S. Schamp, Stephanie WightAbstract:According to traditional theory, success in competition between plant species generally involves a ‘size-advantage’. We predicted therefore that plants with larger body size should impose greater limits on the number of species — especially relatively small ones — that can reside within their immediate neighbourhoods. Species composition was compared within local neighbourhoods surrounding target plants of different sizes belonging to one of the largest herbaceous species found within old-field vegetation in eastern Ontario Canada — Centaurea jacea. Resident species density was generally greater within immediate ‘inner’ target neighbourhoods than within adjacent circular ‘outer’ neighbourhoods, and mean body size of resident neighbour species was unrelated to increases in target plant size. As target plant size increased, the proportion of resident neighbour species that were reproductive increased. Relatively big plants of C. jacea do not limit the number or the proportion of reproductive species that can coexist within their immediate neighbourhoods, nor do they cause local exclusion of relatively small species from these neighbourhoods. These results fail to support the ‘size-advantage’ hypothesis and are more consistent with the ‘reproductive economy advantage’ hypothesis: success under intense competition is promoted by capacity to recruit offspring that — despite severe suppression — are able to reach their minimum body size needed for reproduction, and hence produce grand-offspring for the next generation. The latter is facilitated by a relatively small minimum reproductive threshold size, which is generally negatively correlated with a relatively large maximum potential body size.
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effects of target plant species body size on neighbourhood species richness and composition in old field vegetation
PLOS ONE, 2013Co-Authors: Brandon S. Schamp, Lonnie W. Aarssen, Stephanie WightAbstract:Competition is generally regarded as an important force in organizing the structure of vegetation, and evidence from several experimental studies of species mixtures suggests that larger mature plant size elicits a competitive advantage. However, these findings are at odds with the fact that large and small plant species generally coexist, and relatively smaller species are more common in virtually all plant communities. Here, we use replicates of ten relatively large old-field plant species to explore the competitive impact of target individual size on their surrounding neighbourhoods compared to nearby neighbourhoods of the same size that are not centred by a large target individual. While target individuals of the largest of our test species, Centaurea jacea L., had a strong impact on neighbouring species, in general, target species size was a weak predictor of the number of other resident species growing within its immediate neighbourhood, as well as the number of resident species that were reproductive. Thus, the presence of a large competitor did not restrict the ability of neighbouring species to reproduce. Lastly, target species size did not have any impact on the species size structure of neighbouring species; i.e. they did not restrict smaller, supposedly poorer competitors, from growing and reproducing close by. Taken together, these results provide no support for a size-advantage in competition restricting local species richness or the ability of small species to coexist and successfully reproduce in the immediate neighbourhood of a large species.
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Effects of large target individuals on neighbour plant species diversity.
2013Co-Authors: Brandon S. Schamp, Lonnie W. Aarssen, Stephanie WightAbstract:Relationship between target species size index and [(species richness in target neighbourhood) (species richness in random neighbourhood)] for: (a) inner neighbourhoods only; (b) outer neighbourhoods only; and (c) total neighbourhoods. The 3 replicates of each of the 10 target species are coded by numbers 1–10 in order of increasing average species size index: 1- Rudbeckia hirta, 2- Solidago juncea, 3- Erigeron philadelphicus, 4- Asclepias syriaca, 5- Aster umbellatus, 6- Daucus carota, 7- Verbascum thapsus, 8- Cirsium vulgare, 9- Solidago canadensis, 10- Centaurea jacea. Dashed line indicates where species richness in target neighbourhood = species richness in random neighbourhood. rs and associated P-values are from Spearman Rank correlation analyses. With Centaurea jacea (species 10) omitted,(a) rs = −0.366, P = 0.062, n = 27; (b) rs = −0.376, P = 0.0533, n = 27; (c) rs = −0.143, P = 0.472, n = 27.
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Effects of target individual size on neighbour plant species reproduction.
2013Co-Authors: Brandon S. Schamp, Lonnie W. Aarssen, Stephanie WightAbstract:Relationship between target species size index and [(proportion of reproductive species in target neighbourhood) (proportion of reproductive species in random neighbourhood)] for (a) inner neighbourhoods only; (b) outer neighbourhoods only; and (c) total neighbourhoods. The 3 replicates of each of the 10 target species are coded by numbers 1–10 in order of increasing average species size index: 1- Rudbeckia hirta, 2- Solidago juncea, 3- Erigeron philadelphicus, 4- Asclepias syriaca, 5- Aster umbellatus, 6- Daucus carota, 7- Verbascum thapsus, 8- Cirsium vulgare, 9- Solidago canadensis, 10- Centaurea jacea. Dashed line indicates where species richness in target neighbourhood = species richness in random neighbourhood. rs and associated P-values are from Spearman Rank correlation analyses.
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Effects of target individual height on mean neighbour plant species height.
2013Co-Authors: Brandon S. Schamp, Lonnie W. Aarssen, Stephanie WightAbstract:Relationship between target species size index and (mean height of species in target neighbourhood-mean height of species in random neighbourhood) (species height is the typical maximum from published data) for: (a) inner target neighbourhoods only; (b) outer target neighbourhoods only; and (c) total target neighbourhoods. The same relationship is also depicted for only reproductive species in target and random neighbourhoods (d-f). The 3 replicates of each of the 10 target species are coded by numbers 1–10 in order of increasing average species size index: 1- Rudbeckia hirta, 2- Solidago juncea, 3- Erigeron philadelphicus, 4- Asclepias syriaca, 5- Aster umbellatus, 6- Daucus carota, 7- Verbascum thapsus, 8- Cirsium vulgare, 9- Solidago canadensis, 10- Centaurea jacea. Dashed line indicates where species richness in target neighbourhood = species richness in random neighbourhood. rs and associated P-values are from Spearman Rank correlation analyses.