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Experimental evidence suggests that larger plant species generally have a competitive advantage and thus should dominate communities where competition for limiting resources (i.e. water, soil nutrients, light, space and/or mutualists) is intense. Additionally, researchers have postulated that competition can generate negative co‐occurrence patterns. However, neither expectation is strongly supported by empirical evidence. In a relatively undisturbed old‐field plant community, we explored the interdependency of these expectations by examining pairwise species co‐occurrences, and pairwise species maximum height differences (measured here as the maximum potential height of the species), and by estimating competition intensity for sample plots using field transplants grown with and without neighbours. Specifically, we tested whether maximum species height differences were greater for negatively co‐occurring species pairs than for positively co‐occurring pairs found growing together. We also tested whether the maximum species height differences and co‐occurrence status of species pairs were associated with the average competitive environment they tended to be found growing in. Negatively co‐occurring species pairs found growing together differed more in maximum height than in positively co‐occurring species pairs that were more similar in maximum height. Additionally, species maximum height differences between negatively co‐occurring species pairs where higher in less competitively intense plots, whereas species maximum heights were more similar between positively co‐occurring species pairs found growing together in less competitively intense plots. Synthesis. Our results suggest an association between estimates of competition intensity generated using transplant experiments and co‐occurrence patterns. These findings contribute to our understanding of the association between size‐mediated competition and spatial organization in plant communities and provide a framework for future research in this area. [ABSTRACT FROM AUTHOR]

Aims Studies of community structure and co-occurrence patterns rely on the premise that community data reflect where species successfully grow and which species they grow with. However, plant censuses generally do not distinguish between species with reproductive individuals and those only represented by non-reproductive individuals. We tested whether inclusion of non-reproductive species, which may not reflect success in that location, significantly impacts evaluations of community structure and co-occurrence. Location Queen's University Biological Station, Ontario, Canada, old-field plant communities. Methods We quantified the impact of non-reproductive species in two plant communities by comparing community structure and co-occurrence patterns when non-reproductive species were included or excluded. Results Including non-reproductive species significantly increased plot-level species richness in both communities (54% and 13% increases), altered species evenness in both communities, significantly impacted beta-diversity among plots in one site, and disproportionately impacted assessments of diversity in species-rich plots. Excluding non-reproductive species resulted in reduced negative co-occurrence patterns in both communities, with a substantially larger impact in one community. In that community, the impact of non-reproductive species was even more pronounced when abundance data were used in analysis, and when pair-wise co-occurrence patterns were assessed. Additionally, including non-reproductive species drastically decreased the number of species pairs with perfect negative co-occurrence across sites, indicating that these species can add 'noise' to co-occurrence patterns. We examined possible explanations for the presence of non-reproductive species. In one community, non-reproductive species were 22 times less abundant (per plot) than reproductive species within plots, although they were not rare overall. Differences in the number of non-reproductive species per plot across our focal communities were not clearly driven by differences in clonality, stress from extreme weather or low N. While these patterns are consistent with the interpretation that non-reproductive species are present due to mass effects, this possibility requires further research. Conclusions Including non-reproductive plant species in censuses can significantly impact assessments of community structure and species co-occurrence. The divergent impact of their inclusion on our two communities highlights the possibility that excluding non-reproductive species from surveys may remove noise from community data and clarify theories of plant species co-existence. [ABSTRACT FROM AUTHOR]

The selection consequences of competition in plants have been traditionally interpreted based on a 'size-advantage' hypothesis - that is, under intense crowding/competition from neighbors, natural selection generally favors capacity for a relatively large plant body size. However, this conflicts with abundant data, showing that resident species body size distributions are usually strongly right-skewed at virtually all scales within vegetation. Using surveys within sample plots and a neighbor-removal experiment, we tested: (1) whether resident species that have a larger maximum potential body size (MAX) generally have more successful local individual recruitment, and thus greater local abundance/density (as predicted by the traditional size-advantage hypothesis); and (2) whether there is a general between-species trade-off relationship between MAX and capacity to produce offspring when body size is severely suppressed by crowding/competition - that is, whether resident species with a larger MAX generally also need to reach a larger minimum reproductive threshold size (MIN) before they can reproduce at all. The results showed that MIN had a positive relationship with MAX across resident species, and local density - as well as local density of just reproductive individuals - was generally greater for species with smaller MIN (and hence smaller MAX). In addition, the cleared neighborhoods of larger target species (which had relatively large MIN) generally had - in the following growing season - a lower ratio of conspecific recruitment within these neighborhoods relative to recruitment of other (i.e., smaller) species (which had generally smaller MIN). These data are consistent with an alternative hypothesis based on a 'reproductive-economy-advantage' - that is, superior fitness under competition in plants generally requires not larger potential body size, but rather superior capacity to recruit offspring that are in turn capable of producing grand-offspring - and hence transmitting genes to future generations - despite intense and persistent (cross-generational) crowding/competition from near neighbors. Selection for the latter is expected to favor relatively small minimum reproductive threshold size and hence - as a tradeoff - relatively small (not large) potential body size. [ABSTRACT FROM AUTHOR]

Abstract: 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. [Copyright &y& Elsevier]

Background : We explored evidence for spatial patterning in vegetation across a sand-dune chronosequence spanning 453 years of primary succession to test for indications of a temporal signature of niche versus neutral processes during community assembly. Methods : The study was conducted on a series of dune-capped beach ridges located in Wilderness State Park in Emmet County of northern Lower Michigan, United States (45° 43′ N, 84° 56′ E). Nearest-neighbour survey data were subjected to species dispersion and association analyses. Results : Both species richness and species evenness showed the classic ‘hump-shaped’ relationship across successional time, suggesting that competition is an important force in structuring this community. However, the only evidence of intra-specific aggregation occurred in the youngest dune. Older dunes displayed random spatial structure and the oldest dunes tended toward a mix of random patterns and intra-specific segregation, i.e. with nearest neighbours more likely than by chance to be inter-specific. In addition, the frequency of species having lower neighbour-species richness than expected by chance was higher in younger, not older dunes. Species with significantly low neighbour richness (based on absolute nearest neighbours) on later dunes tended to have a high proportion of nearest neighbours that were intra-specific. Conclusions : The data provided no evidence that resident species in later succession have greater intra-specific aggregation or more niche differentiation based on horizontal niche space. The data are more consistent with theories of community assembly involving competitive equivalence and/or effects of facilitation. [ABSTRACT FROM PUBLISHER]

Abstract: The composition of natural vegetation typically includes a wide range of species sizes, but usually, relatively small size classes are represented by the greatest number of resident species. This is the case even within habitat types where traditional theory predicts that the intensity of plant competition should be most severe. Here we show that distributions of species size metrics are generally right-skewed also at the within-genus level. Our analyses are based on published reports of maximum plant height, leaf size, and seed size for an unbiased sample of fourteen angiosperm genera, involving eleven families – including representation of both endemics and cosmopolitans, plus genera that differ widely in both size (number of species) and growth form diversity. Pair-wise relationships for these three traits were, in most cases, also generally positively correlated for con-generic species. These results indicate that the right-skewed species size distributions commonly reported for natural vegetation reflect a pervasive signal for adaptive size metrics, essentially unstructured by phylogenic relationships. From these data therefore, we infer that most episodes of species origination within virtually all angiosperm lineages must have generally involved favouring effects of natural selection associated with relatively small plant body size, leaf size, and seed size. This represents a striking disconnect with the conventional view that superior competitive ability in plants requires relatively large body size/biomass production. Our interpretation of why ‘small is better’ focuses on two recently developed hypotheses – the ‘physical-space-niche’ hypothesis, and the ‘fecundity allocation premium’ hypothesis. In future research, detailed phylogenies at the species level might be used to test the prediction that relatively small size metrics are especially well represented in the most recently evolved species within angiosperm lineages. [Copyright &y& Elsevier]

1 The prominently right-skewed distribution of species sizes has been the subject of a large literature in animal ecology, but has received comparatively little attention from plant ecologists. It is evident that not all explanations that have been offered for animals are directly applicable to plants. 2 We suggest three hypotheses that require further study in the interpretation of size-dependent species richness in plants. 3 These hypotheses are all based on mechanisms that have generated, for smaller plants, a greater historical opportunity for speciation: (i) large adult plant size confers significant adaptation primarily in habitat types that have been relatively uncommon in space, across evolutionary time; (ii) relatively small species are more widely differentiated from each other in the environmental qualities defining their niches, many of which are made possible by the mere presence of larger species residing in the same habitat; and (iii) compared with large species, smaller species generally have higher fecundity allocation, i.e. they can produce a greater number of offspring per unit plant size per unit time, which generally confers a higher premium under most circumstances of natural selection, thus generating a potentially greater number of descendant individuals, and derived species. 4 We discuss the implications of these hypotheses in addressing an underlying paradox in plant competition/coexistence theory, i.e. that large adult size is assumed to be the principal trait that confers competitive ability yet, even in those habitat types where competition is assumed to reach the highest levels of intensity within vegetation, the vast majority of the resident species are, nevertheless, relatively small. [ABSTRACT FROM AUTHOR]

Pollen extracts from five locally-common grassland species in eastern Ontario, Canada were tested for in vitro allelopathic effect on the pollen germination of 17 sympatric target species in 1987. The flowering phenologies of the test and target species were recorded to determine if any phenologically-divergent species showed evidence of in vitro pollen-allelopathic interactions. Phleum pratense L. demonstrated pollen-allelopathic effects on all targets except Linaria vulgaris Hill. P. pratense L. also exhibited significant phenological divergence with seven of the target species. Pollen extracts from Agrostis stolonifera L., Melilotus alba Desr., and Vicia cracca L. inhibited pollen germination in some of the target species, but the results suggest that these effects were pH-mediated. The pollen extract of Erigeron annuus (L.) Pers. did not cause inhibition of pollen germination in any of the target species. Further tests with pollen extract of P. pratense and another 23 sympatric target species were performed in 1988. Pollen germination was significantly inhibited in all but two of these species: A. stolonifera and P. pratense. Hence, the pollen of P. pratense was not auto-allelopathic. The breeding system, relatively tall growth habit and relatively large quantity of pollen produced all support the in vitro evidence of P. pratense as a Pollen-allelopathic species. [ABSTRACT FROM AUTHOR]