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Having capacity for a relatively large plant body size is usually regarded as a key functional trait associated with success under competition between resident species within natural vegetation. This traditional ‘size-advantage’ hypothesis, however, generally fails to find support from several lines of recent research. Possible interpretations are considered here, including one in particular that has been largely overlooked: a larger species generally also needs to grow to a larger threshold size before it can reproduce at all, and the latter may not be generally attainable when neighbourhood resources are severely contested. The implications of this are explored in calling for a revised model of the selection consequences of competition on body size in plants, where success is defined not just (or even most importantly) by capacity to capture resources and deny them to neighbours, but more fundamentally by the capacity to transmit genes to future generations, despite severe resource deprivation by neighbours . For this latter capacity, a growing body of evidence is pointing to an alternative hypothesis based on ‘reproductive economy advantage’: under conditions of extreme and protracted neighbourhood crowding/competition (where virtually all resident plants are necessarily forced to remain, until death, at only a small fraction of their maximum potential body sizes), it is the relatively small species that are more likely to leave descendants here – simply because they need to reach only a relatively small body size in order to produce at least some offspring. Resident plants of most larger species, however, are more likely to die here producing none at all. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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]

Explores the measurement of competitive ability in plants. Failure to account for other sources of variation in the size suppression of plants; Variation in the intensity of concurrent facilitative interactions between competitors; Characteristics of the competitive ability.

Theory has produced contrasting predictions related to flowering time overlap among coexisting plant species largely because of the diversity of potential influences on flowering time. In this study, we use a trait-based null modelling approach to test for evidence of deterministic assembly of species according to flowering time in an old-field plant community., Plant species coexisting in one-metre-square plots overlapped in flowering time significantly more than expected. This flowering synchrony was more pronounced when analyses focused on bee-pollinated species. Flowering synchrony was also observed for wind-pollinated species, although for only one of our two null model tests, highlighting the sensitivity of some results to different randomization methods. In general, these patterns suggest that relationships between pollinators and plants can influence community assembly processes., Because our study community is composed of approximately 43% native plant species and 57% exotic species, and because the arrival of new species may complicate plant-pollinator interactions, we tested whether flowering time overlap was altered by introduced species. Flowering synchrony was greater in plots with a higher proportion of introduced species. This pattern held for both null model tests, but was slightly stronger when analyses focused on bee-pollinated species. These results indicate that introduced species alter community flowering distributions and in so doing will inevitably affect pollinator-plant interactions., Finally, we tested whether our results were influenced by variation among study plots in above-ground biomass production, which some theory predicts will be related to the importance of competition. Our results were not influenced by this variation, suggesting that resource variation among our plots did not contribute to observed patterns., Synthesis: Our results provide support for predictions that coexisting species should display flowering synchrony, and provide no support for species coexistence via temporal niche partitioning at this scale in this study community. Our results also indicate that introduced species significantly alter the community assembly process such that flowering synchrony is more pronounced in plots with a greater proportion of introduced plant species. [ABSTRACT FROM AUTHOR]