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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]

Plant competition experiments commonly suggest that larger species have an advantage, primarily in terms of light acquisition. However, within crowded natural vegetation, where competition evidently impacts fitness, most resident species are relatively small. It remains unclear, therefore, whether the size advantage observed in controlled experiments is normally realized in habitats where competition is most intense. We characterized the light environment and tested for evidence of a size advantage in competition for light in an old‐field plant community composed of perennial herbaceous species. We investigated whether larger species contributed to reduced light penetration (i.e., greater shading), and examined the impact of shade on smaller species by testing whether their abundance and richness were lower in plots with less light penetration. Light penetration in plots ranged from 0.3% to 72.4%. Significant effects were more common when analyses focused on small plants that reached reproduction (i.e., flowering rooted units); focusing on only flowering plants (i.e., excluding nonflowering rooted units) can clarify community patterns. Plots with a greater mean species height had significantly lower light penetration, and plots with lower light penetration had significantly lower flowering abundance and richness of small species. However, the impact of shade on the flowering abundance and richness of small species was relatively small (R2 values between 8% and 15%) and depended on how we defined "small species." Synthesis: Our results confirm that light penetration in herbaceous vegetation can be comparable to levels seen in forests, that plots with taller species cast more shade, and that flowering smaller species are less abundant and diverse in plots where light penetration is low. However, variation in mean plot height explained less than 10% of variation in light penetration, and light penetration explained between 5 and 15% of variation in the flowering abundance and richness of small species. Coupled with the fact that flowering small species were present even within the most heavily shaded plots, our results suggest that any advantage in light competition by large species is limited. One explanation is that at least some small species in these communities are shade‐tolerant. Shade tolerance in predominantly herbaceous communities, particularly among small plant species, requires further research. [ABSTRACT FROM AUTHOR]