| We tested the influence of competition on plant community structure by investigating the dispersion of traits related to species competitive ability both among forest communities within a single region, and among plots within a single old-field plant community. We found evidence that forests are assembled non-randomly with respect to maximum species height, and explored three possible mechanistic explanations for these results. We also found evidence that maximum height, maximum biomass, and seed mass were neutrally dispersed among plots in an old-field plant community. Dispersion patterns changed with scale, quadrat type (fixed size vs. fixed ramet number), and abundance weighting. We concluded from these results that either competition, driven by these traits, does not play an important role in driving within-community organization of species, or these traits contribute to both competitive interactions and functional niche separation for different species within the community. Next we reviewed evidence for transitive hierarchies between competing species, and for variation in the relationship between inter- and intraspecific competitive affects and species size. Based on this, we developed a theoretical framework for the changing relationship between inter- and intraspecific competition among pairs of competing species, with respect to growth, survival, and reproduction. Predictions arising from this model include dispersal differences between large and small species, and a trade-off between size and clonality. Using a glasshouse experiment with 10 perennial plant species, we tested one of the predictions arising out of the framework developed above, that relatively large species will experience a disproportionately large cost of intra- relative to interspecific competition. Comparing monoculture to mixture performance, we find evidence that larger species have a disproportionately greater intra- relative to interspecific competitive effect on mortality per unit area. Finally, we tested whether large species monocultures, left to themselves for three growing seasons, were more susceptible to invasion from the surrounding plant community. We found evidence that species with a larger maximum size are increasingly susceptible to invasion, signalling that even if large species possess a competitive advantage, they may, through the process of strong intraspecific competition, leave space available for colonization by smaller species. |
