Dispersion of traits related to competitive ability in an old-field plant community

Summary 1. We investigated patterns in the dispersion (i.e. spread and spacing) of plant species traits that are frequently associated with competitive ability, in an old-field plant community. In contrast with previous studies, we found no evidence for significant over- or under-dispersion of maximum plant height, maximum plant biomass, or seed mass. 2. These findings hold across three plot sizes (10 × 10 cm, 30 × 30 cm, 50 × 50 cm), and when plot size is measured in terms of number of ramets (50 ramets, 250 ramets or 500 ramets) rather than per unit area. Plot size, however, significantly affected the direction of dispersion observed across test statistics, supporting previous studies that have observed that trait dispersion patterns are sensitive to plot size. 3. While no significant dispersion was detected, dispersion direction, measured as the tendency for a two-tailed test to indicate trends for a given trait, was more frequently observed when analyses were weighted by abundance. Abundance weighted analyses had significantly different dispersion directions compared with presence/absence analyses for one of three traits considered. These findings suggest that abundance weighted analyses may yield more consistent trait dispersion patterns. 4. Synthesis. Our results may be interpreted as evidence for limited, if any, functional niche partitioning between co-occurring species via size-mediated differences in rooting depth or physical space niches (i.e. limiting similarity). Alternatively, some species with large differences in competitive traits may avoid competition through niche separation, while others may simply compete asymmetrically, leading to overall patterns of dispersion that cannot be distinguished from random. This interpretation supports recent perspectives that niche and neutral theoretical concepts are not mutually exclusive; both, working together, can be applied to the interpretation of plant community assembly and structure.