I examine the effects of competitive symmetry on annual plant populations through growth rate analysis, simulation modeling and experiments. I distinguish symmetric from asymmetric competition at several spatial and temporal scales. Local symmetry is defined by the way competitors divide resources in spaces where they overlap. Whole-plant symmetry is defined by the comparison of the total resource interception of competitors. Through plant growth analysis I show that local symmetry of competition is not identical to whole-plant symmetry of competition. Whole plant symmetry of competition also depends on the allometry of space occupancy and, under some circumstances, on the spatial distribution of the limiting resource. If plants capture space more slowly than they accumulate biomass, competition is symmetric at low density and asymmetric at high density. The local symmetry of competition determines the magnitude of the density response. These results extend to the population-wide, seasonally-integrated level, where the symmetry of competition is expressed in the slope of empirical distribution-modifying functions (DMFs: regression functions of the log-transformed relative biomass increments of individuals between the seedling stage and maturity on the log-transformed seedling biomass). A neighborhood-based simulation model shows that the slopes of empirical DMFs depend mostly on density and local competitive symmetry. Random variation of height, site quality and plant spacing affect DMFs only slightly, but affect population size structure greatly. I confirm these simulation results in field experiments with populations of millet (Pennisetum americanum) and cowpea (Vigna unguiculata). Millet plants were grown isolated, at low density (10,000 plants ha⁻¹, and at high density (20,000 plants ha⁻¹. Cowpea plants were grown isolated, or as single cowpea plants within stands of low and high density millet. In both species and in two experimental years the DMF slopes of high density populations were greater than the DMF slopes of isolated plant populations. However, the DMF slopes of high density plants were not always positive. These results suggest that the population-wide, seasonally-integrated symmetry of competition may vary between years, but that the increase of the DMF slope with density may be quite general
