The effects of density, spatial pattern, and competitive symmetry on size variation in simulated plant populations

Patterns of size inequality in crowded plant populations are often taken to be indicative of the degree of size asymmetry of competition, but recent r esearch suggests that some of the patterns attributed to size-asymmetric co mpetition could be due to spatial structure. To investigate the theoretical relationships between plant density, spatial pattern, and competitive size asymmetry in determining size variation in crowded plant populations, we d eveloped a spatially explicit, individual-based plant competition model bas ed on overlapping zones of influence. The zone of influence of each plant i s modeled as a circle, growing in two dimensions, and is allometrically rel ated to plant biomass. The area of the circle represents resources potentia lly available to the plant, and plants compete for resources in areas in wh ich they overlap. The size asymmetry of competition is reflected in the rul es for dividing up the overlapping areas. Theoretical plant populations wer e grown in random and in perfectly uniform spatial patterns at four densiti es under size-asymmetric and size-symmetric competition. Both spatial patte rn and size asymmetry contributed to size variation, but their relative imp ortance varied greatly over density and over time. Early in stand developme nt, spatial pattern was more important than the symmetry of competition in determining the degree of size variation within the population, but after p lants grew and competition intensified, the size asymmetry of competition b ecame a much more important source of size variation. Size variability was slightly higher at higher densities when competition was symmetric and plan ts were distributed nonuniformly in space. In a uniform spatial pattern, si ze variation increased with density only when competition was size asymmetr ic. Our results suggest that when competition is size asymmetric and intens e, it will be more important in generating size variation than is local var iation in density. Our results and the available data are consistent with t he hypothesis that high levels of size inequality commonly observed within crowded plant populations are largely due to size-asymmetric competition, n ot to variation in local density.