Neighborhood Competition in an Old‐Field Plant Community

A series of field neighborhood experiments was performed to compare the competitive effects of seven plant species on the performance of transplanted individuals of Solidago canadensis. The neighbor species included conspecifics and species of a variety of growth forms that co—occur with S. canadensis in old fields. There were strong competitive effects on the transplants; average S. canadensis growth was reduced 17—62% by the presence of neighbors growing at natural densities. The responses to gradients of increasing neighbor density or biomass appeared to be nonlinear, with decreasing effects of adding more neighbors at higher neighbor density or biomass. Competitive effects were compared among neighbor species at three levels: effects at natural densities, per—individual (population density) effects, and per—gram (biomass density) effects. The magnitude of total effects at natural densities varied with the species of neighbor plants and was stronger for neighbor species with greater total biomass of plants per unit area. The magnitude of effects of each individual also varied with neighbor species and was stronger for neighbor species with larger mean biomass per individual. Consequently, per—gram competitive effects did not differ among the neighbor species. Thus, it appears that the differences in competitive effects among species were primarily due to differences in size or abundance, rather than to more subtle differences in aspects of resource use. The responses of target Solidago canadensis individuals to neighborhood competition were very variable. A maximum of 40% of the variance in individual performance was explained by any one of the measures of abundance of neighbors, and, in half of the neighbor species, <10% of the variance in target performance was explained by abundance of neighbors. Competition intensity seems to limit the maximum potential growth of individuals, but actual growth was often below the boundary determined by competition.