Intensity of intra- and interspecific competition in coexisting shortgrass species

1 Bouteloua gracilis and Buchloe dactyloides are dominant species in the shortgrass steppe. Previous studies have suggested that these species have similar resource requirements and. as there is no temporal segregation in growth, we suggest that competition for below-ground resources should be intense. Classical competition theory predicts that, under stable conditions. coexistence of species with similar requirements can occur if intraspecific competition is more intense than interspecific competition. We therefore compared the competitive abilities of the two species under both inter- and intraspecific conditions, another determinant of coexistence in plant communities. 2 A 3-year experiment was conducted in a small homogeneous area and a separate 2-year experiment was conducted in a larger area. In both experiments, a hexagonal planting design was used to achieve all possible combinations of species and to control plant size and neighbourhood asymmetries at the start of the experiment. Half the target plants were grown within steel cylinders to reduce below-ground competition. 3 We found similar results in the two experiments. Plant biomass and seed production were always higher in plants growing with reduced competition, although seed production was quite variable. Relative competition intensity did not differ between intraspecific and interspecific competition. 4 Our results suggest that competition between these coexisting shortgrasses is intense, but do not support the prediction of a difference between intra- and interspecific competition. We discuss alternative explanations.