The influence of initial size on the competitive responses of six plant species

Plant size may be an important determinant of competitive ability and may affect comparisons of competitive response among species. We examined the influence of initial size on plant competitive response in a field experiment using transplants in three classes of initial mass (class means: 27, 46, and 73 mg) for each of six species (Agropyron cristatum, Bouteloua gracilis, Elaeagnus commutata, Fraxinus pennsylvanica, Melilotus officinalis, and Potentilla pensylvanica). Transplants were grown for one summer in an old field dominated by the perennial grass Bromus inermis. The transplants were grown with no neighbors present, with shoots of neighbors present, with roots of neighbors present, or with both shoots and roots of neighbors present. The existing Bromus served as neighbors. There were 10 replicates of each factor combination. Final transplant mass varied significantly with competition, species identity, and initial size. The mass of transplants grown with only neighbor roots was not significantly different from the mass of transplants grown with both roots and shoots of neighbors, suggesting that competition in the field was primarily belowground. A significant interaction between species identity and competition indicated that species differed in the extent to which they were suppressed by neighbors. There was no significant interaction between size and competition, however, suggesting that initial size had no influence on competitive response. Further, regressions of final transplant mass against neighbor mass provided no evidence that initially smaller transplants were weaker competitors. Our results suggest that the competitive responses of the six species during the first summer of growth were not influenced by their initial size under conditions of intense belowground competition.