Complex Competitive Relationships Among Genotypes of Three Perennial Grasses: Implications for Species Coexistence

Competitive relationships were studied among genotypes of Agropyron repens, Poa pratensis, and Phleum pratense collected from a grassland community in southeastern Ontario. Field surveys revealed no significant correlations of abundance among the three species within randomly placed survey plots. The greater performance of target plants in removal plots versus control plots for all three species in a vegetation-removal experiment, however, suggests that each of these three species was suppressed by competitive interactions in the field. One clone (genotype) of each species was collected from each of 10 sites (7-cm2 neighborhoods) within the community. Each genotype was propagated vegetatively and grown in the greenhouse in monocultures and in pairwise mixtures with all other genotypes collected from the community. Differences in relative competitive ability among species or genotypes were measured as significant differences between the yield-suppression coefficients of the two components of a mixture. A significant interaction of target site of origin and associate site of origin (i.e., mixture effect) for yield-suppression coefficients of intraspecific mixtures for each species suggests genotypic variability among the collected clones. Competitive relationships at the species level were transitive. Genotypes collected from the same neighborhood exhibited competitive standoffs or transitive competitive relationships. Competitively subordinate genotypes from different neighborhoods, however, belonged to different species. Competitive relationships at the genotype level (across neighborhoods) were complex and often intransitive. These data provide preliminary support for the competitive-combining-ability hypothesis for species coexistence, which predicts that intransitive competitive abilities at the genotype level (spanning taxonomic boundaries) may prevent competitive exclusion at the whole-population level and hence contribute to the maintenance of both species and genotypic diversity within vegetation.

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