The effect of initial density on interactions between bryophytes at individual and community levels

Abstract

1. A glasshouse experiment was conducted with seven bryophyte species to determine the effects of competition. We tested whether competitive hierarchies varied with initial abundance (density and biomass) and between two different experimental approaches. 2. Relative competition intensities were calculated based on proportional growth (G) and relative biomass (R). The standards for comparison (i.e. no interactions) were species' performance in monocultures at one of two sowing abundances (the combined monocultures method; CM) and in a low-density mixture of equal proportions of all species (the community density series method; CDS). 3. Proportional growth decreased with increasing initial abundance for all species community effects (relative biomass of each species) were generally weaker and more variable than individual effects. R increased linearly with abundance for only one species, while three species showed a quadratic response (of which two were negative). 4. Competitive hierarchies derived by the CM method differed with abundance, and we argue that the CDS method is likely to provide a more reliable comparison. 5. With CDS, competitive hierarchies were similar along the community abundance gradient, implying that non-linear competitive effects are not likely to be a mechanism of coexistence in this community 6. There were significant competitive effects on community composition, but not on diversity as measured by evenness. At the community level individual species tended to show either competitive or positive effects throughout the gradient of initial community abundance, with decreasing values for most species at high initial community abundance, as were the effects of interactions on community composition.