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

1 A glasshouse experiment was conducted with seven bryophyte species to det ermine the effects of competition. We tested whether competitive hierarchie s varied with initial abundance (density and biomass) and between two diffe rent experimental approaches. 2 Relative competition intensities were calculated based on proportional gr owth (G) and relative biomass (R). The standards for comparison (i.e. no in teractions) were species' performance in monocultures at one of two sowing abundances (the combined monocultures method; CM) and in a low-density mixt ure of equal proportions of all species (the community density series metho d; CDS). 3 Proportional growth decreased with increasing initial abundance for all s pecies. Community effects (relative biomass of each species) were generally weaker and more variable than individual effects. R increased linearly wit h abundance for only one species, while three species showed a quadratic re sponse (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 comp arison. 5 With CDS, competitive hierarchies were similar along the community abunda nce gradient, implying that non-linear competitive effects are not likely t o 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 t he gradient of initial community abundance, with decreasing values for most species at high initial community abundance, as were the effects of intera ctions on community composition.