Addressing biases in replacement series: the importance of reference density selection for interpretation of competition outcomes

Abstract Replacement series are used by researchers to understand how competition-related variables influence dynamics from the individual to the population and community levels, but this approach has been criticized because of inherent biases associated with plant size differences and density-dependent responses. The use of functional densities instead of demographic densities was proposed to minimize those biases. This work explored three models to determine reference densities for replacement series experiments based on 1) maximum biomass, 2) biomass at onset of diminishing returns (i.e., inflection point), and 3) N uptake equivalency. Replacement series experiments were conducted using redroot pigweed ( Amaranthus hybridus L.):maize ( Zea mays L.) and giant foxtail ( Setaria faberi Herrm.):maize proportions of 1:0, 0.75:0.25, 0.5:0.5, 0.25:0.75, and 0:1. The monoculture density for each species was established according to the three models. Density selection criteria resulted in major differences in competitive interactions between species. The use of functional densities at which the biomass accumulation inflection point for the smaller species allowed both species to exhibit either increases or decreases in biomass production depending on competitive interactions for all inter-specific mixtures. Conversely, the maximum biomass model favored the larger species almost completely inhibiting the growth of the smaller species, which resulted in a poor characterization of competitive responses of the smaller species. The N uptake equivalency model resulted in interactions closer to the predicted neutral competition. The model based on the biomass accumulation inflection point was the most sensitive and informative across all inter-specific mixtures for both species. We propose that to reduce bias associated with species size differences when determining reference densities for replacement series experiments, at least two criteria must be met: 1) the experiment sensitivity allows measuring and quantifying the competitive responses for both species in all mixtures, and 2) the balance between density and carrying capacity of the system minimizes intra-specific competition.