Do plant traits predict the competitive abilities of closely related species?

Invasive species are a threat to every ecosystem. There is a strong incentive to predict which species will become invasive before they become too widespread and unmanageable. Different approaches have been advocated to assess invasive species potential. These include examining plant functional traits, quantifying competitive ability and phylogenetic comparison. In this study, we conducted experiments based on the above approaches in a multi-year, temporally replicated, set of experiments to compare these assessment methods to determine the invasive potential of Japanese chaff flower (Achyranthes japonica). We compared plant traits and competitive ability of Japanese chaff flower with two agricultural invasive species, Palmer amaranth (Amaranthus palmeri) and tall waterhemp (Amaranthus tuberculatus), and one endangered plant species, bloodleaf (Iresine rhizomatosa), in the Amaranthaceae. Additionally, we assessed the invasive potential based on each of these approaches and determined the degree of agreement between them. A relatively conservative assessment integrating all three approaches would be that the competitive ability of closely related individuals with similar functional traits would share invasive potential. In a greenhouse experiment, each of the study species and soya beans were grown as monocultures and were evaluated to assess the drawdown of an aboveground (light) and a belowground (nitrogen) resource. In a field experiment, each study species was grown at varying densities per 15-cm-diameter pot with or without one or two soya bean plants, to simulate relative densities for soya beans grown in 38- and 76-cm-wide row spacing, respectively. In addition, Japanese chaff flower seedlings were planted either as un-manipulated seedlings or as a seedling cut back to the soil surface at the four-node stage (cut Japanese chaff flower) at which point seedlings have reached a perennial growth stage. The greenhouse experiment showed that each species drew down light differently, but not nitrogen. Shading decreased the aboveground biomass of the species in comparison with unshaded controls. Nitrogen, however, increased the aboveground biomass of Palmer amaranth and Japanese chaff flower. In the field experiment, a competitive effect ranking was determined to be: tall waterhemp ≥ Palmer amaranth = cut Japanese chaff flower ≥ uncut Japanese chaff flower ≥ bloodleaf, with the competitive response ranking being the inverse. These results suggest that under specific conditions, these closely related species do exhibit similar competitive abilities. Furthermore, the invasiveness and not the life history or habitat of these closely related species appeared to be the driving factor of competitiveness.