Abstract
Ageratum conyzoides is a malignant invasive plant in China. In this study, we sought to clarify the phenotypic variation of this plant and its competitive interaction with the indigenous species Perilla frutescens . Therefore, we established single-species planting groups for each species, with plant densities of one, two, four, or eight individuals. In addition, we set up mixed-species planting groups comprising one, two, or four individuals of both species. The results revealed significant differences in phenotypic characteristics of A. conyzoides , such as the mean aboveground biomass (AB) in all planting, specific stem length (SSL), and specific leaf area (SLA) in single-species planting. Under the mixed-species planting, the height of P. frutescens was significantly lower than A. conyzoides . The initial leaf length, plant height, and planting pattern of A. conyzoides were found to have a significant influence on AB, whereas the initial plant height had a significant influence on growth, and the planting pattern had a significant influence on SLA. For A. conyzoides , the coefficient of variation (CV) values of SSL in the low-density mixed-species planting (HZ(2)), flower bud intensity (FBI) in high-density mixed-species planting (HZ(8)), and AB in all mixed-species planting patterns were greater than 20.0%, thereby indicating that A. conyzoides has strong plasticity. This comparison of competitiveness indicated that the interspecific competition between A. conyzoides and P. frutescens was greater than the intraspecific competition between the respective species and that the competitive capacity of A. conyzoides was greater than that of P. frutescens , particularly under conditions of the medium-density mixed-species planting. Based on these findings, we conclude that A. conyzoides can adapt to intraspecific and interspecific competition via phenotypic characteristics variation and maintain a competitive advantage. In addition, we established that in the presence of sufficient resources, the competitiveness of A. conyzoides is strongest at medium plant densities.