Response of phenotypic plasticity of invasive Ageratum conyzoides L. to interspecific competition

Phenotypic plasticity is an effective way for plants to adapt to environmental changes and one of the mechanisms by which alien plants successfully invade. At present, it is difficult to study how invasive plants respond to complex biological environments to show adaptability plasticity. To explore the response of invasive plant phenotypic plasticity to interspecific competition, we tested the phenotypic plasticity of functional traits of invasive Ageratum conyzoides L. under competition with native P. frutescens at different competitive distances. Results showed that: (1) Higher initial height of A. conyzoides significantly reduced its subsequent relative height growth (regression coefficient, β = −0.137), relative height growth of A. conyzoides was significantly negatively correlated with initial height, and height growth rate of A. conyzoides was significantly higher than that of P. frutescens. (2) The distance between A. conyzoides and competitors (competitive distance) had a significant effect on aboveground biomass, specific stem length, and flower bud intensity of A. conyzoides at harvest (P < 0.05). Specific stem length and flower bud intensity of A. conyzoides were significantly higher at a competitive distance of 5 cm than at other distances. (3) Competitive asymmetry only affected the specific stem length of A. conyzoides. The greater number of P. frutescens with higher initial height than A. conyzoides, the longer the specific stem length of A. conyzoides (regression coefficient, β = 3.374). (4) Except for height growth, seven other A. conyzoides traits showed high phenotypic plasticity, with CV values between 0.17 – 0.55. There were significant differences in the comprehensive phenotypic plasticity of A. conyzoides under different competitive distances. The phenotypic plasticity index of A. conyzoides was the lowest under a competitive distance of 2.5 cm and was significantly improved at competitive distances of 10 and 20 cm. Overall, under close competition with P. frutescens (2.5 cm), A. conyzoides growth and reproduction were generally inhibited with low phenotypic plasticity. However, with the increase in competitive distance, A. conyzoides quickly adapted to interspecific competition and exhibited high phenotypic plasticity. At a competitive distance of 5 cm, A. conyzoides showed a strong shade avoidance response and gained an advantage in light resources through the increase of specific stem length.