Objective The alien invasive plant Alternanthera philoxeroides is often sympatric with native congener A. sessilis, but occupies an ecological advantage over A. sessilis in China. This study aimed to explore the relationship between clonal integration and the strong competitiveness of A. philoxeroides.

Method In a common garden experiment, the stolon connection between the apical and the basal ramets of A. philoxeroides and A. sessilis were left intact (clonal integration) or disconnected (without clonal integration), and the growth, photosynthesis, and biomass distribution of the apical ramets, the basal ramets, and the whole fragments of the two plants under different clonal integration treatments were examined to compare the clonal integration abilities of the two plants.

Result The stem length of the apical ramets, the number of leaves of the basal ramets as well as the leaf number and the stem length of the whole fragment of A. philoxeroides all significantly increased under clonal integration treatment. Moreover, the number of fine roots, total roots, and some photosynthetic indicators (such as light compensation point, stomatal conductivity, etc.) of A. philoxeroides all significantly increased under clonal integration treatment. Similarly, the aboveground/belowground biomass, the total biomass, the number of coarse/fine roots, and total number of roots of the apical ramets, the basal ramets, and the whole fragment of A. sessilis also significantly increased under clonal integration treatment. However, the aboveground/belowground biomass, the total biomass, and some photosynthetic indicators (such as net photosynthetic rate, transpiration rate, and stomatal conductance) of the apical ramets, the basal ramets, and the whole fragment of A. philoxeroides were significantly higher than those of A. sessilis under clonal integration treatment.

Conclusion Both A. philoxeroides and A. sessilis can partly benefit from clonal integration, and A. philoxeroides has a stronger clonal integration ability than A. sessilis. A. philoxeroides might occupy the spatial niche through clonal integration, thus forming competitive advantage in natural habitats.