Effects of Agasicles hygrophila herbivory on the clonal integration of Alternanthera philoxeroides and A. sessilis

Abstract

Many alien invasive plants were clonal species. Examining the relationship between clonal integration characteristics and invasiveness of alien clonal plants is important for clarifying their ecological adaptability and invasion mechanisms. Here, with the invasive plant species Alternanthera philoxeroides and its native congener A. sessilis as the studying objects, we compared the effects of clonal integration on the growth and the biomass allocation of the apical ramets, basal ramets, and the whole fragment of both species under herbivory by the biocontrol beetle Agasicles hygrophila. The results showed that under herbivory by A. hygrophila, leaf number, stolon length, and ramet number of the apical ramets as well as the ground diameter of the whole fragment of A. philoxeroides were significantly higher under clonal integration treatment compared to that without clonal integration, whereas belowground biomass and total biomass of the basal ramets and the whole fragment of A. philoxeroides were conversely decreased by 78.2%, 60.9 % and 48.7%, 37.2%, respectively, under clonal integration treatment compared to that without clonal integration. Ground diameter of the apical ramets and leaf number of the whole fragment of A. sessilis were significantly higher, but the number of basal ramets was 21.7% lower under clonal integration treatment compared to that without clonal integration. The biomass of the apical ramets, basal ramets, and whole fragment of A. sessilis did not significantly differ between clonal integration and without clonal integration treatments. The results of cost-benefit analysis showed that the ramet number and biomass of the apical ramets of A. philoxeroides as well as the ramet number of the apical ramets of A. sessilis were significantly benefited from clonal integration, but the cost-benefit of the ramet number and biomass of the basal ramets of both species were not affected by clonal integration treatment. These results suggested that clonal integration could partly alleviate herbivory pressure by A. hygrophila on the apical ramets of both species, and that the clonal integration ability of A. philoxeroides was stronger than A. sessilis. However, both species seemed not able to gain significant benefits from cloning integration at the whole fragment level.