Herbivory and nitrogen availability affect performance of an invader Alternanthera philoxeroides and its native congener A. sessilis

Abstract

Tolerance to herbivory influences the success of invasive species in introduced ranges, and thus a comparison of the difference in herbivory tolerance between invasive and native species may help to understand the mechanisms of plant invasions. We conducted a greenhouse experiment in Beijing, China to examine effects of aboveground herbivory (by a beetle Agasicles hygrophila or not) and nitrogen availability (high vs. low) on growth and physiology of the invasive plant Alternanthera philoxeroides and its coexisting native congener A. sessilis. Herbivory by A. hygrophila significantly reduced total mass and aboveground development of both plant species, and the negative effects of herbivory on leaf mass, ramet number, and leaf number were more remarkable at the high than at the low nitrogen level. On the other hand, herbivory did not affect root mass and even increased root to shoot ratio of both plant species. The increase in root to shoot ratio and relative ratio of root mass caused by herbivory was also stronger in A. philoxeroides than in A. sessilis. Besides, herbivory exerted neutral or positive effects on the concentrations of starch and total non-structural carbohydrates in stems and roots of A. philoxeroides, but negative effects on the physiological measures of A. sessilis. We conclude that A. philoxeroides may possess a stronger tolerance to herbivory than A. sessilis and that increased resource allocation to roots could potentially contribute to compensatory responses of A. philoxeroides to aboveground herbivory. Our findings also suggest that the control of A. hygrophila may be invalid in belowground invasion of A. philoxeroides, and that belowground herbivores should be considered in the future management of invasive species.