Effects Of Nitrogen Addition On Herbivory Tolerance Of An Invasive Wetland Plant Alternanthera Philoxeroides

The tolerance of plants to herbivores can affect the invasion success of some introduced plants.Due to the loss of specialist herbivores in introduced ranges,introduced plants may undergo rapid evolution of life-history characteristics and thus balance their growth performance and defense capacity.Using the invasive plant Alternanthera philoxeroides,we examined the effects of(real and simulated)herbivory and nitrogen availability on growth and tolerance performance of A.philoxeroides.The results from the real herbivory experiment showed that the herbivory of Agasicles hygrophila significantly reduced total mass and aboveground development of A.philoxeroides and its native congener A.sessilis.The negative effects of herbivory on leaf mass,number of ramets and number of leaves of both plant species were more significant at the high than at the low nitrogen level.Herbivory of A.hygrophila did not affect root mass,and thus increased root to shoot ratio of both plant species.Herbivory had a neutral or positive effect on the concentration of starch and total non-structural carbohydrates in the stems and roots of A.philoxeroides,but a negative effect on these physiological traits of A.sessilis.The results from the simulated herbivory experiment showed that total mass,aboveground mass,leaf area,number of ramets and root mass of A.philoxeroides were significantly lower under leaf-removal treatments than under the control treatment.Aboveground growth of A.philoxeroides was also high at the high than at the low nitrogen level.Root mass and root to shoot ratio of A.philoxeroides increased significantly with increasing growth time.However,there were no three-way interactions of simulated herbivory,nutrient level and growth time.In conclusion,the increase of resource allocation to root system may contribute to the compensatory response of A.philoxeroides after aboveground herbivory.Besides,the herbivory intensity of aboveground herbivores is strongly negatively related to the aboveground development of A.philoxeroides,and the growth time is positively associated with the performance of A.philoxeroides.The results sugest that releasing a high amount of aboveground herbivores(e.g.,A.hygrophila)at the early plant growth stage may be critical to the bio-control of A.philoxeroides,and underground herbivores should be also considered for management of this invasive species.