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RESEARCH PRODUCT

Growth and reproduction responses of French invasive and North American native populations of common ragweed, Ambrosia artemisiifolia L. to defoliation

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Enemy release and Evolution of Increased Competitive Ability hypotheses have been put forward to explain invasion success of introduced plant species. These hypotheses assume that the loss of natural enemies could induce an evolutionary change in plant defence and in resource allocation allowing the introduced plant populations to increase in vigour and fitness. To detect changes in functional traits following defoliation, we compared native and introduced populations of common ragweed, grown in a common garden environment. We compared six populations of Ambrosia artemisiifolia L., three from the North American native range and three from the French introduction range. We explored the effect of leaf herbivory on plant vegetative and reproductive capacities. Plants were defoliated by hand in early summer, simulating different degrees of insect grazing with removing 0%, 50% or 90% of each leaf-limb, one month after transplantation. Total dry biomass was not affected by defoliation treatments whereas root: shoot ratio decreased significantly for native and introduced populations. These findings suggest an efficient reallocation of plant resources in shoots at the expense of roots following defoliation, which allows Ambrosia artemisiifolia to tolerate herbivory. Defoliation treatments did not affect plant reproductive abilities. Furthermore, we did not detect any difference in herbivory tolerance between native and introduced populations. However, French invasive populations had a higher seed production than North American native populations. As a result, the invasion success of A. artemisiifolia could be explained by a high constitutive tolerance to herbivory and an increased seed production in the introduction range.