Interspecific competition with the American Xanthium orientale L. as a possible cause of the decline of the Old-World X. strumarium L.

Xanthium is represented in Europe by three species complexes: X. strumarium L., X. orientale L., and X. spinosum L. The former two complexes are similar, in both morphology and ecological requirements. Xanthium strumarium is native to the Old World, whereas X. orientale originates from America and was accidentally introduced into Europe about two centuries ago. Since then, it has colonized the whole continent, while the native congener has become increasingly rare.Over two years, we conducted competition experiments to assess the impact of the introduced X. orientale on the fitness of the native X. strumarium. Germination time, dry biomass, number of burs (pistillate flower heads) and bur biomass were measured as proxies of fitness. Xanthium strumarium was grown alone (control), together with conspecifics (intraspecific competition) or with X. orientale plants (interspecific competition). We also evaluated the allelopathic effect of X. orientale over X. strumarium, by watering Xanthium seedlings with exudate of X. orientale dry leaves.Growth and reproductive traits of X. strumarium were significantly lower in individuals growing in proximity of X. orientale compared to the control, whereas intraspecific competition has a lower but still significant effect. Xanthium orientale, although, germinates and grows faster than the Old-World congener, and under interspecific competition regime, X. strumarium produces significantly lower biomass, number of burs and bur biomass. Watering with exudates negatively influences the germination and the growth of the two species. We therefore believe that interspecific competition of the introduced congener may be one of the causes explaining the drastic decline of X. strumarium populations in Europe in the past century.