Plant-soil feedback as a mechanism of invasion by Carpobrotus edulis

Invasive plant species have been suggested to change the composition of the soil community in a way that results in a positive feedback for them and a negative feedback for the native plant community. Carpobrotus edulis, a species native to South Africa, is one of the most aggressive exotic species in Mediterranean Europe. Although several aspects of its invasion biology have been studied, the occurrence of plant-soil feedback has been scarcely investigated. We first checked for the existence of biotic resistance in soils from two invaded sites of Mediterranean Europe and one site in the native area. Secondly, we evaluated the effects of soil conditioning on the germination and plant growth of C. edulis and two key species of native dunes. Finally, we tested the effects of short- and long-term soil conditioning on the performance and reproductive effort of C. edulis. Our results show that at first there is a natural resistance to invasion by the soil biota. Later, biotic resistance in invaded soil is suppressed by the establishment of a soil community that enhances the growth of C. edulis and that negatively influences the growth and survival of the native plants. Long-term soil conditioning in the field resulted in shifts in the balance between vegetative growth and sexual reproduction. Long-term invasion was also reflected in high levels of endophyte colonization by chytrids in roots, although the physiological consequences of this colonization remain unknown. The results obtained illustrate a mechanism that explains how C. edulis breaks the initial biotic resistance of newly-invaded landscapes. Finally, this study highlights the importance of studying plant-soil interactions on different members of the plant community and temporal stages in order to fully understand invasion.