Constraints to native plant species establishment in coastal dune communities invaded by Carpobrotus edulis: Implications for restoration

Carpobrotus edulis, an alien chamaephyte species from South Africa, severely invades and represents one of the greatest threats to coastal plant biodiversity in regions with Mediterranean climate worldwide. Although actions have been promoted to eliminate it, these efforts have failed to restore dunes to the natural, preinvasion stage. We tested, by means of field and laboratory experiments, how C. edulis alters soil chemistry by causing residual effects on soil, and examined whether these effects decrease germination, survival and growth of a common native chamaephyte dune species Malcolmia littorea. We also recorded species diversity to find out which species can establish after the removal of the invasive C. edulis. To link both measures, we monitored changes in soil characteristics over 1 year after the removal of the invader, by analyzing both soil chemical properties and extracellular enzymes. C. edulis lowers soil pH, Ca and Na content and increases organic content, salinity and nitrogen and phosphorus concentration. The effects of the invader on the growth of M. littorea during the first growing season were very weak, despite strong negative effects in the early stages of this species’ population development that decreased total germination and survival. Overall, the soil characteristics and activity of the microbial community tend to recover back to preinvasion conditions in plots from which C. edulis is removed. In contrast, the establishment of native dune vegetation is constrained, as indicated by lower species diversity in restored compared to non-invaded areas. This is because regenerating dunes are occupied by opportunistic ruderal species that compete with native dune plants of conservation value and restrict their establishment